R1.1.4 – Measuring enthalpy changes
📌 How enthalpy is measured
- Standard enthalpy (ΔH°) is measured at a pressure of 100 kPa, all reactants in their standard state and all solutions at a concentration of 1 mol dm-3
- Standard enthalpy is used to calculate total enthalpy of any reactions. A negative sign implies that ‘x’ kJ of energy is released during a reaction while a positive sign implies ‘x‘ kJ of energy is absorbed during a reaction.
📌 Specific heat capacity
- Specific heat capacity is the amount of energy required to raise 1 unit mass of a substance by exactly 1K. It is denoted by the letter ‘c‘
- Specific heat capacity can be used to calculate the total energy used/produced by a reaction
Q(energy) = m (mass) x c (specific heat capacity) x ΔT (change in temperature)
📌 Enthalpy of combustion
- The enthalpy of combustion Δ𝐻c can be calculated using the increase in temperature of a solvent (usually water)
- This process requires a copper calorimeter, a solvent, a thermometer and the substance being tested
EXAMPLE : Calculate the enthalpy of combustion of ethanol using the following data :
| Mass of water (g) | 350.00 |
| Temperature of water increase (K) | 6 |
| Mass of ethanol (g) | 0.25 |
- Calculate moles of ethanol : 0.25/46.07 = 0.005
- Calculate Q (knowing the specific heat capacity of water is 4.18 J g-1K-1)
- Q = 350 x 4.18 x 6 = 8778
- Use the formula
Δ𝐻c = – Q/n (where n = number of moles)
5. Δ𝐻c = -8778/0.005 = 1755600J = 1756kJ