1. Ch. 11 Thermochemistry: Heat and Chemical Change
Ch The Flow of Energy – Heat
Ch Measuring and Expressing Heat
Changes
Ch Heat in Changes of State
Ch Calculating Heat Changes

2. Ch. 11.1 The Flow of Energy – Heat
Energy transformations
Thermochemistry is concerned with the heat changes that occur during chemical reactions
Energy is the capacity to do work or supply heat
Energy can be kinetic or potential
Heat (q) is the energy that transfers from one object to another because of a temperature difference between the objects
Heat alone cannot be measured
Only changes caused by heat can be detected
Heat always flows from warmer to cooler

3. Ch. 11.1 The Flow of Energy – Heat
Exothermic and endothermic processes
All reactions involve the release or absorption of heat
A defined area or system is necessary to study heat
The surroundings include the rest of the universe
The law of conservation of energy is observed
All energy in a system must be accounted for

4. Ch. 11.1 The Flow of Energy – Heat
Exothermic and endothermic processes
Reactions that absorb heat from the surroundings are endothermic
The system gains heat as the surroundings cool down
rH is positive
Reactions that lose heat to the surroundings are exothermic
The system loses heat as the surroundings warm up
rH is negative

5. Ch. 11.1 The Flow of Energy – Heat
Heat capacity and specific heat
A calorie is the amount of heat needed to raise the temperature of 1g of H2O 10C
A calorie is equal to J
The amount of heat need to increase the temperature of an object 10C is the heat capacity of that object
The greater the mass of the object, the greater the heat capacity
The specific heat capacity is the amount of heat it takes to raise the temp of 1g of substance 10C

6. Ch. 11.1 The Flow of Energy – Heat
Heat capacity and specific heat
Calculating specific heat capacity
C = q / mass x rT
Specific heat is C, heat is q, mass is m, and change in temperature is rT

7. Ch. 11.2 Measuring and Expressing Heat Changes
Calorimetry
The accurate and precise measurements of heat change for chemical/physical processes
For systems at constant pressure, heat content is the enthalpy (H)
The terms heat and enthalpy can be used interchangeably
q = rH = m x C x rT (rT = Tf – Ti)
To measure the heat change for a rxn in aqueous soln in a foam cup calorimeter, you dissolve the reacting chemicals (the system) in known volumes of water (the surroundings)
Measure initial and final temperatures
Use equation to calculate

8. Ch. 11.2 Measuring and Expressing Heat Changes
Thermochemical equations
An equation that includes the change in heat
A heat of reaction is the heat change
Heats of reaction are reported as rH, which can be positive or negative
Problems involving enthalpy are similar to stoichiometry problems
The amount of heat released/absorbed depends on the number of moles of reactants involved
All chemicals have a heat of combustion for the complete burning of one mole of the substance

9. Ch. 11.3 Heat in Changes of State
Heats of fusion and solidification
All solids absorb heat as they melt to become liquids, or lose heat as they solidify from a liquid to a solid
The heat absorbed by one mole of substance when it melts from a solid to a liquid at a constant temperature is the molar heat of fusion (rHfus)
The heat lost when one mole of substance solidifies at a constant temperature is the molar heat of solidification (rHsolid)
The quantity of heat absorbed by a melting solid is equal to the quantity of heat lost when a liquid solidifies (rHfus = -rHsolid)

10. Ch. 11.3 Heat in Changes of State
Heats of vaporization and condensation
The amount of heat necessary to vaporize one mole of a liquid substance is called the molar heat of vaporization (rHvap)
The amount heat released when 1 mole of vapor condenses is called its molar heat of condensation (rHcond)
Energy is conserved, rHvapor = -rHcond

11. Ch. 11.3 Heat in Changes of State
Heat of solution
The heat of change caused by the dissolution of of 1 mole of substance is called the molar heat of solution (rHsoln)
Some are heat of solution changes are exothermic, some are endothermic

12. Ch. 11.4 Calculating Heat Changes
Hess’s Law
Most reactions take place in a series of steps
Direct measurements of heats of reaction for a chemical reaction cannot always be made
Hess’s law makes it possible to measure a heat of reaction indirectly
If you add two or more thermochemical equations to get a final equation, then you can also add the heats of reaction to give the final heat of reaction
Known as Hess’s law of heat summation

13. Ch. 11.4 Calculating Heat Changes
Standard heats of formation
Sometimes it is difficult to measure the heat change for a reaction
Instead, you can calculate the heat of reaction (rH0) from the standard heats of formation
The standard heat of formation (rHf0) of a compound is the change in enthalpy that accompanies the formation of 1 mole of compound from its elements
In standard states at 250C
rH0 = rHf0 (products) - rHf0 (reactants)