1. CHEMISTRY – UNIT 12 Thermochemistry

2. Energy Energy – ability to do work or produce heat  Potential energy – energy due to composition or position of an object  Ex: Water stored behind a dam  Kinetic energy – energy of motion  Law of conservation of energy – in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed  Chemical potential energy – energy stored in a substance b/c of its composition  Ex: Octane in gasoline  Heat (q) – energy that is in the process of flowing from a warmer object to a cooler object

3.  Measuring heat  Calorie (cal) – amount of heat required to raise the temp. of 1 g of pure water by 1 0 C (metric system) Nutritional C alorie is 1000 calories (1 kcal)  Joule (J) – SI unit of heat and energy 1 cal = 4.184 J Specific Heat – amount of heat required to raise the temp. of 1 g of that substance by 1 0 C  Water has a high specific heat so it can absorb and release large quantities of heat

4.  Heat absorbed or released by a substance during a temp. change depends on the specific heat, mass of the substance, and amount by which the temp. changes. q = c x m x ΔT q = heat absorbed/released c = specific heat of substance m = mass of sample in grams ΔT = change in temp. in 0 C

5. Heat in Chemical Reactions and Processes Measuring Heat  Calorimeter – insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process  Calorimeter can be used to determine the specific heat of an unknown metal Chemical Energy and the Universe  Thermochemistry – study of heat changes that accompany chemical reactions and phase changes  Heat pack – O reacts w/ Fe in an exothermic reaction  Cold pack – ammonium nitrate dissolves in water (endothermic reaction)

6.  System – in thermochemistry, this is the specific part of the universe that contains the reaction or process you wish to study  Surroundings – everything in the universe other than the system  Universe – system plus the surroundings Universe = System + Surroundings  Enthalpy (H) – heat content of a system at constant pressure  You can’t measure actual energy or enthalpy of a substance but you can measure the change in enthalpy, which is the heat absorbed or released in a chemical reaction  Heat of reaction: ΔH rxn  ΔH rxn = H products - H reactants

7.  Sign of enthalpy of reaction: Heat pack reaction is exothermic 4Fe + 3O 2 2Fe 2 O 3 + 1625 kJ 4Fe + 3O 2 2Fe 2 O 3 ΔH rxn = -1625kJ Cold pack reaction is endothermic 27kJ + NH 4 NO 3 NH 4 + +NO 3 - NH 4 NO 3 NH 4 + +NO 3 - ΔH rxn = 27kJ

8. Thermochemical Equations Thermochemical equation – balanced equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy (ΔH)  Ex: Highly exothermic combustion of glucose occurs in the body as food is metabolized to produce energy ΔH comb = -2808 kJ  Energy released is enthalpy of combustion – enthalpy change for the complete burning of one mole of a substance

9. Changes of State  Molar enthalpy of vaporization (ΔH vap ) – heat required to vaporize one mole of a liquid  Molar enthalpy of fusion (ΔH fus ) – heat required to melt one mole of a solid substance  Both are endothermic, so ΔH is positive

10. Calculating Enthalpy Change Can not always measure heat change, so there is a way to theoretically determine ΔH Hess’s law – states that if you can add 2 or more thermochemical equations to produce a final equation for a reaction, the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction  Enables you to calculate enthalpy changes for an enormous number of chemical reactions by imagining that each reaction occurs through a series of steps for which the enthalpy changes are known

11. 2S + 3O 2 2SO 3 ΔH = ? First, chemical equations are needed that contain the substances found in the desired equation and have known enthalpy changes a. S + O 2 SO 2 ΔH = -297 kJ b. 2SO 3 2SO 2 + O 2 ΔH = 198 kJ The desired equation shows 2 moles of sulfur reacting, so Equation a must be rewritten for 2 moles of sulfur by multiplying the coefficients by 2. The enthalpy change (ΔH) must also be doubled b/c twice the energy will be released if 2 moles of sulfur react (Equation a becomes Equation c) c. 2S + 2O 2 2SO 2 ΔH = -594 kJ

12. B/c you want to determine ΔH for a reaction in which SO 3 is a product rather than a reactant, Equation b must be reversed (sign of ΔH changes) and becomes Equation d. d. 2SO 2 + O 2 2SO 3 ΔH = -198 kJ Now add Equations c and d. 2S + 2O 2 2SO 2 ΔH = -594 kJ 2SO 2 + O 2 2SO 3 ΔH = -198 kJ 2SO 2 + 2S + 3O 2 2SO 2 + 2SO 3 ΔH = -792 kJ

13. Entropy Entropy – measure of the disorder or randomness of the particles that make up a system Law of disorder – states that spontaneous processes always proceed in such a way that the entropy of the universe increases  Entropy is always increasing  The more time that passes, the messier your room gets until you clean it again