Chapter 15 Energy and Chemical Change Section 15.1 Energy Section 15.2Heat Section 15.3Thermochemical Equations Section 15.4 Calculating Enthalpy Change Section 15.5Reaction Spontaneity

Energy is the ability to do work or produce heat. The Nature of Energy Energy exists in two basic forms: a) Potential energy is energy due to composition or position. b) Kinetic energy is energy of motion. The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed -also known as the first law of thermodynamics.

Energy stored within chemical substances is called chemical potential energy. Heat (q) is energy that transfers from a warmer object to a cooler object because of a difference in temperature. Heat, itself, cannot be detected by the senses or by instruments. Only changes caused by heat can be detected. One of the effects of adding heat is a rise in the temperature of objects. If two objects remain in contact, heat will flow from the warmer object to the cooler object until the temperature of both objects is the same.

A calorie (cal) is a unit for heat, defined as the quantity of heat needed to raise the temperature of 1 g of pure water by 1 0 C. Dietary Calorie, written with a capital C, always refers to the energy in food. 1 Calorie =1 kilocalorie = 1000 calories. Joule (J) is the SI unit of heat and energy One calorie is joule. Or 1 Joule = calories Use as conversion factors

Specific heat (C) or specific heat capacity, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance by 1  C. unit for specific heat is (J/g· 0 C) or (cal /g· 0 C) Look at data tables.

A Calorimeter is an insulated device used for measuring the amount of heat absorbed or released in a chemical reaction or physical process. Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes. Section15.2

A system is the part of the universe on which you focus your attention, e.g. your reaction mixture. The surroundings include everything else. The universe is defined as the system plus the surroundings. universe

a) A process that absorbs heat from the surroundings is called an endothermic process. b) A process that releases heat to its surroundings is called an exothermic process. EndothermicExothermic

Enthalpy is the heat content of a system at constant pressure. Enthalpy (heat) of reaction is the change in enthalpy during a reaction symbolized as ΔH rxn. ΔH rxn = H final – H initial ΔH rxn = H products – H reactants Enthalpy changes for … exothermic reactions are always negative. endothermic reactions are always positive. (heat and enthalpy can be used interchangeably if pressure constant) q =  H = m x C x  T

Endothermic or Exothermic?

CaO(s) + H 2 O (l)  Ca(OH) 2 (s) kJ or CaO(s) + H 2 O (l)  Ca(OH) 2 (s)  H = kJ 2 NaHCO 3 (s) kJ  Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) or 2 NaHCO 3 (s)  Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g)  H = 129 KJ A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products, and energy change. Section15.3

The (standard) enthalpy or heat of combustion is the heat of reaction for the complete burning of one mole of a substance. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2H 2 O (l)  H = -891 kJ

Molar enthalpy (heat) of vaporization (Standard….) is the heat required to vaporize one mole of a liquid substance. (  H vap ). Molar enthalpy (heat) of fusion is the amount of heat required to melt one mole of a solid substance. (  H fus ).  H fus = -  H solid (fusion) (solidification)  H vap = -  H cond (Vaporization) (condensation)

Which of the following processes are exothermic? Endothermic? a) C 2 H 5 OH(l) → C 2 H 5 OH(g) b) Br 2 (l) → Br 2 (s) c) C 5 H 12 (g) + 8O 2 (g) → 5CO 2 (g) + 6H 2 O(l) d) NH 3 (g) → NH 3 (l) e) NaCl(s) → NaCl(l)

Hess’ law (of heat summation) If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction Section15.4

Hess’ Law can be used to calculate the heat of complicate reactions that may be hard to measure.

The standard heat of formation (  H f °) of a substance is the change of enthalpy that accompanies the formation of one mole of a compound from its elements (with all sub- stances in their standard states at 25  C )

Standard enthalpies of formation can be used to calculate the enthalpies for many reactions under standard conditions by using Hess’s law. (Elements may be above or below the compounds) Elements in their standard states :  H f ° = 0.0 KJ/mol  H° rxn =   H f °(products) -   H f ° (reactants)

15.5 Spontaneous Processes A spontaneous process is a physical or chemical change that once begun, occurs with no outside intervention. Many spontaneous processes require some energy from the surroundings to start the process. The second law of thermodynamics states that spontaneous processes always proceed in such a way that the entropy of the universe increases.

Example: On the left only one way, but after opening the valve one the right there are 4 different ways of randomness. Entropy is the quantitative measure of disorder (or degrees of freedom or randomness) in a system. The more spread out the “particles” are, the more disorder. Making a mess increases the entropy Entropy is a measure of all the ways that the energy of a system can be distributed.

Phase changes: Entropy increases as a substance changes from a solid to a liquid and from a liquid to a gas. Dissolving a gas in a solvent always decreases the entropy. Entropy Changes No Phase change: Entropy of a system usually increases when the products have a greater number of gaseous particles than the reactants. Often entropy increases when solids or liquids dissolve in a solvent. The random motion of particles of a substance increases as its temperature increases. In nature, the change in entropy ∆S tends to be positive when: − The entropy of the system increases. − The reaction or process is exothermic, which raises the temperature of the surroundings

Free energy (Gibbs Free energy) is energy that is available to do work. If the free energy change, ΔG°, is … positive, the reaction is nonspontaneous. negative, the reaction is spontaneous.

The higher the temperature, the more important ∆S is