Practical investigations
Read our general notes on Risk Assessment
Investigating the rate of the reaction of metals with acids
Judging by the number of questions we receive on this site, this investigation is very popular.
We have found from your questions that simple practical methods produce results which are not always reproducible and the interpretation of these results raises questions which are not easy to answer.
All the advice here is a response to the many questions we have been asked about this type of investigation over the last few years.
All references to the Williams & Hacker article, and to this what you find on this website, must be acknowledged in the write-up of your investigation.
See also Metal-acid reactions: a second tutorial
Look through our substantial collection of FAQs too:
2. Sorted FAQS on Metal + Acid investigations
Sorted FAQs on Rates of Reaction.
STUDYING THE LITERATURE
When planning your investigations you should start by finding out as much as you can about the reaction. Start with AS textbooks for the basic theory of acids and the reactions of metals with acids.
We have tracked down the article in Education in Chemistry by Iolo Wyn Williams and Roger Hacker 7, 1, pp20 – 24 , 1970. This is the only published paper we know of that relates directly to this type of investigation.
You can download from this page a reformatted copy of the article here (see under Download in the right-hand margin). The paper includes results from investigations into the rates of the reactions of strong and weak acids with magnesium and zinc.
The paper explores the orders of the various reactions and also discusses the possible mechanisms that might account for the observed reaction orders.
When you do your ‘literature review’ do remember to keep a note of all the sources you consult. You should reference them in your final report.
THE REACTION
Most investigations seem to begin with the equations for the reactions. Thinking in GCSE terms gives
Mg(s) + 2HCl(aq) —> MgCl2(aq) + H2(g)
and Mg(s) + H2SO4(aq) —> MgSO4(aq) + H2(g)
together with the equivalents for other acids.
A little more thought, however, suggests that the reaction is more probably between magnesium metal and hydrogen ions, H+(aq):
Mg(s) + 2H+(aq) —> Mg2+(aq) + H2(g)
Even this is misleading because the hydrogen ion concentration in, say, 0.01M sulphuric acid is almost the same as it is in 0.01M HCl since the ion HSO4– is quite a weak acid. If ethanoic acid is used, the hydrogen ion concentration is, of course, very much lower.
It would complicate matters enormously if magnesium reacts with molecules of ethanoic acid or directly with the ions HSO4–. I have no evidence about this, one way or the other. It may be relevant to point out that the reaction is not actually one characteristic of an acid at all, it is a redox reaction in which the hydrogen (in oxidation state +1) is reduced to the element (oxidation state 0). How much does it matter where the hydrogen comes from?
POSSIBLE INVESTIGATIONS
Investigating orders of reaction
This is easily the most popular and several students have obtained results suggesting that if hydrochloric acid is used, the order of the reaction is 2 with respect to hydrochloric acid. There is some suggestion that the order of reaction changes with concentration of the acid but the evidence for this is unclear.
Measuring activation energies
This is also popular and you are asked to contribute your results. There is fair consistency and a mean of the first twelve such results submitted to this web site in 2003 was 23.2 kJ mol–1, One student reported a difference of 8 kJ mol–1 between hydrochloric acid and sulphuric acid but another student reported that the difference was only 1.5 kJ mol–1. Both of these differences lie within the range of values which have been used to calculate the mean value given.
In this type of investigation you use the Arrhenius Equation to interpret results of measuring the rate over a range of different temperatures, all other variables being kept constant (including concentration). Provided that you arrange the conditions for each experiment carefully, you should be able to ensure an approximately constant concentration whilst it takes place.
FACTORS TO INVESTIGATE
Different acids - strong/weak
To make sense of the results here you have to have some idea as to whether the reaction is just between the metal and hydrogen ions or whether the metal can react direct with the unionised acid. Since it is hard to know, this makes the results hard to interpret.
Different acids – mono/di/tri protic
We have had many questions about this. A monoprotic acid is one where one molecule of the acid is able to donate one proton to a base such as water. Hydrochloric acid, HCl, is monoprotic but sulphuric acid, H2SO4, is diprotic because both of its hydrogen atoms can be donated to water as protons. Ethanoic acid is monoprotic because only the hydrogen atom of the carboxylic acid group can be donated as a proton.
Sulphuric acid itself is a strong acid. In water it is almost completely ionised to hydrogensulphate ions. However, the HSO4- ion is a weak acid and is hardly ionised at all in water. If you look up Ka values you will see that much the same is so for other di- and tri-protic acids.
