Chapter 6 THERMOCHEMISTRY West Valley High School AP Chemistry Mr. Mata
Definitions #1 Energy: The capacity to do work or produce heat Potential Energy: Energy due to position or composition Kinetic Energy: Energy due to the motion of the object
Definitions #2 Law of Conservation of Energy: Energy can neither be created nor destroyed, but can be converted between forms The First Law of Thermodynamics: The total energy content of the universe is constant
State Functions State Functions depend ONLY on the present state of the system ENERGY IS A STATE FUNCTION A person standing at the top of Mt. Everest has the same potential energy whether they got there by hiking up, or by falling down from a plane! WORK IS NOT A STATE FUNCTION
E = q + w E = change in internal energy of a system q = heat flowing into or out of the system -q if energy is leaving to the surroundings +q if energy is entering from the surroundings w = work done by, or on, the system -w if work is done by the system on the surroundings +w if work is done on the system by the surroundings
Work, Pressure, and Volume Expansion Compression + V (increase) - V (decrease) - w results+ w results E system decreases Work has been done by the system on the surroundings E system increases Work has been done on the system by the surroundings
Energy Change in Chemical Processes Endothermic: Exothermic: Reactions in which energy flows into the system as the reaction proceeds. Reactions in which energy flows out of the system as the reaction proceeds. + q system - q surroundings - q system + q surroundings
Endothermic Reactions
Exothermic Reactions
Calorimetry The amount of heat absorbed or released during a physical or chemical change can be measured… …usually by the change in temperature of a known quantity of water 1 calorie is the heat required to raise the temperature of 1 gram of water by 1 C 1 BTU is the heat required to raise the temperature of 1 pound of water by 1 F
The Joule The unit of heat used in modern thermochemistry is the Joule 1 joule = calories
A Bomb Calorimeter
A Cheaper Calorimeter
Specific Heat The amount of heat required to raise the temperature of one gram of substance by one degree Celsius. Substance Specific Heat (J/g·K) Water (liquid)4.18 Ethanol (liquid) 2.44 Water (solid) 2.06 Water (vapor) 1.87 Aluminum (solid) Carbon (graphite,solid) Iron (solid) Copper (solid) Mercury (liquid) Lead (solid) Gold (solid) 0.129
Calculations Involving Specific Heat s = Specific Heat Capacity q = Heat lost or gained T = Temperature change OR
Hess’s Law “In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.”
Hess’s Law
Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ Step #1: CH 4 must appear on the reactant side, so we reverse reaction #1 and change the sign on H. CH 4 C + 2H kJ
Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ CH 4 C + 2H kJ Step #2: Keep reaction #2 unchanged, because CO 2 belongs on the product side C + O 2 CO kJ
Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ CH 4 C + 2H kJ C + O 2 CO kJ Step #3: Multiply reaction #3 by 2 2H 2 + O 2 2 H 2 O kJ
Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ CH 4 C + 2H kJ C + O 2 CO kJ 2H 2 + O 2 2 H 2 O kJ Step #4: Sum up reaction and H CH 4 + 2O 2 CO 2 + 2H 2 O kJ
Calculation of Heat of Reaction Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O H rxn = H f (products) - H f (reactants) Substance H f CH kJ O2O2 0 kJ CO kJ H2OH2O kJ H rxn = [ kJ + 2( kJ)] – [-74.80kJ] H rxn = kJ