CHEMICAL EQUATIONS and STOICHIOMETRY

5.1 Balancing simple equations

Chemists write chemical equations to express chemical reactions that occur. These equations show the type and number of molecules which react and the type and number of molecules which are produced. They are used extensively by chemists and we shall look at how to write chemical equations and how to use them.

When hydrogen gas and oxygen gas are mixed together and a spark ignites them, water is produced. This reaction was demonstrated in spectacular fashion by the Hindenburg dirigible. We can write this

H2 + 02 ------> H20

Although this shows the reactants and products, the above equation is not con1plete because it is not "balanced". The left hand side of the equation contains 2 oxygen atoms in the 0 2

molecule, while there is only one oxygen atom in the water molecule shown as a product.

Each molecule of 0 2 can produce two molecules of H20, so we write a 2 in front of the water molecule. (Note that we can NOT write 2 subscript after the O of H20; that would produce H20 2, which is hydrogen peroxide, a totally different molecule!) By writing a 2 in front of the water molecule we now have 4 H atoms in the product in the 2 water molecules and only 2 H atoms shown in the H2 molecule on the left hand side of the equation. To correct this we can write a 2 in front of the H2 molecule. Thus

2 H2 + 02 -----> 2 H20

This equation is balanced. There are equal number of each type of atom in the reactants and the products.

Let's look at another example. Ammonia (NH3) is synthesized from nitrogen gas (N2) and hydrogen gas (H2). This reaction is carried out in megaton quantities in the synthesis of ammonia fertilizer.

N2 + H2 -----> NH3

As before, we need to equalize (balance) the number of each type of atom in the reactant and the product side of the equation. Since nitrogen gas contains 2 nitrogen atoms per molecule each N2 molecule can produce 2 molecules of ammonia, NH3• We put a 2 in front of the anunonia formula. This gives us 2 atoms of nitrogen and 6 atoms ofhydrogen in the product. We now need to balance the number of hydrogen atoms. Since there are 6 H atoms in the product, we need 6 H atoms in the reactants. Hydrogen molecules come in units of 2 hydrogen atoms, so we need 3 H2 molecules.

5.2 Balancing combustion equations

Let's look at the combustion of natural gas, CH4, with oxygen. This combustion produces carbon dioxide and water as products.

CH4 + 02 ------> CO2 + H20

There is 1 C atom on each side of the equation, so C atoms are balanced. There are 4 hydrogen atoms in a methane molecule and only 2 H atoms in a water molecule, so we need

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2 water molecules to balance H atoms. We have 2 0 atoms in the 0 2molecule in the reactants, but a total of 4 0 atoms in the products ( 2 from carbon dioxide and 2 from the two water molecules) so we need two molecules of 0 2 to balance 0 atoms. The equation is now balanced.

Now try balancing the combustion ofpropane, C3H8.

Balance the equation for the combustion ofbutane, C4H10.

-----> CO2 + H20

When you get this situation, go ahead and balance the equation with a fraction, but when you are through, get rid of the fraction by multiplying the whole equation by 2.

Now balance the equation for the combustion of octane:CsH1s-

-----> CO2 + H20

Look at what happens to the ratio of fuel to oxygen ratio as one goes from bu1ning natural gas, propane, butane, and octane. What sort of problems do you think would occur if you tried to bu1n propane in a device designed for natural gas (methane)?

Let's look at the combustion of ethanol, C2H60.

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The major difference in this equation is that there is an 0 atom in the ethanol fuel that must be accounted for in balancing atoms. We put a 2 in front of the carbon dioxide to balance C atoms, and a 3 in front of the H20 to balance H atoms. As a result we have 7 0 atoms in the product side of the equation and we need to have a total of 7 0 atoms on the reactant side of the equation. In contrast to previous examples, we have O atoms in both the ethanol and 0 2

molecules. We want a total of 7 0 atoms in the reactants, and we have one incorporated into the ethanol molecule. Ifwe put a 3 in front of the 0 2 molecule we will get 6 0 atoms from 0 2

and one more from the O in the ethanol molecule for a total of 7 0 atoms in the reactant side of the equation. (Since we balanced the rest of the equation based on one C2H60 ethanol molecule we do not want to change the number of ethanol molecules.)

. Note the ratio of fuel to oxygen ratio for ethanol and for octane. Would you be able to drive a car designed for octane with ethanol substituted as a fuel?

Try balancing the equation for the bu1ning of methanol (CH40)(produced from breakdown of wood cellulose) with oxygen.

Balance the equation for metabolizing sucrose, C12H220 11 ,

given that it is metabolized the same way as a combustion reaction (i.e. it reacts with 0 2 and produces CO2 and H20 as products

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5.3. Balancing Metathesis Reactions

We can also balance equations involving the reaction of ionic chemicals. When we do so, we balance the equation ion by ion, analogous to the way we balanced the above equations. When we have polyatomic ions in the equation it is easier to balance the equation treating the polyatomic ion as a unit, rather than balancing the atoms within the polyatomic ion atom by atom. Try balancing the following equations.

