To investigate the differences in order of reaction and activation energy of the reactions between magnesium and weak and strong acids.

GCSE Science

Plan

Aim - To investigate the differences in order of reaction and activation energy of the reactions between

magnesium and weak and strong acids.

Background Information

Activation Energy and collision theory

Collision theory is a model used to explain the dependence of rate of reaction on temperature. The theory

is that reactant particles are moving and colliding with one another. Collisions may be successful i.e.

resulting in a reaction depending on whether their combined kinetic energy is equal to or greater than the

activation energy for that specific reaction.

The activation energy, or EA is the amount of energy required to break the bonds in reactant chemicals

before a reaction can take place between them. It provides the reactants with enough energy to reach the

transition state.

It is expected then that, at higher temperatures, reactant particles will have more kinetic energy resulting in

more successful collisions. This being true, it can further be expected that, as the temperature is increased,

there will be a proportionate increase in the number of successful collisions and thus an increase rate.

The Maxwell Bolzman distribution curve below shows how at higher temperatures more particles have a

kinetic energy greater than the activation energy. Therefore at higher temperatures more collisions will be

between particles with energy greater than or equal to the the activation energy, therefore more collisions

will be successful therefore the rate of reaction will be higher.

The Arrhenius equation gives a link between the factors involved in the kinetics of a reaction between

one mole of colliding particles:

Where:

const. = constant,

k = rate constant of the reaction,

R = the gas constant, 8.31 J K-1mol-1,

EA = the activation energy of the reaction, J mol-1 ,

and T = temperature in kelvins.

A graph which uses this information is:

The gradient of which is:

-EA / R

Rate and order of a Reaction

For a reaction between two substances A and B the rate of the reaction can be taken as the

rate at which A is used up. In a reaction in solution the rate of the reaction would therefore be

the rate of change of concentration of the solute A, or rA.

(The method used to determine the rate of the reaction will entail measuring the amount of time

taken to produce a certain amount of hydrogen gas.)

In the reaction

The rate of change of concentration of A should, in practise, follow the rate equation:

rA = k[A]a.[B]b

( '[substance]' denotes 'the concentration of that substance') i.e. [A] = concentration of

substance A in mol dm-3)

The indeces a and b are the order of the reaction with respect to the corresponding substances

A and B respectively. The overall order is the sum of these.

(In this investigation I will try to find out the order of the reaction in respect to the acid and

compare the effect of changing the strength of the acid from weak to strong.)

The order of reaction can be shown in a graph of rate against concentration the orders zero, 1st

and 2nd have the following characteristic shapes:

zero order:

1st order:

2nd order:

The orders of each reaction with respect to the acids are being compared because this gives an

insight into the effect a certain substance has on the rate of the reaction. It gives information

about the mechanism of the reaction - this will be compared for strong and weak acids.

Weak and strong acids - ethanoic and hydrochloric.

A strong acid is fully ionised in aqueous solution, meaning that it exists entirely as ions in

soulution. It will therefore not require any further bond breaking to react. A weak acid however,

is only partially ionised in aqueous solution so during a reaction energy must be spent on

breaking the bonds in the molecules in solution. It is expected then that reactions of weak acids

generally have a higher activation energy than those of strong acids because a certain amount of

energy must be put inm to break bonds in the weaker acids.

The chosen acids are ethanoic acid (CH3COOH(aq)) and hydrochloric acid (HCl(aq)).

Ethanoic acid was chosen because it is a cheap and easily obtainable weak acid. Hydrochloric

acid was chosen becuase it is also easily obtainable. An alternative to hydrochloric acid was

sulphuric acid (H2SO4(aq)) however the use of sulphuric acid would have made the experiment

invalid in the sense that the outcomes would be controlled by two varibles, the strength of the

acid, and the number of moles of H+ ions produced ...

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energy must be put inm to break bonds in the weaker acids.

The chosen acids are ethanoic acid (CH3COOH(aq)) and hydrochloric acid (HCl(aq)).

Ethanoic acid was chosen because it is a cheap and easily obtainable weak acid. Hydrochloric

acid was chosen becuase it is also easily obtainable. An alternative to hydrochloric acid was

sulphuric acid (H2SO4(aq)) however the use of sulphuric acid would have made the experiment

invalid in the sense that the outcomes would be controlled by two varibles, the strength of the

acid, and the number of moles of H+ ions produced for each mole of acid. Ethanoic acid is a

monobasic acid meaning that for each mole of acid there is one mole of H+ ions, sulphuric acid

however, is dibasic submitting two moles of H+ ions for every mole of H2SO4(aq).

