Section 15-5 Section 15.5 Reaction Spontaneity Differentiate between spontaneous and nonspontaneous processes. vaporization: the energy-requiring process by which a liquid changes to a gas or vapor Explain how changes in entropy and free energy determine the spontaneity of chemical reactions and other processes.

Section 15-5 Section 15.5 Reaction Spontaneity (cont.) spontaneous process entropy second law of thermodynamics free energy Changes in enthalpy and entropy determine whether a process is spontaneous.

Section 15-5 Spontaneous Processes A spontaneous process is a physical or chemical change that once begun, occurs with no outside intervention.spontaneous process Many spontaneous processes require some energy from the surroundings to start the process.

Section 15-5 Spontaneous Processes (cont.) Entropy (S) is a measure of the possible ways that the energy of a system can be distributed.Entropy Entropy is sometimes considered a measure of disorder or randomness of the particles in a system. The second law of thermodynamics states that spontaneous processes always proceed in such a way that the entropy of the universe increases. The universe wants to be a mess!second law of thermodynamics

Section 15-5 Spontaneous Processes (cont.) Entropy increases as a substance changes from a solid to a liquid and from a liquid to a gas. In equations, the side with more moles of gas generally has a higher entropy. H 2 O (l)  H 2 O (g) +ΔS 3 H 2 (g) + N 2 (g)  2 NH 3 (g) -ΔS

Section 15-5 Spontaneous Processes (cont.) Dissolving a gas in a solvent always results in a decrease in entropy. N 2 (g)  N 2 (aq) -ΔS With some exceptions, entropy increases when a solid or liquid dissolves in a solvent. NaCl (s)  Na + (aq) + Cl - (aq) +ΔS Random motion of particles increases as temperature increases. (+ΔS)

ΔS o rxn = Σ S o (prod) – Σ S o (react) The entropy change for a reaction can be calculated directly from standard data in the same manner that enthalpy changes are calculated:

The change in enthalpy (ΔH) and the change in entropy (ΔS) both have an influence on the spontaneity of a process. moving to a lower energy state (- ΔH) helps make a process spontaneous moving to more random state (+ ΔS) helps make a process spontaneous

Section 15-5 Entropy, the Universe, and Free Energy (cont.) Free energy is energy that is available to do work.Free energy Gibbs free energy equation ΔG system = ΔH system – TΔS system To assess the influence of both enthalpy and entropy, we consider a quantity called free energy.

Section 15-5 Entropy, the Universe, and Free Energy (cont.) If the sign of the energy change is positive, +ΔG°, the reaction is nonspontaneous. If the sign of the energy change is negative, -ΔG°, the reaction is spontaneous. ΔG o rxn = Σ ΔG f o (prod) - Σ ΔG f o (react) We can also calculate the free energy change by using standard values:

Section 15-5 Entropy, the Universe, and Free Energy (cont.) When ΔH r and ΔS r are both ‘+’ or both ‘-’, you can solve for the range of spontaneity by setting ΔG r = 0 and finding the temp. where it switches.

A.A B.B C.C D.D Section 15-5 Section 15.5 Assessment What is the measure of randomness in a system? A.enthalpy B.entropy C.free energy D.Gibbs free energy

A.A B.B C.C D.D Section 15-5 Section 15.5 Assessment Energy that is available to do work is ____. A.enthalpy B.entropy C.random molecular motion D.free energy

End of Section 15-5

Resources Menu Chemistry Online Study Guide Chapter Assessment Standardized Test Practice Image Bank Concepts in Motion

Study Guide 1 Section 15.1 Energy Key Concepts Energy is the capacity to do work or produce heat. Chemical potential energy is energy stored in the chemical bonds of a substance by virtue of the arrangement of the atoms and molecules. Chemical potential energy is released or absorbed as heat during chemical processes or reactions. q = c × m × ∆T

Study Guide 2 Section 15.2 Heat Key Concepts In thermochemistry, the universe is defined as the system plus the surroundings. The heat lost or gained by a system during a reaction or process carried out at constant pressure is called the change in enthalpy (∆H). When ∆H is positive, the reaction is endothermic. When ∆H is negative, the reaction is exothermic.

Study Guide 3 Section 15.3 Thermochemical Equations Key Concepts A thermochemical equation includes the physical states of the reactants and products and specifies the change in enthalpy. The molar enthalpy (heat) of vaporization, ∆H vap, is the amount of energy required to evaporate one mole of a liquid. The molar enthalpy (heat) of fusion, ∆H fus, is the amount of energy needed to melt one mole of a solid.

Study Guide 4 Section 15.4 Calculating Enthalpy Change Key Concepts The enthalpy change for a reaction can be calculated by adding two or more thermochemical equations and their enthalpy changes. Standard enthalpies of formation of compounds are determined relative to the assigned enthalpy of formation of the elements in their standard states.

Study Guide 5 Section 15.5 Reaction Spontaneity Key Concepts Entropy is a measure of the disorder or randomness of a system. Spontaneous processes always result in an increase in the entropy of the universe. Free energy is the energy available to do work. The sign of the free energy change indicates whether the reaction is spontaneous. ∆G system = ∆H system – T∆S system

A.A B.B C.C D.D Chapter Assessment 1 Which represents the larger amount of energy? A.1 calorie B.1000 calories C.10 kilocalories D.they are all equal

A.A B.B C.C D.D Chapter Assessment 2 What is the universe when using a bomb- calorimeter to measure heat absorbed by a substance in a process? A.the substance in the calorimeter B.the calorimeter itself C.the water in the calorimeter D.the calorimeter and all its contents

A.A B.B C.C D.D Chapter Assessment 3 In which example is the ΔH° positive? A.an ice cube melting B.condensation forming on cold glass of water C.molten lava cooling to form solid rock D.water vapor changing directly into frost on windows

A.A B.B C.C D.D Chapter Assessment 4 The standard enthalpy of an element in its standard state is ____. A.0.00 calories B.0.00°C C.0.00 kilocalories D.0.00 kJ

A.A B.B C.C D.D Chapter Assessment 5 Spontaneous processes always result in: A.an increase in entropy B.a decrease in entropy C.a decrease in randomness D.an increase in enthalpy

A.A B.B C.C D.D STP 1 The specific heat of ethanol is 2.4 J/g ● C. How many degrees Celsius can a 50.0g sample be raised with 2400 J of energy? A.10° B.20° C.30° D.40°

A.A B.B C.C D.D STP 2 What is the measure of the number of possible ways that the energy in a system can be distributed? A.free energy B.Hess's law C.enthalpy D.entropy

A.A B.B C.C D.D STP 3 Energy stored in chemical bonds is ____. A.free energy B.kinetic energy C.specific heat D.chemical potential energy

A.A B.B C.C D.D STP 4 The first law of thermodynamics states: A.matter is neither created nor destroyed in chemical reactions B.energy is neither created nor destroyed in chemical reactions C.spontaneous processes always proceed in such a way that the entropy of the universe increases D.free energy is random motion of particles

A.A B.B C.C D.D STP 5 ____ energy is energy of motion. A.Free B.Kinetic C.Enthalpy D.Chemical potential