Ca(N03)2(aq) + K.3P04(aq) -----> CaJ(P04)2(ppt) + KN03(aq) (The symbol aq stands for aqueous and means that the chemical is

'dissolved in a water solution; ppt stands for precipitate)

Mg(N03)2caq) + Na2S04(aq) -----> MgS04(ppt) + NaN03(aq)

AgN03(aq) + MgCh(aq) -----------> AgCl(ppt) + Mg(N03)2(aq)

You should note that in each of the above reactions the soluble ions exchange partners with each other to form an insoluble precipitate. These are called metathesis reactions.

5.4 Balancing Oxidation-Reduction Reactions

We saw earlier that atoms on the left side of the periodic table tend to lose electrons, while atoms on the far right hand side of the periodic table tend to gain electrons. Reactions involving gain and loss of electrons are called reduction-oxidation reactions or redox reactions. You observed one of these reactions in the "Physical and Chemical Processes" lab.

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Zn+ CuSO4 ----> ZnSO4 + Cu

In this reaction the zinc was oxidized: it lost 2 electrons to become a zn+2 ion. The copper ion started as a +2 ion (can you see why?) and accepted the two electrons from the neutral Zn atom to become a neutral copper atom. Thus the Cu ion was reduced. The sulfate ion acts as spectator ion which balances a mdal ion on each side of the equation.

This equation is a balanced redox equation. It also tells us something about the relative ability of Zn and Cu ions to lose electrons. Although Zn and Cu ions are both transition metals which have a tendency to lose electrons, the above reaction tells us that Zn has a higher tendency to lose electrons than does Cu. Thus Zn has a higher tendency to oxidize than does Cu; we say that Zn is higher in activity than Cu. To say it still another way, Zn has a higher tendency to corrode than does Cu.

Suppose we put a strip of Zn metal in a solution of iron(II) sulfate; We would see that a powder would form on the surface of the Zn metal. Analysis of this powder shows that it is neutral iron n1etal. Zn +z ions are dissolved into solution and their charge is balanced by sulfate ions .. The balanced equation for this reaction is:

Zn+ FeSO4 ---> Fe + ZnSO4 Comparing this equation with the previous one, which atom was

oxidized in this reaction? Which atom was reduced? Which atom is higher in activity?

The activity demonstrated in the above reaction is useful when one galvanizes steel(iron). Galvanization is the process of putting a thin coat of Zn metal on the surface of steel. Which metal will oxidize the most easily? Can you see the advantage of this?

Let's try still another redox reaction. When we put a strip of iron in a solution of copper(II) nitrate, copper metal powder plates out on the

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surface of the iron and some of the original iron on the strip is lost and appears in solution as iron(II) ions.

The equation for this reaction is: Fe + Cu(N03)2 ----> Cu + Fe(N03)2

Can you explain why there are two nitrate ions balancing both the Cu ion and the iron ion?

Which atom was oxidized? Which atom was reduced?

Which atom has higher activity, Fe or Cu? If we accumulate all the information from the previous three

reactions, we can make the following observations.

1) The first reaction tells us that Zn is higher in activity than Cu. ,. 2) The second reaction tells us that Zn in higher in activity than Fe.

3) The third reaction tells us that Fe is higher in activity than Cu. Hence Zn is highest in activity, Fe is inte1mediate, and Cu is the

·1~ast.

Which of the above metals corrodes most easily? Which will have the least tendency to corrode?

If we put a strip of Cu metal in with Zn(N03)2 solution, we observe that no reaction occurs. Can you explain why this is reasonable?

Try writing the redox equation for the following experiment: A strip of Zn is placed in a solution of tin(II) nitrate. Neutral Sn

metal plates out and Zn+z appears in solution.(The zinc(II) ion is balanced by nitrate ions). Write the equation for this redox equation.

Which metal is higher in activity, Zn or Sn?

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When a strip of Sn is put in a solution of zinc(II) nitrate, nothing happens. Explain.

A strip of Sn is placed in a solution of copper(II) nitrate and neutral Cu plates out on the surface of the Sn and Sn +z ions dissolve into solution. Write the equation for this redox reaction.

Which is higher in activity, Sn or Cu? When a strip of Cu is placed in a solution of tin(II) nitrate, nothing

happens. Explain. From the above reactions, rank Cu, Zn, and Sn in order from highest

activity to lowest activity.

When a strip of iron metal is placed in a solution ofAgNO3, silver powder metal plates out on the surface of the iron and the iron corrodes, producing iron(II) ions in solution. Write the equation for this redox reaction.