Hydrochloric acid, like ethanoic acid is monobasic.

Chemical Equations

Strong acid: hydrochloric acid and magnesium

Equation for the reaction:

2HCl(aq) + Mg(s) MgCl2(aq) + H2(g)

Ionic equation ommiting spectator ions (2Cl-):

2H+(aq) + Mg(s) Mg2+(aq) + H2(g)

Weak acid: Ethanoic acid and magnesium

Equation for the reaction:

2CH3COOH(aq) + Mg(s) (CH3COO)2Mg(aq) + H2(g)

Ionic equation ommiting spectator ions (2CH3COO- (aq)):

2H+(aq) + Mg(s) Mg2+(aq) + H2(g)

Predictions

Activation Energy:

Ethanoic acid exists in solution mainly as CH3COOH molecules. To produce 2CH3COO- (aq)

+ 2H+(aq) ions it requires energy to be spent on breaking bonds therefore the reaction

ethanoic acid and magnesium is likely to have a higher activation energy than that between

magnesium and hydrochloric acid which is fully ionised in solution

Order of reaction

Ionisation of CH3COOH:

CH3COOH(aq) CH3COO-(aq) + H+

Order gives information about the rate determining step, which for weak acids may be the

ionisation process. If the mechanism of the reaction invoving ethanoic acid is different to that of

hydrochloric acid they might be expected to have different rate equations and overall orders if

the ionisation of the CH3COOH molecules is the rate determining step. I predict that the orders

of the two reactions with respect to the acids are different for each reaction. This prediction is

based on the fact that the ionisation of ethanoic acid may be the rate determining step.

Iplementing

Based on the the above plan an experiment to find the activation energies and order (with

respect to the acids) of the reactions between hydrochloric and ethanoic acid and magnesium

was carried out using the following apparatus and method.

Apparatus

* Conical flask

* 500cm3 beaker

* Bunson burner

* Tripod and gauze

* Gas syringe

* Delivery tube

* Thermometer

* Stop clock

* Measuring cylinder

* Large test-tube or boiling cylinder

Chemicals

For order of reaction:

* Hydrochloric acid: 60cm3 of each of 0.5, 1, 1.5, 2, 2.5 and 3 Molar solutions.

* Ethanoic acid: 60cm3 of each of 0.5, 1, 1.5, 2, 2.5 and 3 Molar solutions.

* Magnesium ribbon - 0.5 m.

For actication energy:

* Hydrochloric acid: 100cm3 of 2 Molar solution.

* Ethanoic acid: 100cm3 of 2 Molar solution.

* Magnesium ribbon - 0.5m.

Important information

Magnesium ribbon has a mass of 1g per metre its dimensions and density are near constant

along it's length it is to be assumed that any piece of the ribbon contains the same number of

moles of magnesium as another piece of the same length.

As only 4M solutions of both acids are being provided, the required concentrations can be

made by mixing water (tap water - contains less H+ ions than distilled water) and acid in the

following ratios for the adjacent concentrations:

0.5M - ratio of acid to water = 1:7

1M - ratio of acid to water = 1:3

1.5M - ratio of acid to water = 3:5

2M - ratio of acid to water = 1:1

2.5M - ratio of acid to water = 5:3

3M - ratio of acid to water = 7:1

Diagrams

Diagram 1

Diagram 2

Methods

Order of reaction

Remove any magnesium oxide from the surface of the magnesium ribbon using wire wool and

prepare 2cm pieces using clean scissors. Set up equipment as show in diagram 1 above except

without the conical flask. Make sure the gas syringe is set to 10cm3. Measure 20cm3 of 0.5 M

Ethanoic acid in a measuring cylinder and transfer to the conical flask. Clamp the conical flask in

place and add 2cm of Magnesium ribbon. Instantly seal the conical flask securely with the bung

and start the stop clock. Stop the clock as soon as the gas syringe is at 30cm3 - and 20cm3 of

hydrogen gas have been collected. record the time and reset the stopclock. Wash the conical

flask with tap water and dry. Make sure the gas syringe is set to 10cm3. Repeat three times for

each concentration of both acids and record data in a table like those on the following page

marked 'order of reaction'.

Activation energy

Remove any magnesium oxide from the surface of the magnesium ribbon using wire wool and

prepare 2cm pieces using clean scissors. Set up equipment as show in diagram 2 above except

with the bunson off and without the test tube. Make sure the gas syringe is set to 10cm3.

