Chapter 16: Thermochemistry
I Thermochemistry
A. “Thermo” refers to heat
B. Thermochemistry: the study of the transfers of energy as heat in chemical reactions and physical
changes.
Energy is either gained or lost
II Temperature and heat are related, but not identical
A. Heat
1. Heat is a form of energy
2. Heat released or absorbed is measured with a calorimeter
3. The SI unit of heat and other forms of energy is the Joule
4. Heat is the energy transferred between samples of matter because of a difference in their
temperatures.
5. Heat is always transferred from the warmer object to the cooler object.
B. Temperature
1. Temperature is a measure of the average kinetic energy of the particles in a sample of matter.
2. For calculations in thermochemistry, we use the Celsius and Kelvin scales.
K = 273 + oC
C. Specific Heat
1. Substances have a unique amount of energy that they can absorb or release per gram. This is called
the specific heat.
2. Specific heat is the amount of energy required to raise the temperature of one gram of a substance
by one Celsius degree.
3. The unit of specific heat is J/(g ∙ oC)
4. Solving specific heat calculations
Cp = q / (m ∙ ∆ T)
Where Cp = specific heat
q = the energy lost or gained
m = mass in grams
∆T = change in temperature
To find the heat lost or gained, use:
q = Cp ∙ m ∙ ∆ T
Example: Determine the specific heat of a material if a 35 gram sample absorbed 96 J
as it was heated from 293 K to 313 K.
5. Different substances have different specific heat values
a. Metals usually have low values. They transfer or absorb heat easily.
b. Water has a high value. This is why water takes so much energy to heat or cool and is a
stabilizing factor in our climate.
III Heat energy is transferred during a reaction
A. Enthalpy
1. The energy absorbed as heat during a chemical reaction at constant pressure is the enthalpy
change ( ∆ H)
2. The enthalpy of reaction is the quantity of energy transferred as heat during a chemical reaction.
3. This is the “stored” energy in a chemical bond “released” in a reaction.
4. This is referred to as the “heat of reaction”
a. Exothermic reactions release heat. Heat is a product.
b. Endothermic reactions absorb heat. Heat is a reactant.
c. In thermochemical equations, the coefficients represent the number of moles.
5. The products of exothermic reactions tend to be more stable than products of endothermic
reactions
6. The enthalpy of formation is the amount of energy required to form one mole of a compound.
7. Calculating the change in enthalpy.
∆ H = Hproducts - H reactants
8. The physical state of the products and reactants is an important factor and must be included
9. The change in enthalpy is calculated using Hess’s Law, which states the overall enthalpy change in a
reaction is equal to the sum of enthalpy changes for the individual steps in the process.
10. Examples:
Find the ∆ H for the reaction and determine if it is endo or exothermic.
H2O(g)
H2O(l) + 44.02J
Calculate ∆ H
CH4(g) + 2O2(g)
CO2(g) + 2H2O(l)
Draw a diagram of an endothermic and an exothermic reaction:
IV. The driving forces in reactions
A. Two factors allow chemists to predict whether a reaction will occur spontaneously
1. The change in energy of a reaction system
2. The randomness of the particles in a system
B. Reactions usually move to a lower –energy state
1. The majority of reactions in nature are exothermic
2. Endothermic reactions occur where the products are less stable than the reactants. How can this
be? There must be another factor besides enthalpy. It is entropy.
C. Entropy
1. Entropy , S, is a measure of the randomness in a system.
2. The entropy of a pure crystalline solid at absolute zero is zero.
3. An increase in entropy or randomness is a positive ∆ S
4. A decrease in entropy (becoming more organized) is a negative ∆ S
5. Dissolving increases entropy
6. States of matter and entropy
a. Solids: Have a greater organization and therefore lower degree of entropy
b. Liquids: Have less organization and slightly higher entropy
c. Gases: Have a high degree or disorganization and randomness and a high value of entropy
7. Calculating ∆ S
The sum of the products- the sum of reactants is calculated using values from tables.
∆ S = Sproducts - Sreactants
Calculate ∆ S for the following:
V. Gibbs free energy
A. Processes in nature are driven in two directions
1. toward least enthalpy
2. toward greatest entropy
B. When both entropy and enthalpy are changing, free energy can be used to predict how a reaction will
respond.
C Free energy Changes determine if a reaction is endothermic or exothermic
D. The unit of G is kJ/K
E. Calculating free energy (Go)
∆G = ∆ H- T∆ S
Summary
∆H
- = exothermic
- = exothermic
+ = endothermic
∆S
+ = more random
= less random
+ = more random
∆G
Always negative
Negative at lower temps
Negative at higher temps