Note that there needs to be a little more balancing going on. Since the iron loses 2 electrons from each atom and the silver ion only needs to gain one electron to become neutral, there needs to be 2 silver nitrates reacting with each one neutral iron atom.

Let's try another one like this. When a strip of Sn metal is placed in a solution of AgNO3, silver metal plates out and tin(II) ions appear in solution. Write the equation for this reaction.

Which metal is higher in activity, Sn or Ag?

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When a strip of magnesium metal is put in solution with Al(N03) 3,

aluminum metal plates out and Mg +2 ions appear in solution(balanced by nitrate ions). Write the equation for this redox equation.

Which is higher in activity, magnesium or aluminum?

Many metals are oxidized( co1Toded) when they are placed in acid solutions such as hydrochloric acid (HCl) nitric acid (HN03) or sulfuric acid (H2S04). As they oxidize they give their electrons to the H+ ion of the acid and form H2 gas. When a strip of iron is placed in hydrochloric acid (HCl), the reaction is:

Fe + 2HC1 ---> FeClz + H2

Note that the iron is oxidized: it staiis neutral and ends up as a fe1Tous Fe +2 ion.

The H+ is reduced: it staiis as 2 H+1 ion and ends up as a neutral H2

molecule.

Since the iron gave electrons to the W of the HCl, we say that iron is higher in activity than hydrogen, which is just another way of saying that iron is co1Toded by acid.

If we put a strip of gold(Au) in HCl, nothing happens. Gold does NOT give electrons away to H+ and is not co1Toded by acid. We say that gold is lower in activity than hydrogen; i.e. gold is not co1Toded by acid .

Tty the following. When magnesium metal is placed in nitric acid(HN03) the

magnesium is co1Toded away and hydrogen gas is evolved. Write the equation for this reaction.

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Is Mg higher or lower in activity than hydrogen?

When platinum metal is placed in nitric acid, nothing happens. Which is highe1: in activity, platinum or hydrogen?

When Sn is placed in nitric acid, tin(II) nitrate is formed in solution and hydrogen gas is evolved. Write the balanced equation for this reaction. Is tin higher or lower than hydrogen?

When aluminum is placed in nitric acid, aluminum nitrate is formed in solution and hydrogen gas is evolved. Write the balanced equation.

Which is higher in activity?

5.5 Balancing Neutralization Reactions

Another important type of ionic reaction occurs when an acid molecule containing a H+ ion combines with a basic

compound containing a OH-. The H+ and OH- combine to form water and the remaining ions balance each others' charge. These are called neutralization reactions. Try balancing the following equations:

HCl + NaOH -----> H2O + NaCl

HF + KOH -----> H2O + KF

When there is more than one H+ or OH- in the formula a little more effo1i is required to write the balanced equation. Try the following.

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HCl + Mg(OH)2---> H2O + MgCh

5.6 MOLES

So far we have discussed atoms and molecules on an individual basis. Unfortunately atoms and molecules are so small that actual experiments with weighable amounts of chemical are going to involve huge numbers of individual atoms and molecules. It is extremely useful to have a defined number of atoms that is large enough to be measured with typical laboratory equipment. We will define that number as the number ofH atoms in 1 gram ofH. That number can be calculated as:

1 gran1 x 1 atom = .602 (1024) aton1s = 6.02 (10-1)(1024)at01ns 1.66(1 o-24)g

= 6.02 (1023)atoms

This number is called Avogadro's number, in honor of an Italian scientist of the 1600's. Avogadro's number of atoms or molecules is the "working unit" that we will use of any atom or molecule and it is referred to as a mole.

Thus one mole ofH atoms contains 6.02 (1023) H atoms and has a mass of 1 gram. The mass of 1 mole of any other so1i of atoms can be found by noting the relative atomic weight in comparison with H. Thus refening to the periodic table, we find that the atomic weight

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of He is 4, which is 4 times the weight ofH. Hence 1 mole of He will weigh 4 times 1 mole of hydrogen, or 4 gran1s. The atomic weight of iron (Fe) is given in the periodic table as 55.8, which is 55.8 times the weight of H, or 55.8 grams for one mole of Pe. The atomic weight of uranium is 238 and hence one mole ofU will weigh 238 grams. Thus we can find the weight of 1 mole of any atom by simply referring to the periodic table and giving the atomic weight in grams.

We can extend mole weights to molecules by simply adding up the weights of the atoms in the fo1mula. Thus the weight of 1 mole ofwater, H2O, will have a weight equal to the weight of 2 moles of H atoms + 1 mole of oxygen atoms = 2 (1 gram) + 16 grams = 18 grams

What will the weight of 1 mole of methane, CH4 be?

What will the weight of 1 mole of butane, C4H10, be?

What will the weight of 1 mole of sodiun1 phosphate, Na3PO4, be?

It should be easy to see that we can calculate the weight of something other than 1 mole of a substance, simply by multiplying the number of moles we have times the weight of 1 mole.

