EXPERIMENTAL: AN INVESTIGATION INTO THE RELATIONSHIP BETWEEN REACTION
SPEED AND THE CONCENTRATION OF REACTANTS IN A DISPLACEMENT REACTION

Introduction
A displacement reaction is the chemical change which occurs when a more
reactive element displaces a less reactive element from its compound;

e.g.,       Mg(s) + 2HCl(aq) ——————————® MgCl2(aq) + H2(g)

In this redox reaction, electrons are transferred from magnesium atoms
to copper(II) ions because magnesium is a better reducing agent (i.e.,
it is more easily oxidized); the redox half equations are:

                     2Mg(s) ———————————® Mg2+(aq) + 2e-

              2H1+(aq) + 2e- ———————————® H2(g)

The aqueous chloride ions are spectators, so the net ionic equation is:

            Mg(s) + 2H1+(aq) ———————————® Mg2+(aq) + H2(g)

Independent variables known to change the speeds of chemical reactions
include concentration, temperature, surface area, and the presence of 
catalysts. However, the determination of quantitative relationships 
between such variables requires careful experimentation ...

[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]

For the displacement reaction of magnesium with aqueous hydrochloric 
acid, you are required to investigate three hypotheses.
Hypothesis 1: The reaction time (T) is in direct proportion to the
acid concentration (C); i.e., T = k × C.
Hypothesis 2: The reaction speed (S) is in direct proportion to the
acid concentration (C); i.e., S = k × C.
Hypothesis 3: The reaction speed (S) is in direct proportion to the
acid concentration squared (C²); i.e., S = k × C².

Notes
1.  The speed of a reaction can be investigated by measuring the time
taken for a fixed amount of either one of the reactants to disappear or
one of the products to appear; speed = 1 ÷ time or, more conveniently,
speed (in ms-¹) = 1000 ÷ time (in s).

2.  You are provided with magnesium ribbon, aqueous hydrochloric acid
(with an initial concentration of 1.00 mol dm-³), and distilled water.
In addition, you will need to use - within reason - suitable apparatus.

3.  The proposed plans of your investigation should be presented in
detail; these plans may be modified as the investigation proceeds.

Dr. R. Peters                  Next                      Contents' List