Rates of Reaction Coursework

-Pour each of the concentrations in turn into a test tube with equal amounts of magnesium every time present.

-Record the reaction time for each concentration (from pouring of acid into test tube to time when magnesium is no longer present)

-For fairness repeat experiment twice for each concentration

Preliminary Experiment

I will first carry out a preliminary experiment to know what to expect for the real thing and be able to make a more accurate prediction.

I used a smaller range of concentrations for the preliminary experiment. Including 1M, 1.5M, 2M and 3M

Results

Concentration

Time (minutes/seconds)

M

3 Minutes 10 Seconds

.5M

0 Minutes 19 Seconds

2M

9 Minutes 1 Second

3M

5 Minutes 40 Seconds

Fair Testing Procedure

For a fair test the following points must be taken into consideration

* Make sure that the surface area of the magnesium is the same for each experiment

* Retain room temperature at a constant heat, failure to do this could lead to variation in temperatures between experiments.

* Do not disturb the testing process and try not to move the test tubes while the reaction is happening (agitation).

* Accurately measure the different concentrations of hydrochloric acid using a measuring cylinder.

* Be precise when recording results from the stopwatch.

Safety Issues

It is important that the experiment is carried out safely and in a safe environment, the following steps should ensure a safe experiment.

* Wear goggles and even an apron to prevent spillage on clothes or in eyes

* Be careful when using hydrogen as it irritates the skin and can be harmful

* Avoid working near flames as hydrogen (g) given off can explode due to its flammability.

Scientific Background

Physical Chemistry. the theory that the number of new compounds formed during a chemical reaction equals the number of molecules colliding, times a factor that corrects for low-energy collisions.

-harcourt.com

When a reaction takes place, particles of the reacting substances must collide with each other there also must be a fixed amount of activation energy(Eª) must be reached for the reaction to take place. If the collision between the particles can produce sufficient energy the reaction will take place. Not all collisions will lead to a reaction.

An increase in temperature produces an increase in the rate of reaction. A rise of 10ºC approximately doubles he rate of reaction, for instance. The particles move faster when a mixture of particles are heated. The kinetic energy of the particles will naturally be increased and activation energy will be exceeded, increasing the amount of collisions and the rate of reaction.

Fewer particles fewer collisions More particles, more collisions

If the concentration of the catalyst increases the reaction will increase as a catalyst with a larger number of particles means there will be more hydrogen-magnesium collisions

The greatest period of reactivity will be at the start of the reaction because there will be more particles present at their highest concentrations. As a reaction progresses the rate will decrease as the substances decrease in concentration.

The rate of reaction can be sped up by breaking up the magnesium since the reaction can only occur on the surface of the solid (reactant), breaking the solid down into a power or smaller bits will mean the surface area is increased and consequently, the reaction time is sped up.

smaller surface area, reaction larger surface area, reaction takes

takes longer less time

Given the above factors and the preliminary results it is accurate to predict that the eventual outcome of the experiment will be along the lines of the reaction being quicker when using a more concentrated acid, the higher the temperature and the surface area of the magnesium.

Table of Results

Time in

Seconds

s-1

Hydrochloric Acid Molar

Test

2 Test

Average

/T

4.0M

4.0

4.4

4.2

0.24

3.5M

5.3

4.9

5.1

0.20

3.0M

7.8

6.3

7.1

0.14

2.5M

8.7

8.7

8.7

0.11

2.0M

2.5

3.3

2.9

0.08

.5M

29.0

27.9

28.5

0.04

.0M

53.4

35.2

44.3

0.02

0.5M

294.8

247.03

271.1

0.003

0M

no reaction

no reaction

no reaction

0.0

Analysis

After carrying out my experiment I have noticed that as the rate of reaction increases the time the reaction takes to complete decreases.

This can be expressed as the following relationship:

Rate ( 1/time

The gradient of the graph concentration / time represents the rate. As I have mentioned before, the reaction is fastest at the beginning of the experiment and for this reason the gradient is steepest at the beginning of the line on the graph.

The graph of 1/time / concentration produces a straight line. Indirect proportion occurs here, the concentration is increased and the time for the reaction to take place decreases.

Rate ( concentration

I also looked at the factor of temperature and how it effected the reaction, I have previously discussed the changes temperature makes during a reaction, heat speeds up movement of the particles which in turn will mean more collisions.

Of course, when you start the reaction experiment there is going to be a greater number of particles in the concentrated acid than at the end or in the middle of the reaction. This will mean that the reaction is faster at the beginning, than at any other point. This is also because of the surface area of the magnesium, at the start of the reaction there is a bigger surface area, but as it reacts the surface area decreases leading to a slower reaction (see effects of surface area).

I measured the temperatures of the hydrochloric acid at regular intervals in any given experiment (the trend applies to all concentrations and all figures will be relational).

M

0s

5s

0s

5s

20s

25s

30s

35s

40s

45s

50s

Temp (0C)

21

21

21

22

23

25

26

25

25

24

24

From the above results I can conclude that on contact with the magnesium the hydrochloric acid heats up until half way through when the concentration of the acid and surface area of the magnesium has decreased enough for the temperature to subside. The reaction is exothermic.

The reaction times did not decrease when using a concentration of exactly 1M more, although this has been backed up by various other sources.

Evaluation

The results I gathered were accurate enough to back up my prediction, I feel they could have been improved however with better equipment, more care and more tests to attain broader results. The measuring of the acids was not an aspect I was pleased with, using measuring cylinders to add the hydrochloric acid to the water was not the most accurate way of doing it. I could have used a pipette to carefully add small increments of acid to the measuring cylinder and get the exact quantity needed.

The magnesium ribbon was supplied in pre-cut strips which may not have been accurate and all the same size. I believe that the strips could have been of more accurate size should I have been given the ribbon itself and allowed to measure and cut each portion myself using measuring devices such as rulers.

The dropping of the magnesium into the acid was one of the most inaccurate parts of my experiment, it is near impossible to co-ordinate the contact of the magnesium with the acid and the starting of the stop watch. This will have lead to the times being slightly inaccurate. Using a different method of recording the reaction possibly would have meant more accuracy (see method and other options) eg- gas syringe.

It was also hard to tell when the reaction had finished exactly and the stopping of the recording of the time would have also been inaccurate.

I used accurate percentages of water and hydrochloric acid which meant that in that respect I was carrying out a fair test. I made sure that the hydrogen chloride was not contaminated before use and that it was straight from the bottle.

The magnesium didn't appear to be pure, on the surface of the magnesium there appeared to be a layer of dirt and grime, this will result in the acid taking a variable amount of time to burn through this tarnished layer and get to the magnesium. This extra time would have lead to the results being inaccurate. I could have combated this by rubbing off the dirt with sand paper or other such erosive material.

The experiment was over the period of one lesson, this meant that I could use the 4M acid knowing that it would be constantly the same concentration. If I had done this over a period of a week from lesson to lesson I could have been forced to use a slightly different concentration of acid. As It was, I feel this was accurate.

Please see the other methods I have suggested previously for more accurate approaches I could take from here to record a better set of results in the future. To improve the experiment other than changing methods I could have recorded the time using a better stop watch or computer and weighed the magnesium making sure the bits weighed the same .