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.
