Energetics

What calorimetry measures

• Calorimetry uses temperature change to measure heat transfer in a chemical reaction
• Two settings appear:
  • Reactions in solution (neutralisation, dissolving, metal displacement) — energy transferred to or from the surrounding solution
  • Combustion of fuels — energy transferred to a known mass of water above a spirit burner

Assumptions used to keep the maths tractable

• The solution has the same density as water (1.00 g cm⁻³), so 100 cm³ of solution has a mass of 100 g
• The solution has the same specific heat capacity as water, 4.18 J g⁻¹ °C⁻¹
• No energy escapes from the cup, and the container itself takes up none of the heat
• The reaction goes to completion

Calorimeter for reactions in solution

• Method:
  • Use a measuring cylinder to add a known volume of one reactant to the cup
  • Note the initial temperature with a thermometer once the reading has steadied
  • Add an excess of the second reactant in one go, stir continuously, and watch the temperature
  • Record the maximum (for exothermic) or minimum (for endothermic) temperature reached
• Why an excess of the second reactant: it ensures the first is the limiting reactant so the full amount of energy is released or absorbed

Calorimeter for combustion of a fuel

• Method:
  • Weigh out a fixed mass of water inside a copper can, and clamp the can above the burner
  • Weigh the spirit burner with the fuel; record the initial mass
  • Read the starting temperature of the water against a thermometer dipping into the can
  • Light the burner and stir the water gently
  • When the water has risen by about 20 °C, blow out the flame and record the final temperature
  • Re-weigh the burner; the loss in mass is the mass of fuel burnt
• Main sources of error:
  • Heat lost to the surrounding air rather than to the water
  • Heat absorbed by the copper can rather than the water
  • Incomplete combustion (visible black soot on the underside of the can confirms this)