Chapter 5 - Thermochemistry Lindblom AP Chemistry

Energy Energy is the ability to do work or transfer heat. – Work - energy used to cause an object that has mass to move. w = F ×d – Heat - energy used to cause the temperature of an object to rise.

Potential vs. Kinetic Potential stored energy – chemical energy – positional Kinetic being used – heat (thermal) – movement

System and Surroundings The system reacting substances or substances being studied. The surroundings are everything else, including water if reaction happens in aqueous solution. © 2009, Prentice-Hall, Inc.

First Law of Thermodynamics Energy is neither created nor destroyed. Energy can be transferred from one form to another or from one substance to another. – Example: – Fire (chemical, potential, energy transferred to thermal, kinetic, energy) © 2009, Prentice-Hall, Inc.

Enthalpy The “heat” absorbed or released in a reaction. Symbolized ΔH In an equation, it is written at the end and proportional to the stoichiometric amounts of products and reactants: 2C (s) + H 2 (g)  C 2 H 2 (g) ΔH = kJ

Endothermic and Exothermic Endothermic Absorbs heat from surroundings + ΔH Requires energy Exothermic Releases heat to surroundings − ΔH Releases energy

Hess’s Law If a processes is carried out in a series of steps, the overall enthalpy change is equivalent to the sum of the enthalpy changes of each step.

Example Using the reactions: NO (g) + O 3 (g)  NO 2 (g) + O 2 (g)ΔH = −198.9 kJ O 3 (g)  3/2O 2 (g) ΔH = − kJ O 2 (g)  2O(g) ΔH = kJ Calculate ΔH for the reaction: NO(g) + O(g)  NO 2 (g) kJ

Standard Enthalpy of Formation Enthalpy associated with making a compound from its elements. Symbolized ΔH f ° when all reactants and products are at their standard states (1atm and 298K) – For example, the equation used to find the standard enthalpy of formation of water would be: 2H 2 (g) + O 2 (g)  2H 2 O (l)

Standard Enthalpy Continued The standard enthalpy of an element is zero (if it is in standard conditions). We can combine this with Hess’s law, such that for any reaction: ΔH rnx ° = ΣnΔH f ° (products) - ΣmΔH f ° (reactants) Standard enthalpies can be found in appendix C at the back of your book.

Example Calculate ΔH for the reaction: CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O (l) [(1 mol × −393.5 kJ/mol) + (2mol × −285.83kJ/mol)] − [(1mol × −74.8 kJ/mol) + (2 mol × 0)] = − kJ For CO 2 and H 2 O For CH 4 and O 2

Specific Heat Capacity Energy required to increase the temperature of 1 gram of any substance by 1°C (or 1-K)

Calorimetry Measure of heat flow (q) Heat flow is impossible to measure directly Temperature is related to heat through specific heat capacity – the energy required to raise 1 gram of a substance 1°C q = C s × m ×ΔT Energy (J) Specific Heat Capacity (J/g  ° C) Mass Temperature change