Rates of Reaction


BACKGROUND INFORMATION

What affects the rate of reaction? 1) The surface area of the magnesium. 2)
The temperature of the reaction. 3) Concentration of the hydrochloric acid. 4)
Presence of a catalyst.

In the experiment we use hydrochloric acid which reacts with the magnesium to
form magnesium chloride. The hydrogen ions give hydrochloric acid its acidic
properties, so that all solutions of hydrogen chloride and water have a sour
taste; corrode active metals, forming metal chlorides and hydrogen; turn litmus
red; neutralise alkalis; and react with salts of weak acids, forming chlorides
and the weak acids.

Magnesium, symbol Mg, silvery white metallic element that is relatively
unreactive. In group 2 (or IIa) of the periodic table, magnesium is one of the
alkaline earth metals. The atomic number of magnesium is 12.

Magnesium(s) + Hydrochloric acid(aq) = Magnesium Chloride(aq) + Hydrogen(g)
Mg + 2HCl = MgCl2
+ H2

In the reaction when the magnesium hits the acid when dropped in, it fisses and
then disappears giving of hydrogen as it fisses and it leaves behind a solution
of hydrogen chloride.
The activation energy of a particle is increased with heat. The particles
which have to have the activation energy are those particles which are moving,
in the case of magnesium and hydrochloric acid, it is the hydrochloric acid
particles which have to have the activation energy because they are the ones
that are moving and bombarding the magnesium particles to produce magnesium
chloride.

The rate at which all reactions happen are different. An example of a fast
reaction is an explosion, and an example of a slow reaction is rusting. In any
reaction, reactants chemical reactions® products.

We can measure reactions in two ways:

1) Continuous:- Start the experiment and watch it happen; you can use a
computer “logging” system to monitor it. I.e. Watching a colour fade or
increase.

2) Discontinuous:- Do the experiments and take readings/ samples from the
experiment at different times, then analyse the readings/samples to see how many
reactants and products are used up/ produced.

Reaction rate = amount of reactant used up
time taken

If the amount used up is the same each time then the only thing that changes is
the time taken.

so, reaction rate µ 1
time taken.
rate = K
time taken.

Where K is the constant for the reaction.

For particles to react:-

a) They have to collide with each other. b) They need a certain amount of
energy to break down the bonds of the particles and form new ones. This energy
is called the “Activation Energy” or Ea.

When we increase the temperature we give the particles more energy which:

1) Makes them move faster which In turn makes them collide with each other more
often.

2) Increases the average amount of energy particles have so more particles have
the “activation energy”

Both of these changes make the rate of reaction go up so we see a decrease in
the amount of time taken for the reaction and an increase in time taken.
= 1

Time taken reflects the rate of reaction.

Because temperature has an effect on both the speeds at which the particles
react and the activation energy they have a greater effect on the rate of
reaction than other changes.

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) Although 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 effects the rate, but b) is a minor effect which
effects the rate very slightly.

In this experiment we are not concerned with whether the reaction is
exothermic or endothermic because we are concerned with the activation energy
needed to start and continue the reaction.


PREDICTIONS

I predict that as we increase the temperature the rate of reaction will
increase.
If we increase the temperature by 100C the rate of reaction will double.

I predict that if we increase the concentration of the acid the reaction rate
will increase.
If the concentration of the acid doubles, the rate of the reaction will also
double.

LINKING PREDICTION TO THEORY

Reaction Rate and Temperature.
The collision theory describes how the rate of reaction increases as the
temperature increases. This theory states that as the temperature rises, more
energy is given to the particles so their speed increases, this increases the
number of collisions per unit