What is the weight of 3.5 moles of methane?

3.5 moles x 16 grams= 56 grams lmole

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What is the weight of 4.4 moles of ammonia, NH3?

How many molecules of ammonia is this?

What is the weight of .7 moles ofNaHCO3?

Instead of converting moles into grams, we can also convert grams to moles, by dividing by the molecular weight.

How many moles are in 100 grams ofmethane, CH4?

100 grams lmole - 6.25 moles 16 grams

How many molecules ofmethane is this?

How many moles ofNa2SO4 are in 50 grams of sodium sulfate?

How many moles ofC4H1o are in 5.5 grams of C4H 10?

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5.7 Calculating chemical formulas from experimental data

Let's see how we can use mole calculations to determine the chemical formulas for some compounds.

If 2.05 grams of nickel reacts with 1.12 grams of sulfur, what is the formula of nickel sulfide? What is the charge on the Ni ion?

2.05 g Ni x 1 mole - .035 moles Ni 58.7 gNi

1.12 g S x 1 mole = .035 moles S 32 g S

This shows that the Ni and S react in a 1 to 1 n1ole ratio and hence the formula is NiS. Since we know that the charge on S ion is -2 and that the compound as a whole must be neutral, the charge on the Ni ion must be +2.

What is the formula of molybdenum sulfide if l.92 grams of Mo react with 1.92 grams of S? What is the charge on the Mo ion?

What is the formula for silver oxide if 9.7 grams of silver react to form 11.1 grams of silver oxide. What is the charge on the silver ion?

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What is the formula for an iron sulfide where 2.45 grams of iron reacts with S to produce 4.56 grams of iron sulfide? What is the charge on the iron ion? Is it fenous or fenic ion?

5.8 Stoichiometry

We will now look at how we can use balanced equations to predict the ratio in which chemicals will react. Let's look at an equation we balanced previously, the combustion of methane.

-----> CO2 + 2 H20

We previously interpreted this equation as saying that 1 molecule of methane reacts with 2 molecules of oxygen molecules and produces 1 molecule of carbon dioxide and 2 molecules of water. As we already indicated, individual molecules are too small to weigh, and so if we are going to use this equation on a scale that is experimentally measurable, we need to use a much larger quantity of atoms, namely the mole or Avogadro's number. Thus we can interpret this equation as stating that 1 mole of methane (CH4) reacts with 2 moles of 0 2 to produce I mole of carbon dioxide and 2 moles of water. The equation thus gives us the mole ratio in which molecules react as well as the molecule ratio.

We can use this equation to calculate how many moles of 0 2

are needed to react with any number of moles of methane that we start with, because the equation tells us that the 0 2 will always react with CH4 in a 2: I ratio. Hence if we staii with 10 moles of methane we will need 20 moles of 0 2 to react with it. We can do this with cancellation of units as shown below

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2 moles 02 10 moles CH4 x 1 mole CH4 = 20 moles 0 2

We can also calculate the number ofmoles of CO2 and H2O that will be produced from that 10 moles of CH4 • The equation tells us that 1 mole of CO2 will be produced for each mole of CH4 that we start with and that 2 moles of H2O will be produced for each 1 mole ofCH4. Using the cancellation of units method

1 mole CO2 10 moles CH4 x 1 mole CH4 = 10 moles CO2

2 moles H2O 10 moles CH4 x 1 mole CH4 = 20 moles H2O

Using the balanced equation of methane combustion above, try the following problem:

If one starts with 13.5 moles of methane, CH4, how many moles of 0 2 are needed to react with it and how many moles of CO2 and H2O are produced?

Balance the following equation:

Now, using that balanced equation, determine how 1nany moles of 02 are needed to react with 4.6 moles of C5H 12. How many moles of CO2 will be produced from 4.6 moles of C5H 12? How many moles of H2O will be produced from 4.6 moles of C5H 12?

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Balance the equation

02 ----->

When you have balanced the equation, calculate how many moles of 0 2 are needed to react with 8.4 moles of C6H14, How many moles of CO2will be produced from 8.4 moles of C6 H14? How many moles of H20 will be produced by 8.4 moles of C6H14?

,

'

Ifwe were actually going to weigh out chemical reagents, we would weigh them in grams, not moles. We need to figure out how many grams of one chemical are needed to react with so many grams of another chemical. Luckily we can get this information by massaging the balanced equation a bit further.

Let's look at our methane combustion equation again CH4 + 2 02 -----> CO2 + 2 H20

grams:

As we indicated before this equation can be interpreted as indicating that 1 mole of methane reacts with 2 moles of oxygen to fo1m 1 mole of carbon dioxide and 2 moles of water. If we calculate

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the weight of 1 mole of methane, 2 moles of 0 2 , 1 mole of CO2

and 2 moles of water, we will have the gram ratio in which they react.