(Note that there is an older terminology which is still used based on the amount of base needed to neutralise an acid. HCl is monobasic because it takes one mole of a base such a NaOH to neutralise one mole of acid. Sulphuric acid is dibasic.)
Acid concentration
Note that all this guidance refers to dilute, aqueous solutions of acids. What happens, for example, to the rate of reaction if conc sulphuric acid is used instead of dilute sulphuric acid in the reaction:
H2SO4 + Zn(s) –> ZnSO4 + H2?
Concentrated sulphuric acid is noted for its three modes of behaviour. It is, of course, an acid in the sense that it donates protons to bases such as water. It is also a dehydrating and drying agent. Thirdly, and this is what is important here, it is an oxidising agent.
When dilute sulphuric acid reacts with zinc, it is really the hydrogen ions in the solution which act as the oxidising agent; there can be very few actual molecules of sulphuric acid in a dilute solution. When the acid is concentrated, however, there are relatively few hydrogen ions present and a totally different redox reaction can take place in which the zinc atoms are oxidised to zinc ions and the sulphur in the sulphuric acid is reduced to sulphur dioxide. At intermediate concentrations it is possible for these two kinds of reaction to happen simultaneously and in varying proportions which would lead to some complex kinetics.
Effect of stirring
One student tried to investigate the effect of agitation. Think about what happens at and near the surface of a piece of magnesium when it reacts with acid. The hydrogen ions have to strike the surface of the metal. If you watch the reaction happening you see bubbles of hydrogen sticking to the surface of the metal. Imagine how easy or difficult it is for the hydrogen ions to find their way to the metal surface with these bubbles in place. If you agitate the mixture, the bubbles are dislodged. Another point is that if you don’t agitate, the acid concentration right next to the metal surface decreases whereas elsewhere in the solution it does not change as much. Agitation makes the concentration uniform throughout.
Can you think of a practical way of quantifying agitation?
Surface area
This is an aspect of the investigation which needs to be thought about. This is because of the way pieces of magnesium of different lengths present themselves to the solution, sometimes laying flat on the surface and sometimes curled and on edge.
When timing the ‘dissolving’ of a piece of magnesium you are also assuming that its surface area remains constant whilst it is reacting.
Varying the temperature
If you are trying to find the activation energy you need a method for measuring the rate of this reaction over a range of temperatures.
PRACTICAL METHODS
A popular starting point is to use a fixed length of magnesium ribbon and time how long it takes for all the metal to react and disappear.
The concentration of the acid
Some students clearly have difficulty in producing solutions of known concentration. Your results will only make sense if you can be confident about the concentrations of the acids you use.
It is best to start with a solution of known concentration rather than with the pure acid. Diluting the pure acid can be a hazardous business (especially with sulphuric acid) and it is next to impossible to get an exactly known concentration directly by dilution.
Carrying out a series of dilutions is probably the way to go. Your laboratory will have a supply of sulphuric acid already diluted to 1 mol per cubic decimetre. Pipette 50 cm3 of this into a 100 cm3 volumetric flask and make up to the mark with pure water and you have a 0.5 mol dm-3 solution. Similarly 20 cm3 made up to 100 cm3 gives a 0.2 mol dm-3 solution and so on.
Measuring the rate
A simple approach is to measure the time for a fixed quantity of metal to react, then‘1/time’ can be taken as a measure of the rate if the conditions are right. This is essentially a variant of the ‘initial rate’ method.
Provided that you are using an excess of acid, by the time the magnesium strip has disappeared and you have stopped the watch, the concentration of acid is still the same as it was at the start, so the rate of reaction throughout the dissolving process remains fairly constant. Therefore, since rate is inversely proportional to time, a measure of the rate can be obtained from the reciprocal of time. Note that for this method to be valid, a large excess of acid must be used.
You have to be aware that if, in subsequent experiments, the initial concentration of the acid is reduced, the concentration may then be unlikely to remain constant during the measured time. Do check by calculating the amount in moles of magnesium used, calculate from this the amount in moles of acid used and to ensure that this amount was negligible compared to the amount of acid initially present.
