I'm doing a experiment in which I'm testing the affect of the concentration of a reactant on the rate of reaction.

The reaction is between Magnesium and Hydrochloric Acid and so I'm changing the concentration of $\ce{HCl}$ ($\ce{0.5~M}$, $\ce{1~M}$ etc.)

In my lab report, I'm writing a list of controlled variables. So far I've mentioned the surface area and temperature of reactants. I would like to know more controlled variables.

I want to know whether the volume of Hydrochloric Acid affects the rate of reaction when added to the same amount of Magnesium metal?

According to some it doesn't and some say it does; I'm confused.

Also, please tell me why or why not the $\ce{HCl}$ volume affects the rate of reaction.

If the concentration of $\ce{HCl}$ changes between trials and the volume changes between the trial as well, then is the rate of reaction affected?

Does volume affect concentration in this case?


1 Answer 1


More volume of HCL does not increase the concentration and therefore does not increase the likelihood and so rate of reaction. Mg and HCl forms a metal salt MgCl and produces Hydrogen gas.

There may be a very slight increase in rate of reaction but this is permissible/ very negligible due to the fact that metals and acids form adducts and these can further react/ delay further reactions between the acid and metal molecules.

The rate would definitely be affected by factors such as temperature of the (aq) HCL. This is until the reactant Mg is used up then the reaction will cease leaving you with a flat curve on a rate graph.

Now, for Higher concentrations- this implies that more reacting molecules are at higher proximity to each other therefore the intermolecular collisions between the Mg and HCl molecules in your case are more frequent therefore forming more product per unit time.

Control variable: mass of Mg to at least 1.d.p, concentration of acid moldm-3, volume of HCl cm3 - even though this doesn't affect the rate of reaction enough - they would want you to keep this constant for valid test etc.

Hope this helps!


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