Chapter 17: Thermochemistry

17.1: Flow of Energy - Heat

• Energy - capacity to do work or supply heat.
• 1. potential energy - stored energy
• a. positional - where it is
• b. compositional - what it is
• chemical potential energy - energy stored in chemical bonds

• 2. kinetic energy - energy of motion

A. heat (q) - energy transferred between two objects of differing temperature.

• -Can't be measured directly.
• -Flows from hot to cold.

B. light, electrical energy, mechanical, sound

17.1: Endothermic and Exothermic Reactions

• System - what you are looking at
• Surroundings - everything else
• Universe = system + surroundings
• Endothermic - energy in, system absorbs heat from the surroundings(+).
• Exothermic - energy out, system loses heat to surroundings(-).

17.1: Units of energy

• calorie (cal) - heat needed to raise 1 g of water 1 C.
• Calorie (Cal) - food Calorie - 1000 cal
• Joule (J) - SI unit of energy
• 1Cal = 1kcal = 1000 cal = 4184 J = 4.184 kJ (SSN)

17.1: Heat Capacity

• Heat Capacity - heat it takes to change a substance’s temperature by 1 C.
• Law of conservation of energy - in ordinary reactions energy is not created or destroyed but converted from one form to another.
• Thermochemistry - study of heat changes during chemical and physical reactions.

• q = m x C x DT (SSN)
• q = heat
• m = mass of object
• C = Specific heat - heat it takes to raise 1 g of a substance 1C.
• T = change in temperature = T_final - T_initial­ (SSN)
• Table 17.1 in SSN

17.2: Heat Changes

• Single State - warming or cooling a solid, liquid or gas
• q=DH (for a system at constant pressure)
• DH = “change in” enthalpy
• q_sys=DH=-q_surr=m x C x DT (SSN)
• DH is positive for endothermic processes
• DH is negative for exothermic processes

17.3: Heat in Changes of State

• Change in State - melting, freezing, boiling, or condensing
• q = (H) x (# moles)
• Or q = DH x mass (must use different value for DH)

17.3: Solid  Liquid

• q = H_fus x (# mol) (SSN)
• q = DH_fus x m (SSN)
• Molar heat of fusion - heat need to melt 1 mol of a solid.
• melting - H_fus is positive(+)
• freezing - H_fus is negative(-)

​

17.3: Liquid  Gas

• q = (H_vap) x (# mol) (SSN)
• q = (H_vap) x m (SSN)
• Molar heat of vaporization - heat needed to vaporize 1 mol of a liquid.
• boiling - H_vap is positive(+)
• condensing - H_vap is negative(-)

17.3: Soild ® Aqueous (dissolving)

• q = (H_soln) x (# mol) (SSN)
• q = (H_soln) x m (SSN)
• Molar heat of solution – the heat change caused by the dissolution of one mole of substance
• Can be either endothermic or exothermic, depending on the substance.

Practice (SSN)

• Both Single and Change of State
• Calculate the energy needed to cool 5.0 g water from 42 C to ice at -23 C?
• A cold pack has ammonium nitrate in a small bag inside a bag of water. When you punch it, you release the AN to dissolve in the water. If a cold pack contains 12.5g of AN, calculate the DH^o
• NH₄NO_3(s) → NH₄⁺_(aq) + NO₃^-_(aq)
• DH_sln = 25.7 kJ/mol

17.2: Thermochemical Reactions

• DH is the “change in” enthalpy of the reaction
• Endothemic is +, treated as reactant
• Exothermic is -, treated as product
• Ex: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g), DH = 129kJ
• 2NaHCO₃(s) + 129kJ → Na₂CO₃(s) + H₂O(g) + CO₂(g)
• Ex: CaO(s) + H₂O(l) → Ca(OH)₂(s), DH = -65.2kJ
• CaO(s) + H₂O(l) → Ca(OH)₂(s) + 65.2kJ

17.4: Standard Heats of Formation

• DH^o = standard heat of reaction
• = standard heat of formation
• is zero for elements in their standard state & for the diatomic molecules
• for compounds must be looked up in a table.

Examples

• Table 17.2 in SSN
• Tells DH^o for several hydrocarbon combustions rxns
• Table 17.4 in SSN
• Tells DH_f^o for several substances
• Calculate the DH^o for the burning of 8.47g of propane (C₃H₈) (SSN)
• Is the rxn endothermic or exothermic?
• Calculate the DH^o for the following rxn:
• 2H₂ + O₂ → 2H₂O
• Is the rxn endothermic or exothermic?

17.4: Calculating Heat Changes

• Hess’s Law of heat summation: If two or more equations are added to give one final equation, then the heats of each reaction can also be added.
• Ex (SSN): Diamond must turn into graphite to react with oxygen and produce CO₂
• C(s, diamond) + O₂(g) → C(s, graphite) + O₂(g)
• C(s, graphite) + O₂(g) → CO₂(g)
• So, C(s, diamond) + O₂(g) → CO₂(g)