Measure 20cm3 of 2M Ethanoic acid in a measuring cylinder and transfer to the test tube.

Clamp the conical flask in place and place the thermometer in the solution and record the

temperature in kelvins after a 1 minute wait. Remove the thermometer, add 2cm of magnesium

ribbon, seal the test tube with the bung securely and start the stop clock. Again, stop the clock

as soon as the gas syringe is at 30cm3 and 20cm3 of hydrogen had been collected. Wash the

test tube with tap water and dry. Make sure the gas syringe is set to 10cm3. Repeat the

experiment, this time heating the water bath so that the temperature of the soution is

approximately 70, 60, 50 and 40°C (343, 333, 323 and 313 kelvins). Recording the

temperature and time taken for 20cm3 of gas to be collected.

Repeat the whole experiment above using 2M hydrochloric acid.

Only one value is taken at each temperature because it is highly unlikely that the solution will be

at that exact temperature in a second attempt.

Justification of Quantities and Concentrations

It was decided that 2cm of magnesium ribbon would provide a substantial mass and surface area

for reaction. It was also decided that concentrations of acid less than 0.5M would have reaction

time which would be too slow to be practical in the given time and concentrations of more than

3M would react to quickly - inviting more chance of error in the recording of reaction time. It

was also required that about five concentrations were investigated.

.................

20cm3 of acid - means the ratios of acid to water can be calculated easily.

.................

A large excess of acid is required so that there is no change in concentration. The following

shows why this was achieved.

2CH3COOH(aq) + Mg(s) (CH3COO)2Mg(aq) + H2(g)

and

2HCl(aq) + Mg(s) MgCl2(aq) + H2(g)

Magnesium: 2cm = 0.02g

Acid = 20cm3, 0.5mol.dm-3 (lowest conc.)

Moles of magnesium = 0.02 / 24 = 8.33x10-4moles

Lowest moles of acid used = (20 x 0.5) / 1000 = 1x10-2

Moles of acid required to react fully with Mg

= 2 x 8.33x10-4 (because ratio is 2:1)

= 1.67x10-3

Lowest moles of acid used (1x10-2) is much greater than that required (1.67x10-2) Therefore

the acid is in large excess.

This has been worked through for the lowest concentration - there will be even bigger excess

for the higher concentrations.

This means that using the quantities used in these reactions does not change much in any of the

experiments (i.e. is the same from start to finish).

Because the the volumes and concentrations of ethanoic acid are the same for those of

hydrochloric acid and also because the molar ration in the equations is the same in both cases,

(1 mole of magnesium to 2 moles of acid for both acids) the same argument applies to both

acids.

.........................

2M acid was chosen for the activation energy experiments because it was easy to produce

more precisely by mixing water and 4M acid solution and should not react too quickly or too

slowly.

Safety

Long hair should be tied back. Do not let ties hang freely. Safety goggles should be worn to

protect eyes and hands should be washed to prevent passing of substances to eyes or mouth.

In the event of any chemicals coming in contact with the eyes, flood the area with large quantites

of water immediately. A heat burn or a scald from liquids or steam, treat area with cold water

for ten minutes. - For further safety precautions see risk assessment sheet.

Controlling variables

Volume and concentration of acid: these have predetermined values (see method) which should

be measured using the same measuring cylinder.

Surface area of magnesium: The surface area of Magnesium in contact with the acid will vary

uncontrollably- decreasing as the magnesium decreases in size. Bubbles of H2 may propell the

magnesium to the surface further limiting the surface area in faster reactions. - This is a variable

whic cannot be controlled and must be considered to be an influence to the outcomes.

Magnesium oxide: The surface area of the magnesium can be

Mass of magnesium: There is a predetermined mass of 0.02g which is equal to a piece of

magnesium ribbon 2cm long. (provided the magnesium is constant see assumptions).

Agitation: Care must be taken to ensure that the apparatus is not shake during the

experiment.This is likely to increase the rate of reaction - decreasing the time.

Equipment: To keep error to a minimum use the same equipment through all experiments.

Although a built in bias may lead to systematic error there will be no error due to slight

differences in the apparatus.

Magnesium oxide: The

Temperature: Because these reactions are exothermic heat will be produced during the

reactions which will affect the energy of the reactant particles, affecting the reaction rate.

Although this cannot be prevented using this method a alternative method may involve

thermostatically controlled water bath which would reduce the effects due to the heat produced.

These were not available and their use was not practical for the time allowed.

Assumptions

Magnesium: It has been assumed that magnesuim ribbon has constant dimensions and density

along it's length.

