C H E M I S T R Y Chapter 3 Mass Relationships in Chemical Reactions

C H E M I S T R Y

Chapter 3Chapter 3Mass Relationships in Chemical Mass Relationships in Chemical ReactionsReactions

Chemical ReactionChemical Reaction

In a chemical reaction

• a chemical change produces one or more new substances.

• there is a change in the composition of one or more substances.



• old bonds are broken and new bonds are formed.

• atoms in the reactants are rearranged to form one or more different substances.

2 NaHCO3 (aq) Na2CO3(aq) + H2O(l) + CO2(g)



• the reactants are Fe and O2.

• the new product Fe2O3

is called rust. Fe(s) + O2 (g) Fe2O3(s)

Chemical EquationsChemical Equations

A chemical equation gives

• the formulas of the reactants on the left of the arrow.

• the formulas of the products on the right of the arrow.

Reactants Product

5

C(s)

O2 (g)CO2 (g)

Symbols Used in EquationsSymbols Used in Equations

Symbols in chemical

equations show

• the states of the reactants.

• the states of the products.

• the reaction conditions.

6

TABLE

Chemical Equations are Chemical Equations are BalancedBalanced

In a balancedchemical reaction• no atoms are lost

or gained.

• the number of reacting atoms is equal to the number of product atoms.

7

Balancing Chemical Balancing Chemical EquationsEquations

A balanced chemical equation shows that the law of conservation of mass is adhered to.

In a balanced chemical equation, the numbers and kinds of atoms on both sides of the reaction arrow are identical. 2NaCl(s)2Na(s) + Cl2(g)

right side:

2 Na2 Cl

left side:

2 Na2 Cl


2. Find suitable coefficients—the numbers placed before formulas to indicate how many formula units of each substance are required to balance the equation.

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

1. Write the unbalanced equation using the correct chemical formula for each reactant and product.

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


Chapter 3/10

3. Reduce the coefficients to their smallest whole-number values, if necessary, by dividing them all by a common denominator.

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

4H2O(l)4H2(g) + 2O2(g)

divide all by 2


Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 3/11

4. Check your answer by making sure that the numbers and kinds of atoms are the same on both sides of the equation.

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

right side:

4 H2 O

left side:

4 H2 O


Do not change subscripts when you balance a chemical equation. You are only allowed to change the coefficients.

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

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

Chemical equation changed!

H2O2(l)H2(g) + O2(g)

Balanced properly

ExamplesExamples

Balance the coefficients from reactants to products.

A. __N2(g) + __H2(g) __ NH3(g)

A. B. __Co2O3(s) + __ C(s) __Co(s) + __CO2(g)

Write a balanced equation for the reaction between carbon dioxide and potassium hydroxide to form potassium carbonate and water.

13

Chemical Symbols on Different Chemical Symbols on Different LevelsLevels

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

0.56 kg of hydrogen gas react with 4.44 kg of oxygen gas to yield 5.00 kg of liquid water.

macroscopic:

2 molecules of hydrogen gas react with 1 molecule of oxygen gas to yield 2 molecules of liquid water.

microscopic:

How can we relate the two to each other?

Avogadro’s Number and the Avogadro’s Number and the MoleMole

2(12.0 amu) + 4(1.0 amu) = 28.0 amuC2H4:

HCl: 1.0 amu + 35.5 amu = 36.5 amu

Molecular Mass: Sum of atomic masses of all atoms in a molecule.

Formula Mass: Sum of atomic masses of all atoms in a formula unit of any compound, molecular or ionic.

Molar Mass of NaMolar Mass of Na22SOSO44

Calculate the molar mass of Na2SO4.

16

Element Number of Moles

Atomic Mass Total Mass in Na2SO4

Na

S

O

Stoichiometry: Chemical Stoichiometry: Chemical ArithmeticArithmetic

Stoichiometry: The relative proportions in which elements form compounds or in which substances react.

aA + bB cC + dD

Moles ofAGrams of

A

Moles ofB

Grams ofB

Mole Ratio Between A and B (Coefficients)

Molar Mass of B

Molar Mass of A

Stoichiometry: Chemical Stoichiometry: Chemical ArithmeticArithmetic

How many grams of NaOH are needed to react with 25.0 g Cl2?

2NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

Aqueous solutions of sodium hypochlorite (NaOCl), best known as household bleach, are prepared by reaction of sodium hydroxide with chlorine gas:

Moles ofCl2

Grams ofCl2

Moles ofNaOH

Grams ofNaOH

Mole Ratio Molar Mass NaOH

Molar Mass Cl2

Stoichiometry: Chemical Stoichiometry: Chemical ArithmeticArithmetic The commercial production of iron from iron ore

involves the reaction of Fe2O3 with CO to yield iron metal plus carbon dioxide:

Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)

Predict how many grams of CO will react with 0.500 moles Fe2O3?

Yields of Chemical ReactionsYields of Chemical Reactions

The amount actually formed in a reaction.

The amount predicted by calculations from thelimiting reactant.

Actual Yield:

Theoretical Yield:

actual yield

theoretical yieldx 100%Percent Yield =

Reactions with Limiting Reactions with Limiting Amounts of ReactantsAmounts of Reactants

Limiting Reactant: The reactant that is present in limiting amount. The extent to which a chemical reaction takes place depends on the limiting reactant.

Excess Reactant: Any of the other reactants still present after determination of the limiting reactant.

