Investigation to find out the factors affecting heat of neutralisation, and then choosing one variable, ascertain its effect on heat of neutralisation.

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AIM: to find out the factors affecting heat of neutralisation, and then choosing one variable, ascertain its effect on heat of neutralisation.


A neutralisation reaction is a reaction between an acid and a base in which the acidity or alkalinity of a substance is destroyed, ie, either removing H+ ions by reaction with a carbonate or metal base, or removing OH- ions by reaction with an acid. To get a perfectly neutral solution of pH7, the H+ and OH- ions must be in exactly equal amounts.

The ionic equation for neutralisation is:

 H+    + OH-                       H2O

Heat of neutralisation or enthalpy change of neutralisation is the amount of heat energy given out when one mole of hydrogen ions is neutralised by one mole of hydroxide ions.


  • Concentration of acid
  • Concentration of alkali
  • strength (pH) of acid
  • strength (pH) of alkali
  • volume of acid
  • volume of alkali
  • basicity of acid (the no of H+ ions that can be replaced to form a salt)


I know that the concentration of the reagents affects the heat of neutralisation, however this is not as advanced an experiment, and would not give me much to investigate/analyse and so I shall not choose this variable.

The basicity of the acid affects the heat of neutralisation too, however I shall not investigate this variable because I am told that sometimes the results for this can be unreliable. Also, as there are not many acids available to me, I may not be able to get a wide enough range of measurements to make my investigation worthwhile.

The volume of the reagents also effects the heat of neutralisation, however I shall not choose this variable as again the investigation would not be so advanced, or give me as much to investigate/ analyse as my chosen variable, nor do I find it as interesting.

My chosen investigation is that into how the strength (pH) of the reagents affects heat of neutralisation.

From past experiments I know that neutralisation is an exothermic reaction, i.e. the energy released in bond formation is greater than the energy taken in and used up in breaking bonds. Because of this the DH will be negative. Therefore I predict that when reacting an acid with an alkali in a neutralisation reaction, energy will be given to the surroundings as heat and so there will be a rise in temperature. From book research I know that if the acid and alkali are fully ionised (there H+ and OH- ions are fully dissociated), the heat of neutralisation is always – 57 KJ. Therefore this should be the heat of neutralisation attained from my experiments with fully ionised reagents, however it may well be slightly less as some heat is bound to be lost to the surroundings. When a weak acid or base is involved however, I predict that the heat produced will be less. This is because some energy must be supplied to ionise the acid or alkali fully first (before the ions can react and neutralisation can occur), and so the overall heat given out over the heat taken in will be reduced.

  • From this I predict that the highest DH will be reached with the two fully ionised reagents (-57KJ)
  • The next highest will be reached by a fully ionised reagent with a partially ionised reagent.
  • The lowest DH will be reached by two weak (only partially ionised) reagents.

Of the two weak acids (ethanoic and propanoic ) I predict that the propanoic will create a lower heat of neutralisation, as it has a longer chain length and is less soluble in water, and from research I believe that the longer the chain length, the less ionised the acid.

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  • Make sure that all other input variables are kept constant, ie:
  1. I shall use the same concentration for all the reagents- 2M solutions of everything.
  2. I shall use equal  volumes of acid and alkali – 30cm3
  3. I shall use only monoprotic/ monobasic acids.
  • Make sure that the temperature has reached its maximum before I record it- I shall wait for it to stay constant for some while or start to drop again, and then I shall see the true maximum temperature.
  • I shall stir ...

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