Quick questions on Calorimetry and q = mcΔT: VCE Chemistry Unit 3

Solution calorimetry. A reaction (often combustion or neutralisation) takes place in or under a known mass of water. The heat released raises the water's temperature. The heat absorbed by the water is calculated with q = mcΔT.

For 250 g of water raised by 5.0°C: q = 250 × 4.18 × 5.0 = 5225 J = 5.23 kJ

The molar enthalpy of combustion is the energy released per mole of fuel burned, reported as a negative ΔH.

A bomb calorimeter is calibrated electrically. A heater of known voltage V and current I runs for time t, supplying:

Solution calorimetry usually underestimates the true ΔH because:

A reaction (often combustion or neutralisation) takes place in or under a known mass of water. The heat released raises the water's temperature. The heat absorbed by the water is calculated with q = mcΔT.

A sample is burned in pure oxygen inside a sealed steel "bomb" surrounded by water. Because heat goes into multiple components (the steel bomb, the water jacket, the stirrer, the thermometer), the whole apparatus is calibrated with a known electrical input first. The calibration factor (CF, in J °C^-1) converts the measured ΔT directly into energy released.

Combustion is exothermic. ΔH must be negative even though the magnitude is reported as a positive q.

m is the mass of water (the thing being heated), not the mass of fuel.

Specific heat capacity is in J g^-1 K^-1, so m must be in grams. If you use kg, divide c by 1000 first, or you will be out by 1000.

For pure water near room temperature this is fine (1 mL ≈ 1 g). For salt solutions or other solvents, density matters.

q is the heat for that specific experiment (a number of joules). ΔH is the heat per mole of reaction. Always divide by n.