18.5 Free Energy and Entropy > 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 18 Reaction Rates and Equilibrium 18.1 Rates of Reaction 18.2 The Progress of Chemical Reactions 18.3 Reversible Reactions and Equilibrium 18.4 Solubility Equilibrium 18.5 Free Energy and Entropy

18.5 Free Energy and Entropy > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. How can a fire start on its own? CHEMISTRY & YOU Sometimes a fire can occur without an external source of ignition, such as a match or an electrical spark. Spontaneous combustion is the term used to describe these fires.

18.5 Free Energy and Entropy > 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions What are two characteristics of spontaneous reactions?

18.5 Free Energy and Entropy > 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Some of the energy released in a chemical reaction can be harnessed to do work, such as pushing the pistons in an internal-combustion engine. The energy that is available to do work is called free energy.

18.5 Free Energy and Entropy > 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Spontaneous Versus Nonspontaneous Reactions You can write a balanced equation for a chemical reaction, but the reaction may not actually take place. CO 2 (g)  C(s) + O 2 (g) Experience tells you that this reaction does not tend to occur. Carbon and oxygen react to form carbon dioxide, not the reverse.

18.5 Free Energy and Entropy > 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Spontaneous Versus Nonspontaneous Reactions The world of balanced chemical equations is really divided into two groups. One group contains equations representing reactions that actually occur. The other contains equations representing reactions that do not tend to occur, or at least not efficiently.

18.5 Free Energy and Entropy > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions A spontaneous reaction occurs naturally and favors the formation of products at the stated conditions. Spontaneous Versus Nonspontaneous Reactions

18.5 Free Energy and Entropy > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Spontaneous reactions produce large amounts of products and release free energy. Fireworks displays are the result of highly favored spontaneous reactions. Spontaneous Versus Nonspontaneous Reactions

18.5 Free Energy and Entropy > 9 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions A chemical reaction that does not favor the formation of products at the stated conditions is called a nonspontaneous reaction. Such reactions produce little, if any, product. Spontaneous Versus Nonspontaneous Reactions

18.5 Free Energy and Entropy > 10 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions In nearly all reversible reactions, one reaction is favored over the other. Reversible Reactions

18.5 Free Energy and Entropy > 11 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Reversible Reactions Consider the decomposition of carbonic acid in water. H 2 CO 3 (aq) CO 2 (g) + H 2 O(l) 99%

18.5 Free Energy and Entropy > 12 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Reversible Reactions Consider the decomposition of carbonic acid in water. The forward reaction is spontaneous and releases free energy. H 2 CO 3 (aq) CO 2 (g) + H 2 O(l) 99%

18.5 Free Energy and Entropy > 13 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Reversible Reactions Consider the decomposition of carbonic acid in water. The forward reaction is spontaneous and releases free energy. The combination of carbon dioxide and water to form carbonic acid is a nonspontaneous reaction. H 2 CO 3 (aq) CO 2 (g) + H 2 O(l) 99%

18.5 Free Energy and Entropy > 14 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions When solutions of cadmium nitrate and sodium sulfide are mixed, the products are aqueous sodium nitrate and solid yellow cadmium sulfide. A precipitate of cadmium sulfide forms spontaneously. The reverse reaction is nonspontaneous. Cd(NO 3 ) 2 (aq) + Na 2 S(aq) CdS(s) + 2NaNO 3 (aq) Reversible Reactions

18.5 Free Energy and Entropy > 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions The Rate of Spontaneous Reactions The terms spontaneous and nonspontaneous do not refer to the rate of a reaction. Some spontaneous reactions are so slow that they appear to be nonspontaneous.

18.5 Free Energy and Entropy > 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Changing the conditions of a chemical reaction can affect whether a reaction will occur. A reaction that is nonspontaneous in one set of conditions may be spontaneous in other conditions.

18.5 Free Energy and Entropy > 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Photosynthesis is a multistep reaction that takes place in plant leaves. Outside of plants, carbon dioxide and water do not normally combine to produce sugar and oxygen. This complex process could not happen without the energy supplied by sunlight and plant pigments such as chlorophyll.

18.5 Free Energy and Entropy > 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Coupled Reactions Sometimes a nonspontaneous reaction can be made to occur if it is coupled to a spontaneous reaction.

