Specific Heat
Temperature: If temperature is measures in Kelvin, then its value is directly proportional to the average kinetic energy of the molecules of a substance.
Heat: Actual energy measured in joules or other energy units
- Heat is a flow of energy due to a temperature difference
Thermal Energy: Flow of energy called heat, transferred from a hot object to a colder object.
- Thermal energy refers to the amount of energy in a sample
- It is dependent on both mass and temperature of the sample
- Because heat is a process of energy transfer, it is incorrect to refer to the "heat energy of a sample" the correct term is "thermal energy of a sample"
Heat Flow
- Heat flow is measured in two common units: the calorie and the joule
- The energy in food is expressed in kilocalories (kcal)
- 1 kcal = 1000 calories
Units of Energy
- The calorie or joule is a unit of energy used to measure heat transfer and to express thermal energy
- It takes one calorie of heat to raise the temperature of one gram of water by one degree Celsius
- It takes 4.184 joules to raise the temperature of one gram of water by one degree Celsius. This is known as the specific heat of water
Specific Heat Capacity
- There are two factors that the heat capacity of an object depend on:
- Mass (g)
- Chemical composition- what the material is made of
Specific Heat Capacity Equation
- We use the following equation: q = m x C x ΔT
- q = heat (joules or calories)
- m = mass (grams)
- ΔT = change in temperature (°C)
Phase Change
6 Types of Phase Change
- Melting = Fusion (solid to liquid)
- Freezing = Solidification (liquid to a solid)
- Boiling = Vaporization (liquid to gas)
- Gas to Liquid = Condensation
- Solid to Gas = Sublimation
- Gas to Solid = Deposition
Specific Heat of Water
- Solid (ice): 2.1 J/g°C
- Liquid (water): 4.184 J/g°C
- Gas (steam): 2.0 J/g°C
The heat (joules) added/removed during a phase change will break intermolecular forces if warming or allow intermolecular forces to form if cooling
- The freezing AND melting point of water is 0°C
- The boiling point of water is: 100°C
Heat of Fusion
- Heat (joules) absorbed when 1 gram of solid turns into a liquid (melting)
- For H₂O, this is 334 J/g (6.02 kJ/mol)
- If you have 1 g of ice at 0°C it will absorb 334 joules of energy before it will melt into liquid water at 0°C
- The temperature remains constant
Heat of Vaporization/Condensation
- Heat (joules) absorbed when 1 gram of liquid turns into a gas (vaporization)
- For H₂O, this is 2260 J/g (40.6 kJ/mol)
- If you have 1 g of water at 0°C it will give up 2260 joules of energy before it will solidify into ice at 0°C
- The temperature remains constant
When we do problems that involve a phase change, we use Enthalpy
- q = mΔHfus (Melting or Freezing)
- q = mΔHvap (Vaporization or Condensation)
Calorimetry Conversions
1.0 calorie = 4.184 J 1 kilocalorie = 1000 calories 1kJ = 1000 Joules
