IB Chem Unit 7 (Heat)

Heat and Temperature

• Heat is the energy transferred from one body to another due to a temperature difference.

  • The total kinetic energy of random motion of particles in a substance

• What is temperature?

  • Measure of particles’ average kinetic energy

  • Energy of random motion of particles in a substance

Heating Curves

• Heating curves show the energy (heat) needed to overcome intermolecular forces and change the state of matter

  • Shows how much energy is needed to melt ice (s → l) or boil water (l → g).

Why does temperature remain constant during a phase change?

• Because energy is being used to overcome intermolecular forces of attraction

• Forces of vibration > forces of attraction

• Energy is used to break out of crystal lattice 

• After particles are free to move, motion and temperature increase

Change in kinetic energy (∆KE) 

• Happens when there are changes in temperature.

  • Slopes

Changes in potential energy (∆PE)

• Happens when there are changes of state of matter

• Plateau or flat lines

Heating Curve Diagram (not a Flashcard)

Heat Equation

• q = heat absorbed or released (J)

• m = mass of sample (g)

• c = specific heat capacity (^J/_{g °C})

• ΔT = change in temperature (T_f – T_i)

Specific Heat Capacity, c

• The amount of energy required to raise the temperature of one gram of a substance by one degree Celsius (1°C)

• This is different for every substance.

Measuring Energy During a Phase Change

• During a phase change, temperature stays constant!

• We need a NEW equation since we cannot use  ΔT

  q = m·ΔH

• q = heat absorbed or released (J)

• m = mass (g) or mole

• ΔH_fusion or  ΔH_vaporization = enthalpy (^J/_g)

Heat of Fusion

• ΔH_fusion is the heat for a phase change from solid to liquid 

• Heat of fusion is energy added to 1 gram or 1 mole of a solid at its melting point 

• All of the added energy is used to increase the kinetic energy of the molecules to cause the change from solid to liquid

• Temperature remains constant

Heat of Vaporization

• ΔH_vaporization is the heat for a phase change from liquid to gas

• Heat of vaporization is energy added to 1 gram or 1 mole of a liquid at its boiling point 

• All of the added energy is used to increase the kinetic energy of the molecules to cause the change from liquid to gas

• Temperature remains constant

Two and Three Steps Heat Equation

• Deals mostly with water (H₂O)

• KEY points: 

  • Water freezes at 0°C

  • Water boils at 100 °C

Measuring Changes in Heat Energy

• Calorimeter or Bomb Calorimeter

  • Used to measure heat energy as it is transferred to water

• Q = mcΔt

  • Δt of water

  • m = mass of water

  • C_p of water

Assumptions in Calorimeter and Bomb Calorimeter

assumed that their is no heat loss and that all heat is absorbed by water.