RP2: Measuring an Enthalpy Change

Solution calorimetry estimates reaction energy from the temperature change of surrounding solution. An insulated polystyrene cup with a lid reduces transfer to the room. Support the cup in a beaker, measure reactant quantities, and use the actual combined solution mass or an explicitly stated density approximation. The reacting amount is not automatically the number of moles in both solutions added together.

For neutralisation, identify the moles of water formed from the limiting acid or alkali. Record initial temperatures and allow reagents to equilibrate where the method requires a common initial temperature. Stir consistently to reduce temperature gradients without splashing. Use the approved concentrations and take care with corrosive solutions and warming mixtures.

Take temperature readings at regular intervals before mixing and after the change. Note the mixing time. A temperature–time plot can extrapolate the post-reaction cooling trend back to mixing time to estimate a corrected rise. This is an approximation: it assumes the chosen trend represents heat loss during the reaction. Merely using the highest observed reading usually misses some heat lost before the probe responds.

Keep total mass, starting temperature, insulation and mixing procedure comparable when investigating a concentration or reactant change. A lid limits evaporation and heat exchange. A higher-resolution probe helps when the rise is small, but does not itself measure the heat absorbed by the cup.

Calculate q(solution) = mcΔT using consistent units, then q(reaction) ≈ −q(solution). Divide by the appropriate reacted amount and convert J to kJ. A positive temperature rise means the solution gains energy and the reaction enthalpy is negative. If the apparatus heat capacity is supplied, include its contribution rather than treating it as zero.

Heat lost to the room during an exothermic reaction reduces the measured rise, making the calculated enthalpy less negative. The assumption that every solution has water’s density and specific heat capacity is another source of error. Repeat independent runs to assess scatter, and propose an improvement addressing a named limitation.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.