Quantities of Quantities of Reactants and Reactants and

ProductsProductsChapter 4Chapter 4

Dr. Victor Vilchiz

Molar Interpretation of Molar Interpretation of a Chemical Equationa Chemical Equation• A balanced chemical equation:A balanced chemical equation:

22HH22 + +11OO22 22HH22OO

can be interpreted to read 2 moles of can be interpreted to read 2 moles of Hydrogen react with one mole of Hydrogen react with one mole of oxygen to produce 2 moles of water.oxygen to produce 2 moles of water.– In the balanced equation the In the balanced equation the 2, 1, and 22, 1, and 2

are known as the stoichiometric are known as the stoichiometric coefficients.coefficients.• At the molecular level they refer to the number of At the molecular level they refer to the number of

molecules reacting.molecules reacting.

Molar Interpretation of a Molar Interpretation of a Chemical EquationChemical Equation

Because moles can be converted to Because moles can be converted to mass, you can also give a mass mass, you can also give a mass interpretation of a chemical equation.interpretation of a chemical equation.

22HH22 + +11OO22 22HH22OO

2(2.02g)H2(2.02g)H22 react with 1(32.0g) O react with 1(32.0g) O22 to yield to yield 2(18.0g)H2(18.0g)H22OO

4.04g H4.04g H22 react with 32.0g O react with 32.0g O22 to yield 36.0g to yield 36.0g HH22OO

Stoichiometric Stoichiometric CalculationsCalculations

• Suppose we wished to determine Suppose we wished to determine the number of moles of NHthe number of moles of NH33 we we could obtain from 4.8 mol Hcould obtain from 4.8 mol H22..

Because the coefficients in the balanced Because the coefficients in the balanced equation represent mole-to-mole ratios, the equation represent mole-to-mole ratios, the calculation is simple.calculation is simple.

)g(NH 2 (g)3H (g)N 322

32

32 NH mol 2.3

H mol 3NH mol 2

H mol 8.4

The law of conservation of mass dictates that the total number of atoms of each element on both sides of a chemical equation must match. The equation is then said to be balanced.

Chemical Reactions: Chemical Reactions: EquationsEquations

• Writing chemical equationsWriting chemical equations

Consider the combustion of propane to produce carbon dioxide and water.

OH CO O HC 22283

Combustion of PropaneCombustion of Propane

For this equation to balance, five molecules of oxygen must be consumed for each molecule of methane, producing three molecules of CO2 and four molecules of water.

Chemical Reactions: Chemical Reactions: EquationsEquations

• Writing chemical equationsWriting chemical equations

OH CO O HC 22283

Now the equation is “balanced.”

5 43

Balancing Chemical Balancing Chemical EquationsEquations• There are some steps that can make There are some steps that can make

balancing equations easier:balancing equations easier:– Translate the problem from words to a preliminary Translate the problem from words to a preliminary

equation.equation.– Balance the atoms using the stoichiometric Balance the atoms using the stoichiometric

coefficients.coefficients.• Start with substance with most atoms or different types of Start with substance with most atoms or different types of

atomsatoms• Leave simple atoms for last specially H and OLeave simple atoms for last specially H and O

– Adjust coefficients so no fractions are present.Adjust coefficients so no fractions are present.

Note: Remember that coefficients affect all atoms in the Note: Remember that coefficients affect all atoms in the formula, subscripts only affect the formula, subscripts only affect the atom or ionatom or ion

preceding it.preceding it.

Balancing Chemical Balancing Chemical EquationsEquations• Example:Example:

– When propane burns in excess oxygen both When propane burns in excess oxygen both water vapor and carbon dioxide are water vapor and carbon dioxide are produced.produced.• TranslateTranslate

CC33HH88(g) + O(g) + O22(g) (g) H H22O(g) +COO(g) +CO22(g)(g)

• Balance the atoms using the stoichiometric coefficients.Balance the atoms using the stoichiometric coefficients.

CC33HH88(g) + O(g) + O22(g) (g) 44HH22O(g) +O(g) +33COCO22(g)(g)

Both C and H are balanced but not the oxygen, we have Both C and H are balanced but not the oxygen, we have 4(1)+3(2)=10 Oxygen atoms on the right so we need a 4(1)+3(2)=10 Oxygen atoms on the right so we need a 55 in in

front of the Ofront of the O22 on the left to balance the equation. on the left to balance the equation.

CC33HH88(g) + (g) + 55OO22(g) (g) 4H 4H22O(g) +3COO(g) +3CO22(g)(g)

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Chemical Reactions: Chemical Reactions: EquationsEquations

• Balance the following equations.Balance the following equations.

332 POCl PCl O

26424 N OP ON P

232232 SO O As O SAs

24243243 )POCa(H POH )(POCa

2 2

6 6

62 29

34

Types of Chemical Types of Chemical ReactionsReactions• Types of ReactionsTypes of Reactions

Most reactions fall into one of the following simple categories:

Combination Reactions

Decomposition Reactions

Displacement Reactions

Combustion Reactions

Figure 4.13: A representation of a combination reaction. Figure 4.13: A representation of a combination reaction.

Figure 4.14: Dissociation Reaction Figure 4.14: Dissociation Reaction Photo Photo

courtesy of James Scherer.courtesy of James Scherer.

Figure 4.15: Figure 4.15: Displacement Displacement Reaction. Reaction. Photo Photo

courtesy of American Color.courtesy of American Color.

Figure 4.16: Combustion Figure 4.16: Combustion Reaction. Reaction. Photo courtesy of Photo courtesy of

James SchererJames Scherer. .

