2 When an acid reacts with an alkali the neutralisation reaction is always exothermic. H⁺(aq) + OH⁻(aq) → H₂O(l) You will determine the enthalpy change of neutralisation, ΔH, for a monoprotic acid X. FA 3 is aqueous sodium hydroxide, NaOH. FA 4 is a 2.00 mol dm⁻³ solution of monoprotic acid X.

1. (a)Method • Support the plastic cup in the 250 cm³ beaker. • Fill the second burette with FA 3. • Use the measuring cylinder to transfer 25.0 cm³ of FA 4 into the plastic cup. • Measure and record the temperature of FA 4 in the plastic cup. • Add 5.00 cm³ of FA 3 from the burette into the plastic cup. Stir the contents of the cup. Read and record the maximum temperature of the solution. • Add a further 5.00 cm³ of FA 3 from the burette into the cup. Stir the contents of the cup. Read and record the maximum temperature of the solution. • Repeat the addition of FA 3, in 5.00 cm³ portions, until 50.00 cm³ have been added. Read and record the maximum temperature of the solution after each addition.[4]
2. (b)On the grid plot a graph of temperature, (y-axis), against volume of FA 3 added, (x-axis). Your scale should allow a temperature of 2°C above the maximum measured to be plotted. On your graph draw two lines of best fit. One line should be for when the temperature was rising and the other for after the maximum temperature had been reached. You should indicate clearly any points you consider to be anomalous. Extrapolate the lines so that they intersect.[4]
3. (c)From your graph, determine the maximum temperature reached in the experiment and the volume of FA 3 added to produce this maximum temperature. maximum temperature reached = .................................................... °C volume of FA 3 added to reach maximum temperature = .................................................... cm³[1]
4. (d)(i)Calculate the energy released during this experiment. [Assume that 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0°C.] energy released = ........................................................ J[1]
5. (d)(ii)Calculate the number of moles of acid X in 25.0 cm³ of FA 4. moles of X = ........................................................ mol[1]
6. (d)(iii)Calculate, in kJ mol⁻¹, the enthalpy change of neutralisation for acid X. enthalpy change = ........................................................ kJ mol⁻¹ (sign) (value)[1]
7. (e)Without changing the apparatus or solutions used, suggest one way in which the experiment could be modified to make the values obtained in (c) more accurate.[1]
8. (f)The enthalpy change of neutralisation of hydrochloric acid with aqueous sodium hydroxide is more exothermic than the enthalpy change of neutralisation of acid X. Explain what this tells you about acid X.[2]