Thermochemistry

The study of the changes in heat energy that accompany

chemical reactions and physical changes

Thermochemistry

• Heat• a form of energy.• can be transferred between samples• heat flows from matter at a higher

temperature to matter at a lower temperature

• Temperature• a measure of the average kinetic energy of

the particles in a sample.

Thermochemistry

• Units of Heat• Joule (SI unit) • calorie• cal• the amount of energy required to raise the

temperature of one gram of water one degree Celsius.

• Calorie• Cal • a dietary calorie.• kilocalorie, kcal (1,000 calories)

Thermochemistry

• Enthalpy

• the heat content of a system• represented by H• only changes in enthalpy can be

measured• ∴ ΔH is used

Thermochemistry

• Specific heat capacity, cp

• the amount of energy required to raise the temperature of one gram of a substance one degree Celsius

• used in equationq = m x cp x ΔT

Thermochemistryq = m x cp x ΔT

• note that in the Metric System, Joules are the unit of measure for heat.

Energy, or heat

(J)

mass(g)

Δ temp(°C)

specific heat(J/g•°C)

Heat capacity, cp

q = m x cp x ΔT1. A 45.0-gram sample of iron is heated from

25.0°C to 50.0°C. How much energy is required? (cp iron = 0.449 J/g°C)

q = ?

m = 45.0 gcp = 0.449 J/g°C

ΔT = 50.0°C – 25.0°C = 25.0°C

q = m cp ΔT

q = 45.0g (0.449 J/g°C) (25.0°C) = 505.125q = 505 J

2. What is the specific heat capacity of an object if a 12.5-gram sample is heated from 12.0°C to 28.0°C using 100.0 joules?

q = 100.0 Jm = 12.5 gcp = ?

ΔT = 28.0 – 12.0 = 16.0°Cq = m cp ΔT

TΔmq

=cp )C0.16(g5.12J0.100

=cp

CgJ

500.0=cp