Another approach is to collect and measure the volume of hydrogen produced over measured time intervals. This allows you to plot the volume of product formed against time and determine the rate at selected times by finding the gradient of the graph. Again it is very important to work out whether the acid is in excess or the metal before you can interpret the results.
Have a look at the details for Experiment 11.2a on pages 245 - 247 in the 4th edition of the Nuffield Chemistry Student Book. This is a reaction of a solid with an acid to produce a gas so there are enough similarities with the reaction of magnesium with an acid to give you a lot of ideas as to how to set up your investigation. Bear in mind that hydrogen is very much less dense than carbon dioxide when picking your method.
Varying the temperature
If you are trying to find the activation energy you need a method for measuring the rate of this reaction over a range of temperatures. You will find useful clues about how to proceed by looking in the 4th edition of the Nuffield Chemistry Students Book.
- See experiment 11.4 on pages 256-257 to see how to use results from rate experiments at different temperatures to determine the activation energy for a reaction.
Quantities of chemicals
The important thing is to decide whether you are going to use an excess of acid or an excess of magnesium. You will usually choose to use an excess of one of them so that its concentration does not change significantly during the reaction. You should include your working along these lines in the planning part of your report.
To work out how much of the chemicals you need for the apparatus you are using, start with the equation for the reaction, for example:
Mg (s) + 2HCl (aq) -> MgCl2 (aq) + H2 (g)
so 1 mol of Mg reacts with 2 mol of HCl to give 1 mol of hydrogen.
Amount of HCl: you need to know its volume and concentration to know the amount in moles you are using. If it is (say) 1.0 mol dm–3, then 20 cm3 will contain 0.02 moles.
To estimate the length of Mg - you need to weigh it (after cleaning it thoroughly). Divide the mass by the molar mass of Mg and you will have the amount in moles. Compare the amount in moles of HCl and of Mg. I would suggest you use 10 times as many moles of the one you want in excess.
If you are collecting the gas remember that 1 mol of any gas has a volume of about 24 000 cm3 under laboratory conditions.
INTERPRETATION
The order of reaction
Bear in mind that a rate equation describes how the concentration of the reactants affects the reaction rate. It can only be found experimentally: it cannot be predicted from the balanced equation or in any other way. So if you double the concentration of a reactant and the rate also doubles, you can say the order of reaction (with respect to that reactant) is 1, because you have found that rate is proportional to concentration to the power 1. If you double the concentration and the rate is four times greater, the order is 2 because rate is proportional to concentration to the power 2. And so on, but you need only concern yourself with orders of 0, 1 and 2.
Generally the order of reaction does not change as the concentration changes. In extreme cases however a big change in concentration can lead to a different mechanism operating and this can give rise to a different rate equation.
The mechanism for the reaction
A major factor in the difficulty of interpreting results is the absence of an accepted mechanism for the reaction.
Our effort at mechanistic theory argues by analogy with heterogeneous catalysis, and goes as follows:
- Hydrogen ions migrate to the metal surface
- Hydrogen ions accept electrons from the metal surface
- Hydrogen atoms combine to give hydrogen molecules
- Metal ions hydrate and diffuse away from the metal surface
We proposed that steps 2 and 3 are surely fast steps and that, given efficient agitation, step 4 should also be fast. This leaves step 1 as the most likely rate-determining step.
If this mechanism is correct it is important to remember that the reaction is between H+(aq) and Mg(s). A strong acid is completely ionised but a weak acid is not. Energy will be needed to break bonds and produce the H+ ions in a weak acid, so I would expect the activation energy to be higher. This necessitates a change to Step 1 of the mechanism.
However the energy required to ionise a weak acid is only about 2kJ mol-1, and it is not clear that the methods used to measure the activation energy of the reaction are sufficiently accurate to notice this.
Activation energies
One way to evaluate your results is to compare them with results reported by other students. You will find a lot of values in another tutorial on this site.
Several students have asked recently if it is possible to calculate activation energies theoretically. In response we pointed out that activation energies can only be measured by experiment. In order to calculate it theoretically you would have to know the mechanism of the reaction in very fine detail, and even then you would have to make many assumptions. In the case of some simple organic reactions, such as the hydrolysis of halogenoalkanes by the Sn2 mechanism, you can make an estimate of the activation energy by assuming it to be rather less than the bond energy of the carbon-halogen bond which is being broken. But this sort of estimate is not possible in the case of the metal-acid reaction.
