Nuffield Advanced Chemistry - I have been looking at the order and activation energy of the reactions between hydrochloric acid and magnesium, and sulphuric acid and magnesium. I have found the order of the reaction to be 1 with respect to hydrochloric and 2 with respect to sulphuric. Regardless of whether this is right or not, we are supposed to suggest a reason for our conclusions. I have thought of a mechanism for the reaction but have no idea why the mechanism would differ between the two acids! What would the difference be? Also I got almost double the activation energy for sulphuric acid than hydrochloric. I dont think this correct, but could a possible explanation be that energy is required to remove the second hydrogen ion from the sulphuric acid molecule before the reaction can occur? Please help me, I'm so confused!!!

Practical investigations

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I have been looking at the order and activation energy of the reactions between hydrochloric acid and magnesium, and sulphuric acid and magnesium. I have found the order of the reaction to be 1 with respect to hydrochloric and 2 with respect to sulphuric. Regardless of whether this is right or not, we are supposed to suggest a reason for our conclusions. I have thought of a mechanism for the reaction but have no idea why the mechanism would differ between the two acids! What would the difference be?

Also I got almost double the activation energy for sulphuric acid than hydrochloric. I dont think this correct, but could a possible explanation be that energy is required to remove the second hydrogen ion from the sulphuric acid molecule before the reaction can occur? Please help me, I'm so confused!!!

You do not sound confused, judging by your question.

You will find a discussion of the mechanism in the tutorial on metal-acid reactions on this web site (see 'Practical Investigations') - see also the article you can download from this page. Evaluate your findings against the results reported in that paper.

As for activation energies, you can compare your results with those reported on this web site (see 'What answer did you get?' in the tutorial section of Practical Investigations). Discuss your results in comparison with the range of values reported by others. You can comment on how reliable your values are likely to be juding by the range of values reported by others working in similar circumstances.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

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updated: 16 February 2005

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