Thermochemistry

Thermochemistry Spring Semester Final Exam Review

Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt) CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy THERMOCHEMISTRY The study of heat released or required by chemical reactions

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

Total Energy = Kinetic Energy + Potential Energy E = KE + 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 systemis the part of the world we want to study (e.g. a reaction mixture in a flask) The surroundingsconsist of everything else outside the system

Burning fossil fuels is an exothermic reaction 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)

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 exothermic endothermic endothermic

Heating Curves 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

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

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

The standard enthalpy of reaction(DH0 ) is the enthalpy of a reaction carried out at 1 atm. rxn aA + bBcC + dD - [ + ] [ + ] = - S S = DH0 DH0 rxn rxn mDH0 (reactants) dDH0 (D) nDH0 (products) cDH0 (C) bDH0 (B) aDH0 (A) f f f f f f 6.6

Example Problem Calculate the heat of combustion of methane, CH4 CH4(g) +2O2(g) CO2(g) + 2 H2O(g) H◦fCH4 (g) = -74.86 kJ/mol H◦fO2(g) = 0 kJ/mol H◦fCO2(g) = -393.5 kJ/mol H◦fH2O(g) = -241.8 kJ/mol pg. 316 2 mol(-241.8 kJ/mol) =-483.6 kJ Step #1: multiply the H◦fH2O(g) by 2 since there are two moles of water in the products .

Example Problem Calculate the heat of combustion of methane, CH4 CH4(g) +2O2(g) CO2(g) + 2 H2O(g) H◦fCH4 (g) = -74.86 kJ H◦fO2(g) = 0 kJ/ H◦fCO2(g) = -393.5 kJ H◦ fH2O(g) = -483.6 kJ pg. 316 H◦f=[-393.5 kJ + (-483.6 kJ)]- [-74.86 kJ + (0 kJ )] H◦f= -802.2 kJ Step #2: sum up all the H◦f. : Hrxn = Hf(products) - Hf(reactants)

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

Choose all that apply... C(s) + 2 S(g) CS2(l) H = 89.3 kJ Which of the following are true? • This reaction is exothermic B) It could also be written C(s) + 2 S(g) + 89.3 kJ CS2(l) C) The products have higher energy than the reactants D) It would make the water in the calorimeter colder

Thermochemistry