The aim of this investigation is to find out the effect of concentration of acid, in the
reaction between dilute hydrochloric acid and magnesium ribbon. The rate of a chemical
reaction is a measure of how fast the reaction takes place. It is important to remember
that a rapid reaction is completed in a short period of time. Some reactions are very fast,
e.g. (the formation of silver chloride precipitate when silver nitrate and hydrochloric acid
solutions are mixed. In this investigation we will test different concentrations of acid
reacting with magnesium and investigate how it affects the rate of reaction.
I predict that as the concentration of the hydrochloric acid increases, the time taken for the magnesium to disappear decreases. I predict that when the concentration of the hydrochloric acid doubles, the rate of the reaction doubles.
The collision theory describes how the rate of reaction increase (the time taken for the magnesium ribbon to disappear when it is reacted with hydrochloric acid) when the concentration of HCL increases. The theory states that if; the more concentrated the reactants, the greater the number of collisions between particles increases. This also explains why the greatest rate of reaction is usually as soon as the reactants have been mixed, i.e. they are both at their highest concentrations. As the reaction continues, the concentration of the reacting substances decreases and so does the rate of reaction. We must consider what happens when a reaction takes place. First of all the particles of the reacting substances must collide with each other, and secondly a fixed amount of energy called activation energy (Ea) must be reached if the reaction is to take place. If the particles can produce the right amount of energy (i.e. if they collide fast enough and in the right direction) a reaction will take place. The reaction is speeded up if the number of collisions is increased. In this investigation we must consider the topic of variables.
Clearly, the time taken for the magnesium to disappear when it is placed in different
concentrations of HCL is related in some way. The higher the concentration of HCL you
use, the less time it will take for the magnesium to disappear and so the rate reaction
increases. The concentration of HCL you use is the independent variable because it will
vary, and the time taken for the rate of reaction to take place (i.e. the magnesium to
disappear) is the dependant variable because it depends upon the concentration of
hydrochloric acid. No other variable will vary. The variables which will remain unchanged are the temperature (room temperature) will stay the same in order for it to be a fair test, because if the temperature changes it will effect the rate of reaction between the reactants, either by speeding it up if the temperature rises because the particles move faster and travel a greater distance in a given time and so will be involved in more collisions. Or the temperature may slow the reaction down if it gets colder due to particles moving slower. The amount of magnesium used will stay the same (2cm long), so that it is a fair test. The time it takes for the magnesium to disappear will be measured accurately using a stop clock as soon as the HCL is poured into test tube with the magnesium in it. This way most results will be accurate. It is important to keep the reactants separate while setting up the apparatus so that the starting time of the reaction will be measured accurately. Factors which may not be easy to control are, how well the solution is mixed when it is diluted, to get the correct concentration. A way to come around could be (stirring the test tube twice, or shaking the test tube twice). This factor is quite important because it determines exactly what the concentration of the solution is each time it is mixed , if the stirring factor is not carefully controlled it could lead to inaccurate results if the concentration of the acid solution is not what we calculated it to
be. During a chemical reaction the particles have to collide with enough energy to first
break the bonds and then to form the new bonds and the rearranged electrons, so it’s safe
to say that some of the particles don’t have enough energy to react when they collide. The
minimum amount of energy that is required to break the bonds is called activation
energy (Ea). If the activation energy is high only a small amount of particles will have
enough energy to react so the reaction rate would be very small, however the activation
energy is very low the number of particles with that amount of energy would be so high,
so start, so the reaction rate would be higher. An example of low (Ea) would be in
explosives when they only need a small input of energy to start their exceedly exothermic
reactions. A change in concentration is a change in the number of particles in a given
volume. If we increase the volume
a) the particles are more crowded so they collide more often.
b) Even though the average amount of energy possessed by a particle does not change,
there are more particles with each amount of energy - more particles with the activation
energy.
A) Is a major effect which affects the rate but b) is a minor effect which affects the rate
only slightly. In this experiment we are not concerned with whether the reaction is
exothermic or endothermic because we are interested with the activation needed to start
and continue the reaction.
