3.1 Flashcards
3.1 Determination of the enthalpy change of neutralisation • To determine the enthalpy of neutralisation of hydrogen chloride and sodium hydroxide by accurate measurement of temperature over time.
You will determine the enthalpy change of neutralisation of hydrogen chloride and sodium hydroxide.
The enthalpy change of neutralisation is defined as the enthalpy change when solutions of an acid and alkali react together to produce one mole of water, for example: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(aq)
You can see that one mole of water is produced when one mole of HCl (or one mole of NaOH) reacts. So in this experiment, you will calculate the molar enthalpy change by dividing the heat energy change by the number of moles of HCl used.
In calorimetry experiments, the temperature change can sometimes be difficult to accurately measure at the moment that neutralisation takes place. For that reason, this method involves extrapolating lines plotted on a graph in order to calculate the temperature change of the reaction.
First, you will take a series of initial measurements to establish the initial temperature. Then, following the neutralisation, you will plot a cooling curve which you will extrapolate to determine the temperature change.
Equipment
- expanded polystyrene cup
- beaker (250 cm3)
- thermometer graduated in divisions of 0.5 °C or less (–10°C – 110°C)
- two measuring cylinders (25 cm3 or 50 cm3)
- timer/stopwatch
Procedure
- Place a polystyrene cup in a 250 cm3 glass beaker.
- Use a measuring cylinder to measure 25.0 cm3 of dilute hydrochloric acid. Transfer the liquid into the polystyrene cup.
- Use another measuring cylinder to measure 25.0 cm3 of sodium hydroxide but do not yet add this to the beaker.
- Start your timer and record the initial temperature of the acid to the nearest 0.5 °C.
- Record the temperature of the water at 1, 2 and 3 minutes. Stir the acid immediately before reading the thermometer.
- At the 4th minute, do not record the temperature. Instead, add the sodium hydroxide and stir.
- Stir the solution and then record the temperature to the nearest 0.5 °C at 5, 6, 7, 8, 9 and 10 minutes.
Analysis of results
- Plot a graph of time (x-axis) against temperature (y-axis). You do not need to start the y-axis at 0 °C, but it should extend to a maximum temperature that is a few degrees higher than your highest reading. Remember that there is no temperature reading at 4 minutes.
- Draw a line of best fit through your points at 0, 1, 2 and 3 minutes. Extrapolate this line to 4 minutes.
- Draw a line of best fit (straight or curved as appropriate for your data) through your points from 5 minutes onwards. Extrapolate this line back to 4 minutes.
- From your graph, calculate the temperature rise of the solution at 4 minutes.
- Calculate the energy absorbed by the solution during this experiment, using the equation q = mcT
Note: if we assume that the density of the solution is 1.00 g cm3, the mass of the solution in g is the same as its total volume in cm3.
Assume that the specific heat capacity of the solution is 4.18 J g1 °K1. - Calculate the amount (in mol) of HCl that you used.
- Calculate the enthalpy change of neutralisation in kJ mol1.
- Explain why your experimental value for the enthalpy of neutralisation must be negative.
- The reaction is exothermic, evidenced by the rise in temperature due to release of energy. The enthalpy change for an exothermic reaction is negative.
- The literature value for the enthalpy of neutralisation for this reaction is 57 kJ mol1. Compare your answer with this and suggest one or more reasons for the difference.
- Learners should recognise that the temperature difference that they measured is too small – a larger difference would have resulted in a greater enthalpy change, closer to the literature value.
The major source of error in this experiment is heat loss to the surroundings. Learners should be able to suggest some improvements to minimise this – for example adding a lid to the polystyrene cup.
If there is time, it is instructive to allow students to attempt to improve their procedure. In addition to improving the insulation, students may wish to e.g. increase the concentrations of the acid and alkali to see if this makes a difference. Can students achieve a smaller error? If not, can they explain this?
- Why is it safe to dispose of the product solution down the sink?
- As per the equation given in the Background to the method, the product of the neutralisation is a sodium chloride solution. The reacting amounts of substance are equal.
- The enthalpy change of neutralisation should be identical regardless of which strong acid and alkali are used. Suggest why.
- The ionic equation for the reaction is: H+(aq) + OH(aq) H2O(l)
This shows that only the hydrogen and hydroxide ions are involved – the nature of the acid and alkali is irrelevant
- The analysis of the data required you to extrapolate the cooling curve after the reaction took place in order to calculate the temperature change at the point of mixing the reactants. An alternative method involves simply measuring the temperature before mixing and the maximum temperature after mixing and calculating the difference. Decide whether the method using graphical extrapolation is more important or less important for reactions that are highly exothermic.
- The extrapolation method is more important when the reaction is highly exothermic because more heat energy is lost at the point of reaction, than compared with a less exothermic reaction. This would mean that simply measuring the temperature difference would significantly underestimate the temperature change and thus the value of ΔH
Hazard informationHCl(aq)
No hazard classification.
Learners should wear eye protection when using this solution.
Hazard informationNaOH(aq)
DANGER
Causes severe skin burns & eye damage
