-The rate is not a constant throughout the reaction-it changes.
- The reaction is fastest at the start, gradually becoming slower as the reaction proceeds.
- From the graph, the fastest part of the reaction is shown by the steepest curve.
- The curve on the graph goes flat when the reaction is complete. This is because, as time goes on, the volume evolved does not change.
The rate of reaction depends on how many successful collisions there are in a given unit of time.
There are 4 methods by which you can increase the rate of a reaction:
- Increase the concentration of a reactant.
- Increase the temperature of the reactants.
- Increase the surface area of a reactant.
- Add catalyst to the reaction.
Before discovering the reasons for the above causing an increase in rate, what is needed to cause a reaction to occur must first be looked at.
If we take the reaction between magnesium and hydrochloric acid, in order for the to react:
- They must collide with each other.
- The collision must be with sufficient energy.
Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen Gas.
The effect of concentration
If the concentration of acid (a reactant) is increased, the reaction proceeds at a quicker rate.
In dilute acid there are less acid particles. This means there is a less chance of an acid particle hitting a magnesium particle as compared with acid of a higher concentration.
In concentrated acid there are more acid particles, therefore there is a greater chance of an acid particle hitting a magnesium particle.
The graph below shows results from two experiments. Experiment A was with concentrated acid and experiment B uses dilute acid.
As you can see, the greater the concentration of the acid in a reaction the steeper the curve and the shorter the reaction. Hence, these results show that an increase in concentration increases the rate of reaction.
The effect of Surface Area.
The rate of reaction between magnesium and hydrochloric acid increases as you increase the surface area of the magnesium.
For example: powdered metal (greater surface area) reacts quicker with acid than strips of metal (lower surface area).
The greater the surface area of the metal, the more of its particles is exposed to the acid. This increase in exposure increases the frequency of successful collisions.
The effect of Catalyst into reaction
The word catalyst means changing the rate of a reaction with some other material 'added to' or in 'contact with' the reaction mixture. There are two phrases involved: A 'positive catalyst' meaning speeding up the reaction or a 'negative catalyst' slowing down a reaction.
Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules. This effectively means the Activation Energy is reduced.
Therefore at the same temperature, more reactant molecules have enough kinetic energy to react compared to the uncatalysed situation. Although a true catalyst does take part in the reaction, BUT does not get used up and can be reused with more reactants. It is chemically the same at the end of the reaction but it may change a little physically if its a solid.
A solid catalyst might change physically by becoming more finely divided, especially if the reaction is exothermic. Enzymes are biochemical catalysts. They have the advantage of bringing about reactions at normal temperatures and pressures which would otherwise need more expensive and energy-demanding equipment.
Prediction
According to the background information, concentration is one of the factors affecting rate of reaction. This means that the rate of reaction is directly proportional to concentration of reactants. As well as, the time will decrease as concentration increases. Since there are more particles in a fixed volume, there will be more collisions and more reactions taking place using less time to complete.
Consider the reaction:
A +B → AB
The rate of the forward reaction would be proportional to [A] and [B]. The rate of backward reaction is proportional to [AB].
Let Rƒ = Rate of Forward Reaction therefore
Rƒ = [A][B] ∴ Rƒ = k１[A][B]
Let Rь = Rate of Backward Reaction therefore
Rь ∝ [AB] ∴ Rь = k２[AB]
At Equilibrium Rƒ = Rь then
k１[A][B] = k２[AB]
k１ [AB]
--------- = ---------
k２ [A][B]
[AB]
Kc = ----------
[A][B]
Kc is called Equilibrium Constant.
Based on this, the Rate of Reaction between Sodium Thiosulphate and Hydrochloric Acid is likely to increase as the concentration of Hydrochloric Acid increases.
Rate of Reaction ∝ [Hcl]
Rate of Reaction = k[Hcl]
Rate of Reaction
k = -------------------------
[Hcl]
Risk Assessment
- While handling quantities of the higher concentrations, I should be careful not to spill the contents.
- Goggles should be worn while reaction takes place.
- The burettes should not be placed near the edge of the table but a little further away.
