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) CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy

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

A release of heat corresponds to a decrease in enthalpy 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 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?

Heating Curves Heating Curves Animation A plot of temperature vs. time that represents the process in which energy is added at a constant rate The magnitude of the temperature change of an object when it absorbs or loses thermal energy depends on both the amount of thermal energy transferred (q) and the heat capacity of the object. Heat capacity (C) — amount of energy needed to raise the temperature of the object exactly 1°C, and the units of C are joules per degree Celsius (J/°C) Change in temperature (ΔT) is: ΔT = q/C, where q is the amount of heat (in units of J), C is the heat capacity (in units of J/°C), and ΔT is Tfinal  Tinitial (in units of °C).  If ΔT and q are positive, heat flows from the surroundings into the object, and if ΔT and q are negative, heat flows from the object into the surroundings. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

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

A plot of temperature vs 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 + bB cC + dD DH0 rxn dDH0 (D) f cDH0 (C) = [ + ] - bDH0 (B) aDH0 (A) DH0 rxn nDH0 (products) f = S mDH0 (reactants) - 6.6

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

Hrxn =  Hf(products) -   Hf(reactants) Example Problem Calculate the heat of combustion of methane, CH4 CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) H◦f CH4 (g) = -74.86 kJ H◦f O2(g) = 0 kJ/ H◦f CO2(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