Practical investigations
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Why is there a faster rate of reaction at low temperatures with magnesium and an acid and vice versa?
Ulex writes
I’m not absolutely sure what you are getting at here but I suspect you may be thinking of the unexpected phenomenon where you raise the temperature in the expectation that the reaction would get faster and you find that it goes more slowly than at the lower temperature.
You may find that the reaction does go faster if you go on raising the temperature.
My students have observed this phenomenon many times and between us we have evolved a theory; I offer it to you for what it is worth but it is only a theory.
When the reaction occurs, hydrogen gas bubbles are produced which escape from the metal surface. When you raise the temperature, the bubbles are produced more quickly but cannot escape any more easily than they could at the lower temperature – they get in the way, in fact, so that the acid cannot ‘get at’ the surface as easily as before – the rate drops. At higher temperature still the metal moves around more, shaking off the bubbles, so the rate rises.
If you conduct the experiment with continuous stirring, increased temperature has the normally expected effect which provides some support for the theory.
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Risk assessment
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updated: 29 September 2006
