THERMOCHEMISTRY
A. Energy and Heat
1. Thermochemistry - heat changes that occur during chemical reactions
2. Energy - capacity to work/supply heat
a. Detected only by effects
b. Chemical Potential Energy - stored energy
1. Determined by type & arrangement of atoms
3. Heat (q) - energy transfer between objects due to temperature differences
a. Flows from hot objects to cool objects
b. Stops when temperatures are the same
B. Exothermic and Endothermic Processes
1. System - part of universe focusing on
2. Surroundings - everything else
3. LAW OF CONSERVATION OF ENERGY
a. Energy is neither created or destroyed, just changed
4. Endothermic reaction - system absorbs energy from surroundings
5. Exothermic reaction - system releases energy to surroundings
1. calorie (cal) - unit of heat and energy
a. Def. - quantity of heat needed to raise the temperature of 1 gram of water 1 degree C
b. 1,00 calories = 1 Calorie (used for dietary measuring)
2. Joule (J) - SI unit of heat and energy
a. 1 cal = 4.18 J
3. Heat capacity info
a. Depends on mass & chemical composition
b. Increase mass, increase heat capacity
c. Effects temperature of object
4. Specific Heat Capacity (Specific Heat)
a. Def. - amount of heat needed to raise 1 gram of a substance 1 degree C
b. Heat effects objects with low specific heat the most
d. Calculations C = q/(m delta T)
1. C = specific heat
2. m = mass
3. delta T = change in temperature (Tf - Ti)
a. Tf = final temperature
b. Ti = initial temperature
II. Measuring and Expressing Heat Changes
A. Calorimetry
1. Accurate & Precise measurement of heat change
2. Calorimeter - device used to measure amount of heat absorbed or lost
3. Enthalpy (H) - amount of heat of a substance at a given temperature and pressure
a. Constant pressure, heat change (delta H) = q
q = delta H = mCdeltaT
1. Exothermic = negative delta H
2. Endothermic = positive delta H
B. Thermochemical Equations
1. Equations that include heat change
CaO (s) + H2O (l) --> Ca(OH)2 (s) + 65.2 kJ
a. State of matter is very important
b. Changes amount of delta H
2. Heat of Reaction - heat change for the equation as written
a. Assumed at 101.3 kPa and 25 degrees C
3. Problems work like stoichiometry problems
Ex. 2 moles of CaO produces how much heat when reacted with water?
Delta H = 2 mol X -65.2 kJ = -130.4 kJ (exothermic)
4. Heat of combustion - heat of reaction for completely burning one mole of a substance
a. Calculate & reported like other heat of reactions
III. Heat in Changes of State
A. Heats of Fusion and Solidification
1. All phase changes involve absorbing or releasing heat at constant temperature
q = delta H = mdeltaH(fus) [NO delta T)
2. Molar heat of fusion (deltaH{fus}) - heat absorbed by one mole of a substance melting at constant temperature
3. Molar heat of solidification (deltaH{solid}) - heat released by one mole of a substance freezing at constant temperature
a. delta H(fus) = - delta H(solid)
B. Heat of Vaporization and Condensation
1. Molar heat of vaporization (delta H {vap}) - heat absorbed by one mole of a substance evaporating at constant temperature
2. Molar heat of condensation (delta H{cond}) - heat released by one mole of a substance condensing at constant temperature
a. delta H(vap) = - delta H(cond)
C. Heat of Solution
1. Molar heat of solution (delta H{soln}) - heat change caused by dissolution of on mole of a substance
a. Heat absorbed or released when dissolving solutes in solvents
D. Important note: Molar heats are always in units of J/mol or kJ/mol, if the word Molar is removed (heat of ...) then the units are J/g or kJ/g
IV. Calculating Heat Changes
A. Hess's Law
1. Hess's Law of heat summation - If you add two or more thermochemical equations to make a new equation, then you can add the delta H values from each equation to get the new delta H value
Ex.
B. Standard Heats of Formation (delta H(f)o)
1. change in enthalpy (delta H) from the formation of one mole of a compound from its elements at 25 degrees C
2. Standard heat of reaction (delta Ho) can be calculated from the delta H(f)o of the reactants and products delta Ho = delta H(f)o (products) - delta H(f)o (reactants)
Outline based upon:
Matta, M. S., Staley, D. D., Waterman, E. L., & Wilbraham, A. C. (2000). Chemistry, Addison-Wesley (5th ed.). Menlo Park, CA: Prentice Hall. pp. 293-320.
