Rates of reaction - kinetics
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I am investigating the acid/metal reactions (HCl, H2SO4 and magnesium) and I am trying to work out the rate of reaction. I have read 'Measuring the rate' on React and it was very useful. However I am slightly puzzled.
When you measure the rate of reaction by measuring the time taken for the magnesium strip to disappear with excess acid (therefore 1/t as rate), I understand that the acid concentration will remain almost the same as it was at the start. I do not understand how this shows that the rate of reaction throughout the dissolving process remains constant. Could you explain this, please? Can it be explained mathematically or can it be proved with more detail? I thought the reaction rate changes over time, and that is why you have to work out the initial rate of reaction.
Also, the only thing that I can think about for measuring the initial rate of reaction is calculating volume of hydrogen gas per minute (drawing a tangent on the graph), but it seems rather inaccurate to me. Furthermore, how can you decide over how many seconds the rate can be 'called' initial rate? Is it just the gradient of the straight part of the curve on the graph? Does this mean that, if the graphs for different concentrations have straight part of the curve over different amount of time, the initial rate is measured over different time?
Is there another way of measuring the initial rate?
Ulex writes ...
Right! We have three questions here, so let's take them in order.
1. Most methods of observing the way the rate of a reaction varies with concentration do not in fact do what we want. They measure the variation of concentration with time which is not the same thing at all. The form of a rate equation for a reaction involving a reactant A is:
rate = k[A]n
where n is the order of the reaction.
Our task is to find the value of n. If we are to compare actual experimental results with this equation to see which value of n best fits them, we must first contrive a way of simultaneously measuring ‘rate’ and ‘concentration of A’. I’ll come back to this in a moment.
2. If we do what you suggest, and obtain data for the volume of hydrogen collected at known intervals of time, we can plot the graph of volume against time, and estimate the rate of reaction by drawing tangents to the curve, as you say. You are not very impressed with the potential accuracy of this, and you are quite right. However it does work sufficiently well to yield results. From the graph you can get tangents (rates) at various concentrations of A and plot one against the other. If you get a straight line, n must be 1. If not, try plotting rate against [A]2 and see if this gives a straight line – if so then n=2.
3. Now we can come back to Q1 again because there is another way. At the start of the reaction we know the concentration of A exactly: it is the ‘initial concentration’. If we can measure the ‘initial rate’ as well, we can do the experiment several times at several different initial concentrations, and plot a graph of initial rate against initial concentration as in Q2. The problem is that as soon as the reaction starts both the rate and the concentration change. The trick is to fix it so that neither changes by very much. If the A concentration is high and the piece of magnesium small, then while the magnesium is reacting the concentration remains more or less the same (you can’t actually prevent it changing). The rate of reaction does not vary much either – this follows from the rate equation, whatever the value of n. You actually know this because you have observed the initial straight portion of the volume/time graph. Should you vary the technique if the straight portion varies in length between ‘runs’ of the experiment? Well, yes, you could, but you could also choose such a small piece of magnesium for each run that the first bit of the volume/time graph is guaranteed to be straight. Ideally the concentration of A should not vary by more than about 2 to 5%. If it does, your rate is not an ’initial rate’ any more!
To sum up. Two different methods are being describe here. One of them is to do a single experimental run, letting the concentration of the acid decrease with time (there should be an excess of magnesium for this). Plot volume against time, draw tangents etc.
The other is to do several runs, each with a different initial concentration of acid. Each run gives one point for a rate/concentration graph. This method requires you to use a small piece of magnesium for each run.
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updated: 19 December 2003