Grams

Given the above equation calculate how many grams of 0 2 are needed to react with 10 grams ofmethane. How many grams of CO2

are produced? How many grams of water are produced?

Balance the following equation

(

+ 02 ------>

grms

Using this balanced equation, calculate how many grams of oxygen are needed to react with 50 grams of butane, C4H10. How many grams of carbon dioxide are produced? How many grams of water are produced?

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Balance the following equation:

02 ----->

a) Starting with 5.5 moles of C6H14, how many moles of02 are required to react with it? How many moles of CO2 are produced? How many moles ofH20 are produced?

b) Staiiing with 50 grams of C6H14, how many grams of 0 2 are needed to react with it? How many grams of CO2 are produced? How many grams ofH20 are produced?

5.9 Energy and Chemical Reactions

Besides balancing chemical equations and calculation of amounts of each reagent needed, there is one other aspect of chemical reactions we need to take a look at. When we carry out the combustion ofmethane, CH4, heat is released. Production of heat is the reason for burning methane. Reactions which release energy are called exergonic or exothermic reactions.

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Chemists frequently look at energy changes during reactions in terms of REACTION PROFILES. They show the chemical potential energy of the reactants and products. The reaction profile for the combustion of methane is shown below.

Chemical Potential Energy

time The potential energy ofmethane and oxygen is higher than that of

carbon dioxide and water. This corresponds to the methane ( and oxygen) being on the top of a plateau, with a high potential energy. When combustion occurs, the potential energy decreases and is converted into kinetic energy i.e. faster moving molecules. A good analogy would be a boulder which rolls down from the top of the hill. As its potential energy (the height on the hill) decreases its

1kinetic energy (how fast it is moving) increases. At the bottom of the hill we have a boulder with low potential energy but high kinetic energy. After the combustion occurs, we have produced carbon dioxide and water which have lower potential energy than methane and oxygen, but they are moving at faster speeds due to the energy released.

There are also reactions which require input of energy. When you boil liquid water and convert it into gas, energy is absorbed. When you heated the blue crystals of copper sulfate pentahydrate in lab, the blue crystals decomposed into anhydrous (without water) copper sulfate and water condensate at the top of the test tube.

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1 This reaction required the absorption of heat. It is referred to as an endergonic or endothermic reaction. The reaction profile is shown below:

Chemical CuS04 + 5 H20 Potential Energy

Let's consider the combustion of methane in more detail. We can mix methane and oxygen and no reaction occurs until we provide a spark to ignite the methane. Why is the spark necessary? Because the reaction requires that we break the C-H and O=O bonds of the reactants before we can start forming new bonds of the products. The breaking of these bonds requires INPUT of energy before the energy of combustion is released. Thus there is an energy hill in between the reactants and products:

Ea Chemical Potential Energy

time

This energy hill in between reactants and products is called the activation energy and is abbreviated Ea,

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The activation energy corresponds to the amount of energy needed to break the bonds of the reactants to start the actual reaction.

The energy needed to get over the activation energy barrier is provided by the kinetic energy of collisions between molecules. As we increase the temperature, the average kinetic energy ofmolecules increases and the number of mok,'ular collisions with enough energy to break bonds increases. This explains why increasing the temperature usually speeds up the rate of reaction. Increasing the concentration ofmolecules also increases the frequency of collisions and hence increases the rate of chemical reactions.

The higher the activation energy barrier, the more energy is needed to break the bonds, and the higher the temperature needed. As the temperature increases, molecules move faster, they collide more violently and more bonds get broken. To use an analogy, the faster cars are traveling the more likely it is that there will be collision of cars (breaking of bonds).

There are substances which tend to destabilize selected bonds of the reactants and make it easier to break the bonds. These substances are called catalysts and they lower the activation energy. It should be emphasized that they are not used up in the reaction.

No catalyst ..---------

Catalyst added

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By lowering the activation energy, they make it easier for molecules to react and the rate of reaction speeds up, without having to increase the temperature! There are many catalysts in nature, the most important ones for health science majors being enzymes, the biological catalysts that catalyze most of the reactions which occur in living organisms.

Label the following reaction profiles as exergonic or endergonic. Also label Ea in each reaction.

Note that although the second two profiles are both exergonic, the third is much more exergonic than the second. The third profile is essentially irreversible. It is unlikely that a product molecule will reform a reactant molecule, unless a great deal of energy is put into the system. In contrast the energy level of reactants and products are fairly similar in the first and second reactions, even though the first is endergonic and the second is exergonic . The Ea that must be overcome is about the same in both the forward and backwards direction. Both of these reactions are easily reversible and there will be an equilibrium between reactant and product molecules.

