8.3 -Measuring Enthalpy Changes

• experiment to measure enthalpy change of combustion

  use calorimeter & spirit burner containing fuel

  calculate mass of water in calorimeter & measure starting temperature

  weigh spirit burner then place it under calorimeter light it

  allow it to heat water by a certain number of degrees

  weigh spirit burner and measure final temperature of water

  calculate temperature change
• sources of error in the experiment

  energy also heats surrounding air

  sometimes fuel burns incompletely producing soot

  flame is affected by draughts
• improvements that could be made to the experiment

  use a bomb colorimeter

  it ensures sample burns completely and that energy losses are avoided by having excess oxygen & high pressure
• equation to calculate enthalpy change

  energy transferred (J) = mass (g) × specific heat capacity × temperature change
• What's specific heat capacity?

  energy needed to raise the temperature of 1 gram of a substance by 1 K
• specific heat capacity of water

  4.18 Jg-1 K-1
• what's meths?

  ethanol made undrinkable by adding methanol to it which is toxic
• What's used to measure enthalpy changes in solution?

  polystyrene cup because polystyrene is a good insulator & has a negligible specific heat capacity
• assumptions mode when measuring enthalpy changes in solution

  solution has the same specific heat capacity & density as water
• what thermometer must be used when measuring enthalpy changes in solution?

  thermometer graduated in tenths of a degree because temperature changes are often quite small
• experiment to measure enthalpy changes in solution

  add set volume of solution of known concentration to polystyrene cup

  put lid on cup and measure starting temperature

  add known mass of a substance to solution so it can react

  measure temperature of reaction mixture every 30 seconds until it has reached a maximum or a minimum

  calculate temperature change of reaction