Always carry out a risk assessment and check with your teacher before starting any practical work.
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Lola
This is more helpful than other sites but still doesn't explain enough - there is no where to find out about orders - for H2SO4 my order seems to be 1 and then 2, and for HCl 1 and then the half lives start to decrese?!? (Why do teachers set c/w over the holidays?)
06 January 2008
R
Thanx Nuffield - Really Helpful Information and Previous Results - Totally understand the rate determining step now -.
15 October 2007
Ben
I found this website really useful in my chemistry project. Im not very good at chemistry but this website really helped me!!!
27 March 2007
O Parks
I found this really helpful with my cw. THanks!
19 March 2007
Miriam Hillyard
Again, this page gives a fantastic overview of the topic - thank you to the authors.
02 January 2007
C
Terrific! Highly informative and the coursework is less daunting and more accessible. Thank you to who spent the time and effort to produce this fantastic page.
03 November 2006
James
Great page really helped with the cwk!
30 October 2006
Paul
Clear and easy to follow. A fantastic resource.
17 September 2006
Caitlin
1M HCl + 0.03 g Mg
Ea = 41.6 kJ/mol
1M H2SO4 + 0.03 g Mg
Ea = 8.3 kJ/mol
03 April 2006
Anon
I found this page very useful, I wish I had known about it earlier.
09 January 2006
Jimmy
Thank you to who ever made this page. I'm sure its helped thousands.
29 November 2005
Melanie
I spent hours on the Internet looking for information to help me with my coursework, and this was the only one that I found to have relevant and useful information. A fantastic resource!
03 November 2005
Kyle
This page has made the coursework much more understandable, thanks to who took the time and effort to produce it.
31 October 2005
Ollie Woolf
This site was very useful to me. It helped me understand what I needed to do. I found this more helpful than my teachers.
12 October 2005
Spence
Thanks so much for the help, reading this page has really clarified things in my mind
09 March 2005
Ana
In my investigation I have found that activation energy of ethanoic acid (1M) is almost double of that of hydrochloric acid (1M).
09 March 2005
Sid
This was quite a useful page. Could you also include details on measuring the order of the reaction wrt acid by titration with sodium carbonate at regular time intervals by quenching?
16 February 2005
David Beaton
At last I know what im going to do my investigation on. This page gave me ideas while still allowing me to choose my own course - a great help
29 January 2005
Julian Brett
I agree with Nicolette; this page is very useful in terms of how to go about your investigation. The varying factors were very useful and have given me some good ideas.
31 October 2004
Rob Davies
This page is very helpful for clarifying the Nuffield Chemistry take on the acid-metal reaction mechanisms. I still feel there are several questions unanswered, but that is the beauty of chemistry.
26 October 2004
Daniel Mckemey
This page is the best document that I have found on the whole internet relating to rates of reaction. It is helping me with my coursework no end! Maybe some links to other sites would aid my studies! Thanks nuffield!
28 June 2004
nicolette francis
i have found this page to be very informative but it could be a little more in depth on what the student is looking for and not just the basics.
01 June 2004
Becca Rogers
This has been a fantastic site for doing a very last minute write-up, as my deadline is tomorrow!
09 May 2004
Matt Ellis
Very helpful for giving you hints and tips about how to go about your investigation and how to write it up afterwards. Full marks for Re:act!
31 March 2004
Alistair Edward James
Thank you so very much for your coursework guide. The sheer mechanics of the coursework was mind boggling, but your site is a ray of sunshine in my life.
25 March 2004
Adamski Hallage
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24 March 2004
karen
A big help on the coursework. After struggling for weeks this has been a big help. Thank you!
04 January 2004
Jack
Wow, after trawling the web hopelessly looking for snippets of info this is a breath of fresh air.
Thank you Re:act!
03 November 2003
Michael Garland
Great web site all round!!!!!! Of great help with all aspects of Nuffield chemistry not just the course work.
20 October 2003
Marvin Daglish
This is perhaps the only informative website/resource for A-level students in chemistry which is especially helpful for investigations
09 October 2003
Stephanie
Very well explained summary of the coursework, but could be improved by making the text into bullet points thus making it easier to understand.
12 September 2003
Stephen Martin
I found this particular article most useful in helping me prepare for my investigation which I am about to undertake in two weeks time
04 September 2003
updated: 12 March 2008
Education in Chemistry article (73 KB)