ENERGY AND REACTIONS 18.2

Chapter Eighteen: Energy and Reactions  18.1 Energy and Chemical Reactions  18.2 Chemical Reaction Systems  18.3 Nuclear Reactions

Chapter 18.2 Learning Goals  Discuss how chemical equations are similar to recipes.  Identify limiting and excess reactants in chemical reactions.  Describe factors that may influence the rate at which a chemical reaction occurs.

18.2 Chemical Reaction Systems  A balanced chemical equation is like a recipe.  If you write the equation for making chocolate cake, you will see it is similar to a real recipe for making water.

18.2 Chemical Reaction Systems

18.2 Information from Balanced Chemical Equations  If the recipe for chocolate cake gives you ratios among the ingredients needed to make eight servings, how many servings are possible if you only have half a cup of flour?

18.2 Information from Balanced Chemical Equations  To make a good-tasting chocolate cake with half as much flour, you would have to use half as much of the other ingredients, too.  By halving the recipe, you can make four servings of chocolate cake.

18.2 Information from Balanced Chemical Equations  A balanced chemical equation shows the ratios of the number of molecules of reactants needed to make a certain number of molecules of products using coefficients.

18.2 What a balanced equation doesn’t tell you  A balanced equation does not describe the exact conditions under which a reaction will occur.  The right conditions for most of the reactions that are used in science and industry are the result of careful research and experimentation.

18.2 Limiting and Excess Reactants  When a chemical reaction occurs, the reactants are not always present in the exact ratio indicated by the balanced equation.  What usually happens is that a chemical reaction will run until the reactant that is in short supply is used up.

18.2 Limiting and Excess Reactants  The reactant that is used up first in a chemical reaction is called the limiting reactant.  The limiting reactant limits the amount of product that can be formed. Which reactant will be used up first?

18.2 Limiting and Excess Reactants  A reactant that is not completely used up is called an excess reactant because some of it will be left over when the reaction is complete. Which reactant will be left over (mixed with) product?

18.2 Percent Yield  Often the amount of product you are able to collect and measure is less than the amount you would expect.  Experimental error often affects how much product is produced.

18.2 Percent Yield  The percent yield (%) is the actual yield divided by the predicted yield and then multiplied by one hundred.

Solving Problems  Aspirin can be made in the laboratory through a series of reactions.  If the actual yield for aspirin was grams when the reactions were performed, and the predicted yield was 500 grams, what was the percent yield?

1.Looking for:  …percent yield of reaction 2.Given  Actual yield = g  Predicted yield = g 3.Relationships:  percent yield = actual yield× 100% predicted yield 4.Solution  percent yield = (461.5 g ÷ g) × 100 = 92.3% Solving Problems

18.2 Reaction Rates  In all phases of matter, atoms and molecules exhibit random motion.  This concept is part of the kinetic theory of matter.  The speed at which atoms or molecules move depends on the state of matter and temperature.

18.2 Reaction Rates  The reaction rate for a chemical reaction is the change in concentration of reactants or products over time.  Reaction rates can be increased by: 1.adding heat to increase molecular motion 2.increasing the concentration of the reactants 3.increasing the chances that two molecules will collide.

18.2 Catalysts  A catalyst is a molecule that can be added to a reaction to speed it up.  Catalysts work by increasing the chances that two molecules will be positioned in the right way for a reaction to occur.

18.2 Chemical equilibrium  A reaction may reach chemical equilibrium, the state in which the rate of the forward reaction equals the rate of the reverse reaction.  In chemical equilibrium, the reaction can proceed both left and right simultaneously.