Spring Semester Final Exam Review Thermochemistry THERMOCHEMISTRY The

Spring Semester Final Exam Review Thermochemistry

THERMOCHEMISTRY The study of heat released or required by chemical reactions Fuel is burnt to produce energy - combustion (e. g. when fossil fuels are burnt) CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(l) + energy

What is Energy? Energy Kinetic energy (KE) Energy due to motion Potential energy (PE) Stored energy

Total Energy = Kinetic Energy E = KE + + Potential Energy PE Temperature measures the average Kinetic energy & potential energy are interchangeable Heat is the total energy of a system: Kinetic energy + potential energy

Systems & Surroundings In thermodynamics, the world is divided into a system and its surroundings A system is the part of the world we want to study (e. g. a reaction mixture in a flask) The surroundings consist of everything else outside the system

EXOTHERMIC & ENDOTHERMIC REACTIONS Exothermic process: a change (e. g. a chemical reaction) that releases heat to the surroundings. A release of heat corresponds to a decrease in enthalpy Exothermic process: H < 0 (at constant pressure) Burning fossil fuels is an exothermic reaction

Exothermic Reactions

Endothermic process: a change (e. g. a chemical reaction) that requires (or absorbs) heat from the surroundings. An input of heat corresponds to an increase in enthalpy Endothermic process: H > 0 (at constant pressure) Photosynthesis is an endothermic reaction (requires energy input from sun)

Endothermic Reactions

Endothermic or Exothermic? exothermic endothermic

Heating Curves Animation A plot of temperature vs. time that represents the process in which energy is added at a constant rate Courtesy Christy Johannesson www. nisd. net/communicationsarts/pages/chem

Temperature (o. C) Heating Curves 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 Gas - KE Boiling - PE Liquid - KE Melting - PE Solid - KE Time

A plot of temperature vs. time that represents the process in which energy is added at a constant rate

0 ) is the enthalpy of The standard enthalpy of reaction ( Hrxn a reaction carried out at 1 atm. a. A + b. B c. C + d. D H 0 rxn = [ c H 0 f (C) + d H 0 f (D) ] - [ a H 0 f (A) + b H 0 f (B) ] H 0 rxn = S n H 0 f (products) - S m Hf 0 (reactants)

Example Problem Calculate the heat of combustion of methane, CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(g) H◦f CH 4 (g) = -74. 86 k. J/mol H◦f O 2(g) = 0 k. J/mol H◦f CO 2(g) = -393. 5 k. J/mol pg. 316 H◦f H 2 O(g) = -241. 8 k. J/mol 2 mol(-241. 8 k. J/mol) = -483. 6 k. J Step #1: multiply the H◦f H 2 O(g) by 2 since there are two moles of water in the products. 15

Example Problem Calculate the heat of combustion of methane, CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(g) H◦f CH 4 (g) = -74. 86 k. J H◦f O 2(g) = 0 k. J/ H◦f CO 2(g) = -393. 5 k. J H◦ f. H 2 O(g) = -483. 6 k. J pg. 316 H◦f = [-393. 5 k. J + (-483. 6 k. J)]- [-74. 86 k. J + (0 k. J )] H◦f = -802. 2 k. J Step #2: sum up all the H◦ f. : Hrxn = Hf(products) - Hf(reactants) 16

Calculations Involving Specific Heat OR C = Specific Heat Capacity q = Heat lost or gained T = Temperature change Tf - T i

Choose all that apply. . . C(s) + 2 S(g) CS 2(l) H = 89. 3 k. J Which of the following are true? A) This reaction is exothermic B) It could also be written C(s) + 2 S(g) + 89. 3 k. J CS 2(l) C) The products have higher energy than the reactants D) It would make the water in the calorimeter colder