Term paper on What Affects The Rate Of Reaction

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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

1

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 of time. This increase

in collisions increases the rate of reaction.

The collision theory explains how the rate of reaction increases,

but it does not explain by how much or by how fast the rate increases.

The Kinetic energy of a particle is proportional to its absolute (Kelvin)

temperature.

1/2 mv 2µ T

But the mass of the particles remains constant so we can

eliminate that part of the equation so;

Þ V2µT

Therefore we can fit this into a formula:

V21/V22 = T1/T2

If we substitute the temperature into the formula we can work out

the average speed of the formula:

V21/V22 = 310/300

V 1 = Ö310/300V 2

= Ö1.033V2

= 1.016V2

However if we look at this it is only 1.016 times greater than

the speed at 300K, in other words we can see that it has only increased

by 1.6%.

The frequency of the collisions depends on the speed of the

particles, this simple collision theory only accounts for the 1.6%

increase in the rate, but in practice the reaction rate roughly doubles

in a 10K rise, so this simple theory cannot account for an 100% increase

in the reaction rate.

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 is “safe” to assume that some of the

particles do not have enough energy to react when they collide.

The minimum amount of energy that is needed to break down the

bonds is called the 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, if the activation energy

is very low the number of particles with that amount of energy will be

high so the reaction rate would be higher. An example of a low EA would

be in explosives when they need only a small input of energy to start

their exceedingly exothermic reactions.

In gases the energy of the particles is mainly kinetic, however

in a solid of a given mass this amount of energy is determined by their

velocities.

This graph below shows how the energies of particles are

distributed.

This graph is basically a histogram showing the number of

particles with that amount of energy. The area underneath the curve is

proportional to the total number of particles. The number of particles

with * EA is proportional to the total area underneath the curve.

The fraction of particles with * EA is given by the ratio:

Crosshatched area under the curve

total area under curve

Using the probability theory and the kinetic theory of gases,

equations were derived for the distribution of kinetic energy amongst

particles. From these equations the fractions of particles with an

energy * EA J mole-1 is represented by the equation: e -Ea/RT where R=

the gas constant (8.3 J K-1 mole -1)

T= absolute temperature.

This suggests that at a given temperature, T,

The reaction rate µ e -Ea/RT

If we use k as the rate constant, as a measure of the reaction

rate we can put this into the equation also.

kµ e -Ea/RT

Þ k= A e -Ea/RT

The last expression is called the Arrhenius equation because it

was developed by Srante Arrhenius in 1889. In this equation A can be

determined by the total numbers of collisions per unit time and the

orientation of the molecules when the collide, whilst e -Ea/RT is

determined by the fraction of molecules with sufficient amounts of energy

to react.

Putting the probability theory and the kinetic theory together

this now gives us a statement which accounts for the 100% increase in the

rate of reaction in a 10K rise.

Reaction Rate and Concentration.

The reaction rate increases when the concentration of the acid

increases because:

If you increase the concentration of the acid you are introducing more

particles into the reaction which will in turn produce a faster reaction

because there will be more collisions between the particles which is what

increases the reaction rate.

METHOD.

To get the amount of magnesium and the amount of hydrochloric

acid to use in the reaction, we have to use an excess of acid so that all

of the magnesium disappears.

Mg + 2HCl = MgCl2

+ H2

1 mole 2 moles 1 mole

1 mole

So, we can say that one mole of magnesium reacts with 2 moles of

hydrochloric acid.

If we use 1 mole of magnesium and 2 moles of hydrochloric acid we

will get a huge amount of gas, too much for us to measure. We would get

24,000 cm3 of hydrogen produced where we only want 100 cm3 of hydrogen

produced. So to get...

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