It is being assumed that the temperature measure is constnt throughout the solution and that the

temperature remains the same while the reaction takes place.

It is being assumed that there are no H+ ions present in the water used to dilute the 4M acids.

Concluding

Analysis

ORDER

Ethanoic Acid

Results for 0.5M ethanoic acid had to be discarded due to the fact that they took too long and a

20cm3 volume of hydrogen gas was not collected.

Graph 1: Having compared a 'square relationship' trendline and a linear trendline on the same

graph, the square relationship (darker line) can be seen to be most likely - passing through the

origin and coming close to most points. It is unlikely then that there is a linear relationship

between the rate and concentration as there would be if the reaction was 1st order. The linear

trendline crosses the axis more than 0.5 units away from the origin - this is too far away from the

origin to be a product of systematic error. It has therefore been decided that the relationship

between the concentration and the rate is a square relationship and therefore that the reaction is

2nd order with respect to ethanoic acid. To varify this, a rate against concentration2 graph was

plotted, graph 2.

Graph 2 shows a definite linear relationship between the rate and the square of the

concentration which reinforces the conclusion that the reaction is 2nd order.

Anomoly assessment:

Apart from the value for 0.5M ethanoic acid, which was not plotted, there appear to be no

anomolies. All points fall very close to the line.

Hydrochloric Acid

Looking at the graphs in which all points are included there also seems to be a square

relationship between the concentration of hydrochloric acid and the rate at which hydrogen gas

was formed. Although a linear trendline in displayed it is not near the origin ans is not cose to

many points. However questions arose when the raw data table was analysed more carefully. It

was apparent that at high concentrations the times taken to produce 20cm3 of hydrogen were

all quite close, especially considering the fact that a likely error in recording the time is about 2

seconds. This poor spacing between the values suggests that one or more of the times for 3M,

2.5M and 2M could be anomolous. After carefull study (see anomoly assessment) it was

decided that order of the reaction with respect to hydrochloric acid is most probably 2nd

(based on these results) but the reasults are inconclusive.

Anomoly assessment:

It can be seen that the first value for time taken at a concentration of 2.5M (15 seconds) cen be

considered an anomoly (marked by a *) it is more than two times the estimated possible error

of 2 seconds away from the other two values (8s and 9s). This value was discarded and the

average of the other two values taken.

Because of the closeness between the results in the lower three rows (highlighted green) graphs

were plotted including four points - always omitting two of these values. (see anomoly

assessment graphs) These graphs suggested that the reaction was likely to be second order -

but none are really conclusive.

ACTIVATION ENERGY

Main calculation:

gradient = -EA / R = -EA / 8.314

therefore:

EA = -(gradient x 8.314)

Ethanoic Acid

gradient = -2403.8

therefore EA = -(-2403.8 x 8.314) = 19'985.19 J mol-1 = 20.0kJmol-1

Hydrochloric Acid

gradient = -2021.8

therefore EA = -(-2021.8 x 8.314) = 16'809.25 J mol-1 = 16.8kJmol-1

The results for the acitvation energy have been much more successful. There were no anomolies

- all points fell quite close to the lines.

Concluding Summary

Ethanoic acid is a weak monobasic acid which only partially ionises in aqeuous solution.

Hydrochloric acid is a strong monobasic acid which fully ionises in aqueous solution.

The exothermic reaction between ethanoic acid and magnesium metal is 2nd order with respect

to the ethanoic acid.

The investigation provided no conclusive evidence of a possible difference between the

mechanisms of the reactions of ethanoic acid and hydrochloric acid with magnesium. If the

reaction of hydrochloric acid is taken to be 2nd order (although results inconclusive) then it is

probable that the mechanisms of the two reactions are similar or the same - with the same rate

determining step.

The investigation has demonstrated that the activation energy for a reaction between a weak

acid and a metal is likely to be larger than that of a strong acid and a metal because a weak acid

is only partially ionised in solution and needs energy to be put in to break bonds and form ions.

This can be seen from the results of this investigation - ethanoic acid is a weak acid and the

activation energy of the reaction between ethanoic acid and magnesium metal is 20.0kJmol-1

whereas the activation energy of the reaction between hydrochloric acid (a strong acid) and

magnesium metal is much lower at 16.8kJmol-1. this big difference is due to the fact that

ethanoic acid exists mainly as CH3COOH molecules in solution which need energy to break

into H+ and CH3COO- but hydrochloric acid exists entirely as ions in solution.

To investigate the differences in order of reaction and activation energy of the reactions between magnesium and weak and strong acids.

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