Quantities of Reactants and Products Chapter 4 Dr. Victor Vilchiz

Molar Interpretation of a Chemical Equation A balanced chemical equation: A balanced chemical equation: 2H 2 +1O 2  2H 2 O can be interpreted to read 2 moles of Hydrogen react with one mole of oxygen to produce 2 moles of water. –In the balanced equation the 2, 1, and 2 are known as the stoichiometric coefficients. At the molecular level they refer to the number of molecules reacting. At the molecular level they refer to the number of molecules reacting.

Molar Interpretation of a Chemical Equation Because moles can be converted to mass, you can also give a mass interpretation of a chemical equation. 2H 2 +1O 2  2H 2 O 2(2.02g)H 2 react with 1(32.0g) O 2 to yield 2(18.0g)H 2 O 4.04g H 2 react with 32.0g O 2 to yield 36.0g H 2 O

Stoichiometric Calculations Suppose we wished to determine the number of moles of NH 3 we could obtain from 4.8 mol H 2. Suppose we wished to determine the number of moles of NH 3 we could obtain from 4.8 mol H 2. Because the coefficients in the balanced equation represent mole-to-mole ratios, the calculation is simple.

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: Equations Writing chemical equations Writing chemical equations Consider the combustion of propane to produce carbon dioxide and water.propane

Combustion of Propane

For this equation to balance, five molecules of oxygen must be consumed for each molecule of methane, producing three molecules of CO 2 and four molecules of water. Chemical Reactions: Equations Writing chemical equations Writing chemical equations Now the equation is “balanced.” 54 3

Balancing Chemical Equations There are some steps that can make balancing equations easier: There are some steps that can make balancing equations easier: –Translate the problem from words to a preliminary equation. –Balance the atoms using the stoichiometric coefficients. Start with substance with most atoms or different types of atoms Start with substance with most atoms or different types of atoms Leave simple atoms for last specially H and O Leave simple atoms for last specially H and O –Adjust coefficients so no fractions are present. Note: Remember that coefficients affect all atoms in the formula, subscripts only affect the atom or ion preceding it.

Balancing Chemical Equations Example: Example: –When propane burns in excess oxygen both water vapor and carbon dioxide are produced. Translate Translate C 3 H 8 (g) + O 2 (g)  H 2 O(g) +CO 2 (g) C 3 H 8 (g) + O 2 (g)  H 2 O(g) +CO 2 (g) Balance the atoms using the stoichiometric coefficients. Balance the atoms using the stoichiometric coefficients. C 3 H 8 (g) + O 2 (g)  4H 2 O(g) +3CO 2 (g) 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 5 in front of the O 2 on the left to balance the equation. C 3 H 8 (g) + 5O 2 (g)  4H 2 O(g) +3CO 2 (g) Back to Lecture

Chemical Reactions: Equations Balance the following equations. Balance the following equations

Types of Chemical Reactions Types of Reactions Types 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 4.14: Dissociation Reaction Photo courtesy of James Scherer.

Figure 4.15: Displacement Reaction. Photo courtesy of American Color.

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

Types of Chemical Reactions Combination Reactions Combination Reactions. A combination reaction is a reaction in which two substances, usually two elements, combine to form a third substance. Sodium and chlorine combine in a fiery reaction. (see Figure)(see Figure)

Types of Chemical Reactions Combination Reactions Combination Reactions Other combination reactions involve compounds as reactants.

Types of Chemical Reactions Decomposition Reactions Decomposition Reactions A decomposition reaction is a reaction in which a single compound reacts to give two or more substances.

Types of Chemical Reactions Displacement Reactions Displacement Reactions 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 Reactions Combustion Reactions Combustion 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.

Limiting Reagent In the previous example we assumed that we had enough N 2 to react with the 4.8 moles of H 2. However it is possible that we could had ran out of nitrogen before consuming the hydrogen in that case the N 2 will be labeled as the limiting reactant. In the previous example we assumed that we had enough N 2 to react with the 4.8 moles of H 2. However it is possible that we could had ran out of nitrogen before consuming the hydrogen in that case the N 2 will be labeled as the limiting reactant. The limiting reactant (or limiting reagent) is the reactant that is entirely consumed when the reaction goes to completion. The limiting reactant (or limiting reagent) is the reactant that is entirely consumed when the reaction goes to completion.

Limiting Reactant The limiting reagent ultimately determines how much product can be obtained. For example, bicycles require one frame and two wheels. If you have 20 wheels but only 5 frames, it is clear that the number of frames will determine how many bicycles can be made. Pictorial Example

Limiting Reactant Back to Lecture

Stoichiometric Calculations Amounts of substances in a chemical reaction by mass. Amounts of substances in a chemical 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?

First, you write what is given (5.00 g MnO 2 ) 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 Calculations

Limiting Reagent Example Zinc metal reacts with hydrochloric acid by the following reaction. Zinc metal reacts with hydrochloric acid by the following reaction. If 0.30 mol Zn is added to hydrochloric acid containing 0.52 mol HCl, how many moles of H 2 are produced?

Limiting Reagent 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 is the limiting reagent. Since HCl is the limiting reagent, produces the least H 2, the amount of H 2 produced must be 0.26 mol.

Theoretical and Percent Yield The theoretical yield of product is the maximum amount of product that can be obtained from given amounts of reactants. The percent yield is the actual yield (experimentally determined) expressed as a percentage of the theoretical yield (calculated). The percent yield is the actual yield (experimentally determined) expressed as a percentage of the theoretical yield (calculated).

Theoretical and Percent Yield To illustrate the calculation of percentage yield, recall that the theoretical yield of H 2 in the previous example was 0.26 mol (or 0.52 g) H 2. To illustrate the calculation of percentage yield, recall that the theoretical yield of H 2 in the previous example was 0.26 mol (or 0.52 g) H 2. If the actual yield of the reaction had been 0.22 g H 2, then

Reactions in a sequence In the case when you have reactions in a sequence: In the case when you have reactions in a sequence: –balance all reactions in the sequence. –add all reactions (both sides). –Cancel species found on both sides of the overall reaction (make sure they in the same physical state before canceling). –The left over is the net equation.