Types of Chemical Types of Chemical ReactionsReactions• Combination ReactionsCombination Reactions

A combination reaction is a reaction in which two substances, usually two elements, combine to form a third substance..

)(2)()(2 2 sNaClgClsNa Sodium and chlorine combine in a fiery reaction. (see Figure)

Types of Chemical Types of Chemical ReactionsReactions• Combination ReactionsCombination Reactions

Other combination reactions involve compounds as reactants.

)s(CaSO)g(SO)s(CaO 32

Types of Chemical Types of Chemical ReactionsReactions• Decomposition ReactionsDecomposition Reactions

A decomposition reaction is a reaction in which a single compound reacts to give two or more substances.

)g(N)g(OH4)s(OCr)s(OCr)NH( 22327224

Types of Chemical Types of Chemical ReactionsReactions• Displacement ReactionsDisplacement Reactions

)g(H)aq(ZnCl)aq(HCl2)s(Zn 22

A displacement reaction (also called a single- replacement reaction) is a reaction in which an element reacts with a compound, displacing an element from it.

Types of Chemical Types of Chemical ReactionsReactions• Combustion ReactionsCombustion Reactions

A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame.

)g(OH 10)g(CO 8)g(O 13)g(HC 2 222104

Limiting ReagentLimiting Reagent

• In the previous example we assumed that we In the previous example we assumed that we had enough Nhad enough N22 to react with the 4.8 moles of to react with the 4.8 moles of HH22. However it is possible that we could had . However it is possible that we could had ran out of nitrogen before consuming the ran out of nitrogen before consuming the hydrogen in that case the Nhydrogen in that case the N22 will be labeled will be labeled as the limiting reactant.as the limiting reactant.

• The The limiting reactantlimiting reactant (or (or limiting limiting reagentreagent) is the reactant that is entirely ) is the reactant that is entirely consumed when the reaction goes to consumed when the reaction goes to completion.completion.

Limiting ReactantLimiting Reactant

The limiting reagent ultimately determines The limiting reagent ultimately determines how much product can be obtained.how much product can be obtained.

For example, bicycles require one frame and For example, bicycles require one frame and two wheels. If you have 20 wheels but only two wheels. If you have 20 wheels but only 5 frames, it is clear that the number of 5 frames, it is clear that the number of frames will determine how many bicycles frames will determine how many bicycles can be made.can be made.

Pictorial Example

Limiting ReactantLimiting Reactant

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Stoichiometric Stoichiometric CalculationsCalculations

• Amounts of substances in a chemical Amounts of substances in a chemical reaction by mass.reaction by mass.

How many grams of HCl are required to react with 5.00 grams manganese(IV) oxide, according to the following chemical equation?

(g)Cl(aq)MnClO(l)2H(s)MnO HCl(aq) 4 2222

First, you write what is given (5.00 g MnO2) and convert this to moles.

Then convert to moles of what is desired, mol HCl, using the stoichiometric coefficients.

Finally, you convert this to mass of HCl, (g HCl).

Stoichiometric Stoichiometric CalculationsCalculations

HCl mol 1

HCl 36.5gx

MnO mol 1

HCl mol 4x

MnO 86.9g

MnO mol 1x MnO g 5.00

22

22

HCl g 8.40

(g)Cl(aq)MnClO(l)2H(s)MnO HCl(aq) 4 2222

Limiting Reagent Limiting Reagent ExampleExample

• Zinc metal reacts with hydrochloric Zinc metal reacts with hydrochloric acid by the following reaction.acid by the following reaction.

If 0.30 mol Zn is added to hydrochloric acid containing 0.52 mol HCl, how many moles of H2 are produced?

)g(H (aq)ZnCl HCl(aq) 2 Zn(s) 22

Limiting ReagentLimiting Reagent

Take each reactant in turn and ask how Take each reactant in turn and ask how much product would be obtained if each much product would be obtained if each were totally consumed. The reactant that were totally consumed. The reactant that gives the smaller amount is the limiting gives the smaller amount is the limiting reagent.reagent.

Since HCl is the limiting reagent, produces the least H2, the amount of H2 produced must be 0.26 mol.

22 H mol 0.30

Zn mol 1H mol 1

Zn mol 0.30

22 H mol 0.26

HCl mol 2H mol 1

HCl mol 0.52

Theoretical and Percent Theoretical and Percent YieldYield

The The theoretical yieldtheoretical yield of product is of product is the maximum amount of product that the maximum amount of product that can be obtained from given amounts of can be obtained from given amounts of reactants.reactants.

The The percent yieldpercent yield is the actual yield is the actual yield (experimentally determined) expressed (experimentally determined) expressed as a percentage of the theoretical yield as a percentage of the theoretical yield (calculated). (calculated).

%100yield ltheoretica

yield actual Yield%

Theoretical and Theoretical and Percent YieldPercent Yield

• To illustrate the calculation of percentage yield, To illustrate the calculation of percentage yield, recall that the theoretical yield of Hrecall that the theoretical yield of H22 in the in the previous example was 0.26 mol (or 0.52 g) Hprevious example was 0.26 mol (or 0.52 g) H22..

If the actual yield of the reaction had been 0.22 g H2, then

%42%100H g 0.52H g 0.22

Yield%2

2

Reactions in a Reactions in a sequencesequence

• In the case when you have In the case when you have reactions in a sequence:reactions in a sequence:– balance all reactions in the sequence.balance all reactions in the sequence.– add all reactions (both sides).add all reactions (both sides).– Cancel species found on both sides of the Cancel species found on both sides of the

overall reaction (make sure they in the same overall reaction (make sure they in the same physical state before canceling).physical state before canceling).

– The left over is the The left over is the net equationnet equation..