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

Chapter 18.1 Learning Goals Contrast endothermic and exothermic reactions. Explain why activation energy is needed to begin chemical reactions. Describe what happens when ionic compounds are dissolved in water.

18.1 Energy and Reactions Energy is involved in chemical reactions in two ways: to break some (or all) bonds between atoms in the reactants so the atoms can form new bonds or when the atoms or products form new bonds to make new products.

18.1 Two Types of Reactions We classify chemical reactions based on how the energy of the reactants compares to the energy of the products.

18.1 Exothermic reactions If forming new bonds releases more energy than it takes to break the old bonds, the reaction is exothermic.

18.1 Exothermic reactions A good example is the reaction of hydrogen with oxygen. Once started, exothermic reactions tend to keep going as each reaction releases more energy to fuel neighboring molecules.

18.1 Endothermic reactions If forming new bonds in the products releases less energy than it took to break the original bonds, the reaction is endothermic.

18.1 Endothermic reactions An important endothermic reaction is photosynthesis. Plants need energy from sunlight to make glucose and oxygen from carbon dioxide and water.

18.1 Activation Energy Activation energy is the energy needed to begin a reaction and break chemical bonds in the reactants.

18.1 Activation Energy This is why a flammable material like gasoline does not burn without a spark or flame.

18.1 Carbon Reactions Carbon dioxide and other atmospheric gases are called “greenhouse gases.” Scientists believe the rise in amount of greenhouse gases will result in rises in sea level and changes in weather.

18.1 Examples of Endothermic Reactions Most of the reactions used in industry to produce useful materials require more energy than they produce. One process that uses endothermic reactions is the refining of ores to produce useful metals.

18.1 Examples of Endothermic Reactions Most of the reactions used in industry to produce useful materials require more energy than they produce. The reaction taking place inside an instant cold pack is endothermic.

18.1 Examples of Endothermic Reactions When you squeeze the plastic bag the water reacts with the ammonium nitrate crystals, and the reaction dissolves the ionic bonds in the ammonium nitrate.

18.1 Examples of Endothermic Reactions The reaction is also a dissolution reaction. Dissolution occurs when an ionic compound (like ammonium nitrate) dissolves in water to make an ionic solution.

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 461.5 grams when the reactions were performed, and the predicted yield was 500 grams, what was the percent yield?

Solving Problems Looking for: Given Relationships: Solution …percent yield of reaction Given Actual yield = 461.5 g Predicted yield = 500.0 g Relationships: percent yield = actual yield × 100% predicted yield Solution percent yield = (461.5 g ÷ 500.0 g) × 100 = 92.3%

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: adding heat to increase molecular motion increasing the concentration of the reactants 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.

18.2 Chemical vs. Nuclear Reactions For complex reasons, the nucleus of an atom becomes unstable if it contains too many or too few neutrons relative to the number of protons. The forces inside the atom result in it breaking apart or releasing particles.

Radioactive Decay This process of radioactive decay results in an unstable, radioactive isotope like carbon-14 becoming the more stable isotope nitrogen-14.

Radioactive Decay There are three types of radioactive decay: alpha decay, beta decay, and gamma decay.

Two types of Nuclear Reactions There are two kinds of nuclear reactions: fusion and fission. Nuclear fusion is the process of combining the nuclei of lighter atoms to make heavier atoms.

18.3 Fusion Nuclear fusion occurs in the Sun and the resulting energy released provides Earth with heat and light.

18.3 Types of Nuclear Reactions Nuclear fission is the process of splitting the nucleus of an atom. A fission reaction can be started when a neutron bombards a nucleus.

18.3 Fission A nuclear reactor is a power plant that uses fission to produce heat.