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5.10 Reaction profiles of some important reactions

As you know, iron can react with oxygen to form rust. 4 Fe+ 302 ---------> 2Fe203

This reaction is very exergonic and is effectively irreversible. Although it is common to see steel become rusty, one is unlikely to see rust become a piece of fresh steel. Note that the reaction has a fairly high activation energy. If one keeps iron dry, it does not tend to rust unless one increases the temperature to overcome the high activation energy.

However in the presence of moisture (H20) the rate of rusting increases greatly. In some ways the presence of water acts as a catalyst for the rusting of iron (the situation is actually more complicated than this). The activation energy barrier is lowered and the rate of rusting increases. If we add some NaCl, the rate of rust formation goes even faster, as everybody knows who has ever driven a car where they salt the roads. Its reaction profile is shown below.

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4 Fe+ 3 02

~No catalyst Water Water+ NaCl

Why are roads salted? To lower the melting point of ice. In fact there are other substances which also lower the melting point of ice, but they all cost more tan NaCl. Since the budget of most highway departments is limited, they buy the cheapest ice melter, NaCl, and transfer the cost of the conosion onto the vehicle owner. As a whole, automakers have not been unhappy with this situation, since it ensures that new cars will have to be bought on a regular basis. In recents years, they have taken to galvanizing auto bodies to increase their lifetime. How does galvanizing increase the life of the car? A second example with impo1iant physiological implications t~

CO2 + H20 <------> H2C03 carbonic acid

This reaction occurs in the blood and the gas exchange reactions in the lungs. The reaction profile is as shown below:

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This reaction is endergonic and reversible. There is only a slight difference in energy levels of the reactants and the products and the direction of the reaction can be shifted by changing the concentration of reactants and/or products. When carbonated drinks (soda, beer, champagne) are produced a high pressure of CO2

is applied which increases the carbonic acid concentration. When the top of the bottle is taken off, the pressure of CO2 is released and the carbonic acid dissolved in solution starts decomposing to form CO2 and H2O.

The bubbles observed in carbonated beverage are carbon dioxide bubbles resulting from the decomposition of the carbonic acid in solution. As it does so the carbonic acid concentration decreases, and the carbonated beverage goes "flat". If one observes a carbonated beverage carefully, one may notice that bubbles often appear at the same spot on the glass surface. This paiiicular spot on the glass surface is acting as a catalyst for the decomposition of carbonic acid. Some people like to salt their beer and if one observes the NaCl crystals in beer, one may notice that they act as catalytic sites for carbonic acid decomposition.

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OREGON INSTITUTE OF TECHNOLOGY Natural Science Department

Ch 101 Elementary Chemistry

study Guide

1. Balance the following equations:

CH4 + 02 --> CO2+ H20

+ --> CO2 + H20c3H8 o 2 + --> CO2 + H20c2H6 o 2 + o 2 --> CO2 + H20c2H2

c2H60 + o 2 --> CO2 + H2o c3H8o + --> CO2 + H2oo 2 CH4o+ o 2 --> CO2 + H20

+ -->P4 H2 PH3 + o2 --> S02s 8

+ -->s 8 o 2 so3 NaOH + FeC13 --> Fe(OH) 3 + NaCl

( MgC1 2 + Na 3Po4 --> Mg3 (P04 ) 2 + NaCl

Al(N03 ) 3 + (NH4 ) 2so4 -> Al 2{so4 ) 3 +

K3P04 + MgC1 2 --> Mg3(P04 ) 2 + KCl

Li2S04 + AlCl3 --> Al 2 (S04) 3 + LiCl

2. Indicate the significance (if any!) of Avogadro's number.

3. Describe the relationship between a mole of a substance and Avogadro's number.

4. What is the mass of 1 mole of potassium? lithium? argon?

5. How many atoms are in .032 moles of He atoms?

6, Calculate the weight (in grams) of:

a) 1 mole of Ne e) ,07 moles H2 b) 3 moles of He f) 3.2 moles Cl2 c) 0.35 moles NaCl g) 4.8 moles CaF2 d) .035 moles LiI h) 75 millimoles o 2

Ch 101 study Guide, Chapter 5 Page 2

7. Calculate the number of moles in:

a) 15 grams of NaI e) 10 trams F2 b) 42 grams of CaF2 f) .35 kg of NaF

c) 3.2 grams LiF. g) 500 mg of KI

d) l gram H2

8. If 1.20 grams of praseodymium reacts with .135 grams of sulfur, what is the formula for praseodymium sulfide? What is the charge on the Pr ion? Atomic weight of · Pr= 141 g/mole.

9. If 0.9 grams of scandium reacts with .48 grams of oxygen, what is the formula for scandium oxide? What is the charge on the SC ion? Atomic weight of Sc= 45g/mole.

In the following problems, use the balanced equation to calculate how many grams of various chemicals in an equation are needed or produced, given either the number of moles or the number of grams of one chemical.

