Chemical Reaction

• What is a chemical reaction?

• A chemical reaction: is a process in which one set of substances called reactants is converted to a new set of substances called products.

• reactants products

How do we know a chemical reaction has occurred ?

Some physical evidence to look for that shows a reaction has occurred: a color change formation of a solid (precipitate) within a clear solution evidence of a gasevolution or absorption of heatTypes of reactions

Combination reaction A + B AB

Decomposition reactions AB A + B

Displacement reactions AB + C AC + B

Metathesis reactions (double-replacement reactions)

AB + CD AD + CB

Combustion reactions reaction with oxygen

How do we represent the chemical reaction in a way that is convenient and easy to understand?

chemical equation, which is a symbolic way of representing a chemical reaction in terms of chemical formulas. In a chemical equation, formulas for the reactants (starting substances in a chemical reaction) are written on the left sides of the equation and formulas for products are written on the right. The two sides of equation are joined by an arrow ().

E. g. Consider the reaction of colorless nitrogen monoxide and oxygen gas to form red-brown nitrogen dioxide gas.

1 Substitute chemical formulas for names, to obtain the following expression.

nitrogen react oxygen to nitrogen monoxide with gas form dioxidegas gas

NO + O2 NO2

2 Balance numbers of atoms to obtain a chemical equation. In a balanced equation, for each element present, the total number of atoms of the element is the same on both sides. A chemical reaction involves simply a recombination of the atoms; none are destroy and none are created.

Two N atoms and 4 O atoms

2 atoms and 4 O atoms

2NO(g) + O2(g) 2NO2(g)

The numbers in front of the chemical formula are coefficients required to balance a chemical equation and they are called stoichiometric coefficients. Note: In balancing a chemical equation, the equation can be balanced only by adjusting the coefficients of formulas, as necessary.

1 Never introduce extraneous atom or molecules into equation 2 Never change formulas for the purpose of balancing an equation

Balancing a Chemical Equation

Balancing by Inspection: means to adjust stoichiometric coefficients by trial and error until a balanced condition is found.Useful strategies for balancing equations

1 If an element occurs in only one compound on each side of the equation, try balancing this element first.

2 When one of the reactants or products exists as the free element, balance this element last.

3 In some reactions, certain groups of atoms (e.g. polyatomic ions) remain unchanged. In such case balance these groups as a unit.

4 It is permissible to use fractional as well as integral numbers as coefficients. At times, an equation can be balanced most easily by using one or more fractional coefficients and then, if desired, clearing the fractions by multiplying all coefficients by a common multiplier.

E.g. Cu + AgNO3 Cu(NO3)2 + Ag

Cu + 2AgNO3 Cu(NO3)2 + 2Ag

 E.g. Balance the following chemical equations

(a) H2 + O2 H2O

(b) ZnS+ O2 ZnO +SO2

(c) Pb(NO3)2 + KI PbI2 + KNO3

State of Matter

A complete chemical reaction also includes state of matter and the reaction condition.In a chemical equation, the state of matter or physical form of reactants and products are presented through symbols in parentheses.

(g) gas (l) liquid (s) solid (aq) aqueous (water)solution

Therefore, the chemical equation of our first example: Consider the reaction of colorless nitrogen monoxide and oxygen gas to form red-brown nitrogen dioxide gas is

2NO(g) + O2(g) 2NO2(g)

E.g. Write a balanced equation for the combustion of thiosalicylic acid, C7H6O2S, used in the manufacture of indigo dyes.

E.g Solid iron(III) oxide react with carbon monoxide to liberate carbon dioxide gas and to form iron metal.

Reaction Conditions

An important aspect of modern chemical research involves working out the condition for a reaction. The condition of the reaction is written above or below the arrow in an equation. means that a high temperature is required.

E.g. 2NaNO3(s) 2NaNO2(s) + O2(g)

CO2(g) + 2H2(g) — CH3OH(g)

ZnO, Cr2O3

Review Questions

• What is a balanced equation?

• Why does a chemical equation has to be balanced?

Chemical equations are written in terms of molecules or formula units. But to plan for real reaction, you can’t count molecules; they are too tiny. Since the moles can be converted to mass and the number of moles involved in a reaction is proportional to the coefficients in the balanced chemical equation, we can convert the equation to the amounts we can weigh, or measure by some other means.

Number of molecules

Amounts you can measure e.g. mass

Stoichiometry

Chemical Equations and Stoichiometry

The coefficients in the chemical equation

2H2(g) + O2(g) 2H2O(l)

mean that two molecules of H2 react with one molecules of O2 and

produce 2 molecules of H2O. A similar statement involving multiples

of these numbers of molecules is also correct.

2 x molecules H2 + x molecules O2 2x molecules H2O

Suppose that we let x = Avogadro’s number. Then x molecules represents one mole. Thus the chemical equation also mean that

2 moleH2 + 1 mol O2 2 mol H2O

Therefore, on the atomic or molecular scale, the coefficients of a balanced equation represent atoms and molecules. On the macroscopic scale, the coefficients of a balanced equation represent moles of atoms and molecules.