Reactions with Limiting Reactions with Limiting Amounts of ReactantsAmounts of ReactantsAt a high temperature, ethylene oxide reacts with water to form ethylene glycol which is an automobile antifreeze and a starting material in the preparation of polyester polymers:

C2H4O(aq) + H2O(l) C2H6O2(l)

ExamplesExamples How many sandwiches can be made from 10 slices of

bread and 8 slices of cheese? Which one is the limiting reactant?

The balanced chemical equation is

2Bd + Ch Bd2Ch

Limiting reactant and Limiting reactant and theoretical yieldtheoretical yieldConsider the reaction A + B AB

Step1: Grams A moles A moles AB

Grams B moles B moles AB

Step 2: Determine which reactant (A or B) produced the least amount of product (AB) and label it as limiting reactant

Step 3: Calculate the theoretical yield of AB (or least moles of AB grams AB)

Excess reactantExcess reactant Whichever reactant (A or B) produced the most

moles of product (AB) is the excess reactant

most moles of product - least moles of product = moles of product in excess

moles of product in excess --> moles excess reactant --> grams of reactant = left over from reaction

ExampleExample Ammonia, NH3, can be synthesized by the following

reaction:

2 NO(g) + 5 H2(g) 2 NH3(g) + 2 H2O(g)

Starting with 86.3 g NO and 25.6 g H2

a. Determine the limiting reactant

b. Find the theoretical yield of NH3 in grams

Reactions with Limiting Reactions with Limiting Amounts of ReactantsAmounts of Reactants

Li2O(s) + H2O(g) 2LiOH(s)

Lithium oxide is used aboard the space shuttle to remove water from the air supply according to the equation:

If 80.0 g of water are to be removed and 65.0 g of Li2O are available, which reactant is limiting? How many grams of LiOH are produced? How many grams of excess reactant remain?

Percent Composition and Percent Composition and Empirical FormulasEmpirical Formulas

Percent Composition: Expressed by identifying the elements present and giving the mass percent of each.

Empirical Formula: It tells only the ratios of the atoms in a compound.

multiple =molecular mass

empirical formula mass

Percent compisition =

total mass of individual atom

Total mas of the compound

x 100

Steps in determine the Empirical Steps in determine the Empirical formulaformula Step 1: Obtain the mass of each element (in grams)

E.G 100% = 100g therefore mass percent is the same numerical value in grams

Step 2: Determine the numbers moles of each atom present

Step 3: Divide the smallest moles by numbers of each atoms to obtain the closet integer as possible.

Step 4: If the result ended with 0.5, 0.33, 1.125, 1.50 etc… then multiply with a factor to get the nearest integer as possible.

E.g 1.5 x 2 = 3.0 atoms

1.33 x 3 = 3. 99 = 4 atoms

ExampleExample A compound containing nitrogen and oxygen is

decomposed in the laboratory and produces 24.5 g nitrogen and 70.0 g oxygen. Calculate the empirical formula of the compound.

ExampleExample An unknown sample gives the following mass

percent: 17.5% Na, 39.7% Cr and 42.8% O. What is the empirical formula?

ExamplesExamples Ibuprofen, an aspirin substitute, has the following

mass percent composition: C 75.69%, H 8.80% an O 15.51%. What is the empirical formula of ibuprofen?

Molecular formulaMolecular formula Molecular Formula: It tells the actual numbers of

atoms in a compound. It can be either the empirical formula or a multiple of it.

multiple =molecular mass

empirical formula mass

ExampleExample Butanedione – a main component in the smell and

taste of butter and cheese – contains the elements carbon, hydrogen, and oxygen. The empirical formula of butanedione is C2H3O and its molar mass is 86.09 g/mol. Find its molecular formula

Percent Composition and Percent Composition and Empirical FormulasEmpirical Formulas

A colorless liquid has a composition of 84.1 % carbon and 15.9 % hydrogen by mass. Determine the empirical formula. Also, assuming the molar mass of this compound is 114.2 g/mol, determine the molecular formula of this compound.

Determining Empirical Determining Empirical Formulas: Elemental AnalysisFormulas: Elemental Analysis

Combustion Analysis: A compound of unknown composition (containing a combination of carbon, hydrogen, and possibly oxygen) is burned with oxygen to produce the volatile combustion products CO2 and H2O, which are separated and weighed by an automated instrument called a gas chromatograph.

hydrocarbon + O2(g) xCO2(g) + yH2O(g)

carbonhydrogen

Determining Empirical Determining Empirical Formulas: Elemental AnalysisFormulas: Elemental Analysis Step 1: g CO2 moles CO2 moles C g C

Step 2: g H2O moles H2O moles H g H Step 3:

g O (or any missing atom) = grams sample – (g C + g H)

Step 4: determine the moles of O or the missing atom

Step 5: Follow all steps from determining the empirical formula

ExampleExample A compound contains only carbon, hydrogen, and

oxygen. Combustion of 10.68 mg of compound yields 16.01 mg CO2 and 4.37 mg H2O. The molar mass of the compound is 176.1 g/mol.

ExamplesExamples Paradichlorobenzene, a common ingredient in solid

air fresheners, contains only C, H and Cl and has a molar mass of about 147g/mol. Given that combustion of 1.68 g of this compound produces 3.02 g of CO2 and 0.412 g H2O, determine its empirical and molecular formulas

Documents

C H E M I S T R Y Chapter 3 Mass Relationships in Chemical Reactions