18.5 Free Energy and Entropy > 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy and Spontaneous Reactions Coupled Reactions Sometimes a nonspontaneous reaction can be made to occur if it is coupled to a spontaneous reaction. One reaction releases energy that is used by the other reaction. Coupled reactions are common in the complex biological processes that take place in living organisms.

18.5 Free Energy and Entropy > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Decomposition reactions that occur inside a pile of oily rags or a damp stack of hay cause heat to build up. If the heat cannot escape, the temperature within the pile or stack will rise. How can a rise in temperature cause a fire to start on its own? CHEMISTRY & YOU

18.5 Free Energy and Entropy > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Decomposition reactions that occur inside a pile of oily rags or a damp stack of hay cause heat to build up. If the heat cannot escape, the temperature within the pile or stack will rise. How can a rise in temperature cause a fire to start on its own? CHEMISTRY & YOU The combustion reaction is a nonspontaneous reaction that can be made to occur when it is coupled to the spontaneous decomposition reaction. The decomposition reaction releases energy that is used by the combustion reaction.

18.5 Free Energy and Entropy > 22 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following is ALWAYS true of spontaneous reactions? A.They produce heat and are not reversible at the stated conditions. B.They release free energy and favor the formation of products at the stated conditions. C.They are coupled with a nonspontaneous reaction and are easily reversible at the stated conditions.

18.5 Free Energy and Entropy > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following is ALWAYS true of spontaneous reactions? A.They produce heat and are not reversible at the stated conditions. B.They release free energy and favor the formation of products at the stated conditions. C.They are coupled with a nonspontaneous reaction and are easily reversible at the stated conditions.

18.5 Free Energy and Entropy > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy What part does entropy play in a reaction?

18.5 Free Energy and Entropy > 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy You might expect that only exothermic reactions are spontaneous. Some processes, however, are spontaneous even though they absorb heat.

18.5 Free Energy and Entropy > 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy You might expect that only exothermic reactions are spontaneous. Some processes, however, are spontaneous even though they absorb heat. Consider what happens as ice melts. As it changes from a solid to a liquid, 1 mol of ice at 25 o C absorbs 6.0 kJ of heat from its surroundings. If you consider only enthalpy changes, it is difficult to explain why the ice melts.

18.5 Free Energy and Entropy > 27 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Some factor other than the enthalpy change must help determine whether a physical or chemical process is spontaneous.

18.5 Free Energy and Entropy > 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Some factor other than the enthalpy change must help determine whether a physical or chemical process is spontaneous. The other factor is related to order.

18.5 Free Energy and Entropy > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Entropy is a measure of the disorder of a system. The law of disorder states that the natural tendency is for systems to move in the direction of increasing disorder or randomness.

18.5 Free Energy and Entropy > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy A dog walker with several dogs could represent relative order and disorder. All of the dogs are on leashes and are strolling orderly along the path. The dogs are no longer wearing leashes and are running freely. This situation represents disorder.

18.5 Free Energy and Entropy > 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Reactions in which entropy increases as reactants form products tend to be favored. Entropy can affect the direction of a reaction.

18.5 Free Energy and Entropy > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Thus, entropy increases in reactions in which solid reactants form liquid or gaseous products.

18.5 Free Energy and Entropy > 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Entropy increases when a substance is divided into parts. For instance, entropy increases when an ionic compound dissolves in water.

18.5 Free Energy and Entropy > 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules.

18.5 Free Energy and Entropy > 35 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Entropy Entropy tends to increase when the temperature increases. As the temperature rises, the molecules move faster and faster, which increases the disorder.

18.5 Free Energy and Entropy > 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following would have an increase in the entropy of the reaction system? A.2NH 4 NO 3 (s)  2N 2 (g) + 4H 2 O(l) +O 2 (g) B.2H 2 (g) + O 2 (g)  2H 2 O(l) C.C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l) D.2Fe(s) + O 2 (g) + 2H 2 O(l)  2Fe(OH) 2 (s)

18.5 Free Energy and Entropy > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following would have an increase in the entropy of the reaction system? A.2NH 4 NO 3 (s)  2N 2 (g) + 4H 2 O(l) +O 2 (g) B.2H 2 (g) + O 2 (g)  2H 2 O(l) C.C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l) D.2Fe(s) + O 2 (g) + 2H 2 O(l)  2Fe(OH) 2 (s)

18.5 Free Energy and Entropy > 38 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Enthalpy and Entropy What two factors determine whether a reaction is spontaneous?