10. Ammonia is produced according to the following chemical equation:

a) If one has 4.5 moles of N2 , how many moles of H2 are required to react with it?

If one starts.with 50 grams of N2 , how many grams of

H2 are required to react with that 50 grams of N2 ?

How many moles of HN 3 are produced? What weight of

NH3 is produced?

c) If one begins with 30 grams of H2 , how many grams of

N2 are required? How many grams of NH3 are

produced? How many moles of NH3 is that?

11. Ethane (a component of natural gas) burns according to the equation:

a) If one starts with l mole of c 2H6 , how many moles

of oxygen (02 ) are required to react with it? How

many moles of and H2o are produced?co2

J !.

Ch 101 Study Guide, Chapter 5 Page 3

b) If one starts with 4.3 moles of c 2H6 , how many

moles of are required? How many moles ofo 2 co2 and H2o are produced?

11. c) If one produces 25 moles of CO2 by the above

reaction, how many moles of c 2H6 were originally

present as reactants?

d) If one starts with 100 grams of c 2H6 , how many

grams of are required? How many grams of coo 2 2 are produced? How many grams of H20?

12. Given the balanced equation:

2C2H2 + 502 -> 4C02 + 2H2o

a) If one starts with 4 moles of o 2 , how many moles of

(acetylene) will react with it? How manyc 2H2 moles of co2 will be produced? How many moles of

H2o?

b) starting with 15 moles of c 2H2 , how many moles of

o 2 are required to react with it? How many moles of

co2 will be produced? How many moles of H o?2

c) Starting with 130 grams of c 2H2 , how many grams of

are required? How many grams of areo 2 co2 produced? How many grams of H2o?

d) If one produces 360 grams of CO2 , how many grams of

did one start with? How many grams of oc 2H2 2 were required? How many grams of water were produced?

13. Explain the meaning of the following terms: activation energy, exergonic, endergonic, reversible, irreversible, catalyst.

14. Be able to draw re.action profiles for endergonic and exergonic reactions. Label the activation energy and show what effect a catalyst has upon the activation energy.

15. Draw the reaction profile for a reversible exergonic reaction, a reversible endergonic reaction, and an irreversible exergonic reaction.

16. Discuss how temperature and concentration affect the rate of reaction.

SWISHER:sl/101H04/7-7-87

Activity Series Study Guide (

1. Given that the following reactions occur, rank the elements L~ K, Ca and Ba in an activity series· from highest to lowest.

2K + BaCI, ----> 2 KCl + Ba

Li + KC! ---->LiCl +K

Ba +. CaCI, -----'> BaCI, + "a

2. Given that the following reactions occur:

Magnesium reacts with zinc nitrate to form zinc metal and magnesium nitrate.

Zinc metal reacts with iron nitrate to form iron metal and.zinc nitrate.

Iron metal reacts with cobalt nitrate to form iron nitrate and cobalt metal.

a) Write balanced chemical equations for each of the above reactions using the periodic table and the fact that zinc, iron, and cobalt form +2 ions under the conditions of this experiment.

b) Rank the elements in an activity series, highest to lowest

3.Given the following facts:

Mg reacts with HCl to form magnesium chloride and hydrogen gas (H2)

Ga reacts with HCl to form gallium chloride and hydrogen gas.

Sr (strontium) reacts with nitric acid(HNO3) to form strontium nitrate and hydrogen gas

Hg does not react with nitric acid (HNO3)

a) Write the balanced chemical equations for each reaction and indicate which metals are above hydrogen and which below. 4. given the following facts:

Aluminum-reacts with manganese nitrate to form aluminum nitrate and manganese_ metal

Manganese reacts with chromium nitrate to form manganese nitrate and chromiwn metal.

Manganese forms +2 ions and chromium forms +3 ions in the above reactions.

a) Write balanced chemical equations for each reaction. b) Rank the elements Al, Mn, and Cr in an activity series.

3,

'

Homework Problems I. Balance the following equations:

C6H12 + 02 > CO2+ H20

C9H20 + 02 > CO2+ H20

C1H14 + 02 > CO2+ H20

C10H22 + 02 > CO2+ H20

C4H100 + 02 > CO2 + H20

C4H1002 + 02 > CO2 + H20

C6H1206 + 02 > CO2+ H20

Al+ Ss > AliS3

Mg+ HCl > H2 + MgCli

Al + HCI > H2 + AlCh

Al(N03)3 + K2S04 > Ali(S04)J(pptJ + KN03

Mg(N03)2 + K3P04 > Mg3(P04)2(pptJ + KN03

Ba(N03)2 + K2S04 > Ba(S04)(pptJ + KN03

Ca(N03)2 + K3P04 > Ca3(P04)2 (pptJ + KN03

HF+ KOH >

H2S04 + LiOH >

H3P04 + NaOH >

HCl + Mg(OH)2 >

HNO3 + Al(OH)3 ---~>

2. a) How many grams ofCH4 are in 2.5 moles ofCH4? How many grams ofNa3PO4 are in 25 millimoles ofNa3PO4?

b) How many moles of propane are in 15 grams ofpropane (C3H8)? How many moles of Mg(NO3)2 are in 800 mg of Mg(NO3) 2?