Summary

Because moles can be converted to mass, we can also give a mass interpretation of a chemical equation. We may interpret a chemical equation either in terms of numbers of molecules (or ions or formula units) or in terms of numbers of moles, depending on our needs. We summarized these three interpretations as follows:

2H2(g) + O2(g) 2H2O(l)

2 molecule H2 + 1 molecule O2 2 molecules H2O

2 mole H2 + 1 moleO2 2 moleH2O

2 x2.02g H2 + 32.0g O2 2 x18.0g H2O

E.g. How many grams of CO2 is produce by burning 2.50 mole of

CH4 in an excess of O2?

Chemical Reactions in Solution

A solution is a homogeneous mixture of two or more substances. Type of solution: see handout. The common type of solution is a solid dissolved in a liquid. The solid that is dissolved is called solute, and the liquid in which it is dissolved is called solvent.

The most common method of expressing the concentration of the solute is Molarity which is defined as

Molarity(M) = amount of solute in mole __ volume of solution in liter

E.g 15mL of acetic acid, CH3COOH (d=1.048g/mL), is dissolved

in enough water to produce 500.0 mL of solution. What is the molarity of acetic acid in the solution?

E.g. Commercial sulfuric acid is sold as a solution that is 96.7% H2SO4(l) and 3.3% H2O by mass. What is the concentration in

molarity of this solution? Density of the solution is 1.84g/mL.

Solution Dilution

~ all solute taken from the initial, more concentrated solution appears in the final diluted solution. ~ When a solution is diluted, the amount of solute remains constant between the initial(i) solution taken and the final(f) solution produced. That is

MiVi =ni = nf = MfVf

MiVi = MfVf

E.g Compute the volume of concentrated (18.1M) H2SO4(aq)

required to produce 500mL of 0.30M H2SO4(aq).

E.g. A 15mL sample of 0.450 M K2CrO4 is diluted to 100.00 mL.

What is the concentration of the new solution?

E.g. How many mL of water must be added to 30.0 mL of 0.520 M

NaCl to produce a 0.195 M solution?

Stoichiometry of Reactions in Solution

Molarity is used in stoichiometric calculation for reaction that take place in solution.E.g. A 25.00mL of 0.250M of K2CrO4(aq) is added to an excess

of AgNO3(aq). What mass of Ag2CrO4(s) will precipitate from the

solution?

Limiting ReactantWhen all the reactants are completely and simultaneously consumed in a chemical reaction, the reactants are said to be in stoichiometric proportions-in the mole ratios dictated by the coefficients in the balanced equation. In reality, many reactions are carried out using an excess amount of one reactant-more than is actually needed according to stoichiometric. In such case, one of the reactants may be completely consumed at the end of reaction, whereas some amounts of other reactants will remain unreacted.

The reactant that is completely consumed when a reaction goes to completion and thereby limits the amount of product formed is called the limiting reactant (or limiting reagent). Once one of the reactants is used up, the reaction stops. This means that the moles of product are always determined by the starting moles of limiting reactant.

E.g. Reaction of sodium sulfate and carbon yield sodium sulfide and carbon monoxide. What mass of CO(g) forms in the reaction of 30.0g sodium sulfate and 20.0g carbon?

First write a balanced equation

Then determine the limiting reactant

•There are more than one ways to answer this. Here we are going to use a somewhat tidies but maybe less confusing way to you. We take each reactant in turn and ask how much product would be obtained if each were totally consumed. The reactant that gives the smaller amount of product is the limiting reactant.

Other Practical Matters in Reaction Stoichiometry

Theoretical Yield, Actual Yield, and Percent YieldFor many chemical reactions the amount of the desired product obtained is less than the theoretical amount. The percent yield is defined as

the quantity of product that is actually produced

actual yield of product theoretical yield of product

x 100%percent yield =

the calculated quantity of product that one expects from given quantities of reactants

E.g. What is the percentage yield if the reaction of 25.0g P4 with 91.5g

of Cl2 produces 104g PCl3?

Consecutive Reactions, Simultaneous Reactions, and Overall Reactions

Most of time it takes more than one steps to get desired product~ Reactions that are carried out one after another in sequence to yield a final product are called consecutive reactions.~ In simultaneous reaction, two or more substances react independently of one another in separate reactions occurring at the same time. ~Any substance that is produced in one step and consumed in another step of a multistep process is called an intermediate.

E.g. Purification of TiO22TiO2(impure) + 3C(s) +4Cl2(g) 2TiCl4(g) + CO2(g) + 2CO(g)

2x[ TiCl4(g) + O2(g) TiO2(s) +2Cl2(g)]_______

overall reaction: 3C(s) + 2O2(g) CO2(g) + 2CO(g)

E.g. How many gram of NaBr is produced if 55.85g of Fe is consumed?

3Fe + 3Br2 3FeBr2

3FeBr2 + Br2 Fe3Br8

Fe3Br8 + 4Na2CO3 8NaBr + 4CO2 + Fe3O4

Overall reaction 3Fe + 4Br2 + 4Na2CO3 8NaBr + 4CO2 + Fe3O4

  

E.g Simultaneous reactionMagnalium alloys contain 70% Al and 30.0% Mg by mass. How many grams of H2(g) are produce in the reaction of a 0.710g

sample of this alloy with excess HCl(aq)?