18.5 Free Energy and Entropy > 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Enthalpy and Entropy The size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous.

18.5 Free Energy and Entropy > 40 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Enthalpy and Entropy Consider an exothermic reaction in which entropy increases. The reaction will be spontaneous because both factors are favorable.

18.5 Free Energy and Entropy > 41 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Enthalpy and Entropy A decrease in entropy is offset by a large release of heat. An increase in enthalpy is offset by an increase in entropy. A reaction can be spontaneous if:

18.5 Free Energy and Entropy > 42 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Enthalpy and Entropy The table below summarizes the effect of enthalpy and entropy changes on the spontaneity of reactions. How Enthalpy Changes and Entropy Changes Affect Reaction Spontaneity Enthalpy changeEntropy changeIs the reaction spontaneous? Decreases (exothermic) Increases (more disorder in products than in reactants) Yes Increases (endothermic) Increases Only if unfavorable enthalpy change is offset by favorable entropy change Decreases (exothermic) Decreases (less disorder in products than in reactants) Only if unfavorable entropy change is offset by favorable enthalpy change Increases (endothermic) DecreasesNo

18.5 Free Energy and Entropy > 43 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Would the following exothermic reaction be spontaneous? Explain why or why not. 2KClO 3 (s) 2KCl(s) +3O 2 (g)

18.5 Free Energy and Entropy > 44 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Would the following exothermic reaction be spontaneous? Explain why or why not. 2KClO 3 (s) 2KCl(s) +3O 2 (g) Two molecules of solid are transformed into 2 molecules of solid and 3 molecules of gas, so entropy is increased in the reaction. A reaction that is exothermic with an increase in entropy will be spontaneous.

18.5 Free Energy and Entropy > 45 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy Change How is the value of ΔG related to the spontaneity of a reaction?

18.5 Free Energy and Entropy > 46 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free energy is often expressed as Gibbs free energy. Free Energy Change This term is named for Josiah Gibbs, the scientist who defined this thermodynamic property. The symbol for Gibbs free energy is G. Free energy can either be released or absorbed during a physical or chemical process.

18.5 Free Energy and Entropy > 47 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. ΔG = ΔH – TΔS The equation below is used to calculate the change in Gibbs free energy (ΔG). Free Energy Change ΔS is the change in entropy. ΔH is the change in enthalpy. T is the temperature in Kelvins.

18.5 Free Energy and Entropy > 48 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Free Energy Change When the value of ΔG is negative, the process is spontaneous. When the value is positive, the process is nonspontaneous.

18.5 Free Energy and Entropy > 49 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The entropy change for the following reaction at 298 K is 3.0 J/mol·K, and the enthalpy change is –394 kJ/mol. C(s) + O 2 (g) CO 2 (g) Calculate the Gibbs free energy change and determine whether the reaction will occur spontaneously.

18.5 Free Energy and Entropy > 50 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The entropy change for the following reaction at 298 K is 3.0 J/mol·K, and the enthalpy change is –394 kJ/mol. C(s) + O 2 (g) CO 2 (g) Calculate the Gibbs free energy change and determine whether the reaction will occur spontaneously. ΔG = –394 kJ/mol – (298 K  kJ/mol·K) ΔG = –395 kJ/mol The reaction is spontaneous.

18.5 Free Energy and Entropy > 51 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts Spontaneous reactions produce large amounts of products and release free energy. Reactions in which entropy increases as reactants form products tend to be favored. The size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous. When the value of ΔG is negative, a process is spontaneous. When the value is positive, a process is nonspontaneous.

18.5 Free Energy and Entropy > 52 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Equation ΔG = ΔH – TΔS

18.5 Free Energy and Entropy > 53 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms free energy: the energy available to do work spontaneous reaction: a reaction that favors the formation of products at the specified conditions; spontaneity depends on enthalpy and entropy changes nonspontaneous reaction: a reaction that does not favor the formation of products at the specified conditions

18.5 Free Energy and Entropy > 54 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms entropy: a measure of the disorder of a system; systems tend to go from a state of order (low entropy) to a state of maximum disorder (high entropy) law of disorder: it is a natural tendency of systems to move in the direction of maximum chaos or disorder

18.5 Free Energy and Entropy > 55 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chemical Reactions, Matter, and Energy BIG IDEA Changes in enthalpy and entropy can be used to explain why some reactions occur naturally and others do not.

18.5 Free Energy and Entropy > 56 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF 18.5