3. a)How many atoms of He are in 2.5 moles of He? b) How many molecules of ammonia, NH3, are in 8.5 grams ofNH3?

4. a)Given 50 grams of C2F4, calculate the number of molecules ofC2F4.

b) Given that you have 4.0 (1023) molecules of C2F4, how many g ofC2F4 is that?

5. a)Given 5.0 g of diethyl ether, C4H10O, how many molecules of ether is this? b) Given 3.5 (1023) molecules of diethyl ether, how many g of ether is this?

6.Given the balanced equation:

a)How many moles ofH2will needed to react with 4.4 moles ofN2? b)How many moles ofNH3will be produced from 4.4 moles ofN2? c) How many grams ofH2will be needed to react with 12 grams ofN2? d) How many grams ofNH3 will be produced from 12 grams ofN2?

7. Given the balanced equation: 2 CsH12O + 15 02 -----> 10CO2 +

a) Starting with 13.6 moles ofC5H10O, how many moles ofO2is needed to react with it?

b) Starting with 13.6 moles of C5H12O, how many moles of CO2will be produced? c) Starting with 13.6 moles ofC5H12O, how many moles ofH2O will be produced? d) Starting with 20 grams of C5H12O, how many grams of 0 2needed to react with it? e) Starting with 20 grams of C5H12O, how many grams of CO2 will be produced?

3'l

14 CO2 +

a) If one starts with 8.5 moles of C7H14, how many moles of 0 2 are needed to react with it?

b) If one starts with 25 g C7H14, how many grams 0 2needed to react with it? c) Starting with 25 g of C7H14, how many g of CO2 produced?

9. If2.7 grams of silver reacts with 0.40 grams of sulfur, calculate the formula for silver sulfide and calculate the charge on each silver ion.

10. If 2.0 g of titanium reacts with 1.32 grams of oxygen what is the fmmula for titanium oxide! What is the charge on each titanium ion?

11. If 1.0 gram of titanium reacts with sulfur to fmm 2.0 grams of titanium sulfide, calculate the formula for titanium sulfide and the charge on each titanium ion.

12. If 8.0 g ofvanadium reacts with sulfur to fmm 15.5 g of vanadium sulfide, what is the fmmula for vanadium sulfide? What is the charge on the vanadium ion?

13. The reaction of magnesium with oxygen is exothe1mic, irreversible and has a high activation energy when the reaction is carried out dry. Draw the reaction profile. The addition of aqueous NaCl greatly speeds up the reaction. Show the effect on the reaction profile with a dotted line. Label clearly the activation energy before and after addition of the aqueous NaCl.

14. The reaction of 0 2with hemoglobin in the blood is reversible, exergonic, and has a very low( essentially 0) activation energy. Draw the reaction profile.

15. The reaction of glucose with oxygen to f01m CO2and water is exergonic, irreversible, but with a high activation energy barrier in the absence of appropriate enzymes. Draw the reaction profile, labelling E0 • Show the effect of adding appropriate enzymes. What effect will this have on the rate of reaction?

16. The combustion of wood with oxygen is exergonic, irreversible, and has a high activation energy barrier. Draw a reaction profile. Explain the role of a spark or bolt of lightning.

17. CO2dissolving in water results in the solution getting slightly cooler. Label the reaction endergonic or exergonic. Draw the reaction profile. Based on previous discussion, is this reaction reversible or irreversible? b)You reacted iron metal in HC! and evolved H2 gas.The solution temperature increased by 15 degrees Celsius. Draw the reaction profile. Make it clear whether this reaction is reversible or irreversible. 18.Draw the reaction profile and indicate whether the following reactions are exergonic or endergonic. a) the decomposition of crystals of copper sulfate pentahydrate(you did this is lab) b) the mixing of acid with base that you did in lab(hint: what happened to the temperature?).

19.What effect (if any) will heating a reaction mixture have on the activation energy?

20. a)The formula for glucose (blood sugar) is C6H12O6. Write the equation for the metabolism of glucose, given that the net equation is the same as burning glucose. b) Do the same for the metabolism ofpalmitic acid, a fatty acid component of fat. Palmitic acid has the formula C16H32O2, c) Nutritional scientists can detennine whether a person is using primarily glucose (carbohydrate) or fat (palmitic acid) to provide energy by measuring the ratio of inhaled oxygen vs the amount of carbon dioxide exhaled. What is the (molar) Oi/CO2ratio for glucose(carbohydrate) metabolism? For fat metabolism?