Diagram
Apparatus List
- Burettes
- Conical Flask
- Electronic Stopwatch
- Piece of paper with cross
- Safety Goggles
- Stirring Rod
- Thermometer
Chemical List
- 15 ml each of 0.5M, 1.0M, 1.5M, 2.0M, 2.5M, 3.0M Hydrochloric Acid
- 15ml of 1.0M Sodium Thiosulphate
Accuracy of Apparatus
Burette ± 0.1ml
Measuring Cylinder ± 25ml
Conical Flask ± 20ml
Timer ± 0.01s
Method
- First, Hydrochloric Acid (0.5M), and Sodium Thiosulphate (1.0M) will be both measured at 15ml, with the help of a burette.
- Temperature, which is a fixed variable, of both the acid and the Thiosulphate must be measured and kept at constant.
- The measured amounts of the two chemicals will be poured into a conical flask.
- The reaction will be timed until the black cross, under the flask, is no more visible.
- The accuracy of apparatus as listed above, adds to the reliability of the experiment.
- The experiment is repeated varying the concentrations of the acid (1.0M, 1.5M, 2.0M, 2.5M, and 3.0M).
- Again, for accurate results, the same concentration will be repeated twice.
- The obtained evidence will be recorded and presented in a results table.
Results
Analysis
Refer Graph1 shows the relation between concentration of acid and time taken for reaction to be completed. The graph is almost a straight line and as the concentration increases, the average time decreases.
Refer Graph 2. The graph shows the relation between concentration of acid and rate of reaction. There is an almost directly proportional relation between concentration and rate of reaction. That means, as concentration increases, rate of reaction increase along with it.
Rate of Reaction ∝ [Hcl]
Rate of Reaction = k[Hcl] where k is a constant.
Rate of Reaction
k = -------------------------
[Hcl]
The equation of a straight line passing through the origin is y=mx + c
y is the rate of reaction and x was the concentration of acid.
The m (gradient) is shown to be calculated on the last column, using the formula stated on the previous page; however under the label heading of k. K is ‘almost’ a constant.
Conclusion
From my analysing my graph I could see that from Graph 1 there was a strong negative correlation which meant that, as stated before, when I increased the concentration, the time taken for the black cross to disappear was less.
The second graph showed a directly proportional relationship between concentration of acid and rate of reaction.
The results match my prediction and they show that concentration has a significant influence in rate of reaction.
Evaluation
Using the accuracy of the experiment my results are reliable since it is reinforces my scientific explanation. The burette measured my controlled variables; volume of hydrochloric acid and sodium Thiosulphate which had an apparatus accuracy of ±0.1ml. The timing was accurate to the nearest one hundredth of a second and since I used a digital stopwatch it was easy to control. The reliability of the experiment would gather the same conclusion if repeated by another. The procedure was very simple and straightforward. The control variables were easily kept constant and measured with equipment of the highest accuracy possible. However certain improvements would increase the reliability of the results. Only two repeats were carried out for each of the concentrations. The more the no. of repeats the more accurate and the more reliable the overall conclusion. The range of the values needs to be extended because my conclusion is only valid for the range I had used for this experiment. The desired range could start from 3M to 6M. This would be quite risky as you are handling acids of high concentration. Since we proved that increasing concentration increases rate of reaction, we then need to find the optimum molarity. Having more and better equipment would also be a major improvement. Having a light detector underneath the conical flask can measure the amount of light that is passing through the, at first, clear solution. As the reaction takes places and the black cross starts to disappear, the amount of light would, hence, decrease until the light detector reads zero. At that exact moment, the timer will be stopped. This is the most accurate method that can be applied to get the correct time for the reaction’s completion.
There are two extensions I have thought for this investigation:
The variables; volume of sodium Thiosulphate and volume of hydrochloric acid can be altered. The concentrations of both reactants are kept the same as well as temperature. The range of volumes that can be used can be from 10ml to 50ml in 10ml steps. Experiment 1 will carry out changes to the volume of sodium Thiosulphate and Experiment 2 will carry out changes to the volume of hydrochloric acid. The aim of this extension is to find out whether there will be any effect on the rate of reaction if volume is increased.
Another extension is by using a different acid in place of hydrochloric acid. All other variables are kept constant; volume, temperature. So the reaction will be between Sodium Thiosulphate and Sulphuric Acid. I expect to see an increase in rate of reaction since Sulphuric Acid (H2SO4) has 2 hydrogen ions more than Hydrochloric Acid (HCl). Sulphuric Acid can give out more ions so that means more collisions and more reactions at a faster rate. Concentration of acid is the independent variable and the range to be used is the same that of this experiment.
