The effect of Pressure
If one or more of the reactants is a gas then increasing pressure will effectively increase the concentration of the reactant molecules and speed up the reaction. The increased chance of a 'fruitful' collision produces the increase in reaction.
The effect of Surface Area
If a solid, reactant or a solid catalyst is broken down into smaller pieces the rate of reaction increases. The speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid. Therefore, there is more chance that a reactant particle will hit the solid surface and react.
The diagrams below illustrate the acid and marble chip reaction, but they could also represent a solid catalyst in a solution of reactants.
The effect of Temperature
When gases or liquids are heated the particles gain kinetic energy and move faster. The increased speed increases the chance of collision between reactant molecules and the rate increases. However this is not the main reason for the increased reaction speed.
Most molecular collisions do not result in chemical change. Before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the activation energy shown on the energy level diagrams below.
Going up and to top hump represents bond breaking on reacting particle collision. The purple arrow up represents this minimum energy needed to break bonds to initiate the reaction. Going down the other side represents the new bonds formed in the reaction products.
It does not matter whether the reaction is an exothermic or an endothermic energy change. Now when heated molecules have a greater kinetic energy, a greater proportion of them have the required activation energy to react.
The increased chance of ‘fruitful’ higher energy collisions greatly increases the speed of the reaction.
The effect of a Catalyst
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 the two phrases you may come across:
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 un-catalysed 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 it is a solid.
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.
In my experiment; I will be focusing on the effect of the concentration of a substance in the reaction. The rates of chemical reactions can vary greatly. For example: a really fast reaction would be when a chemical explodes-its rate would be very fast, however there are reactions that could take months or even years, this would be like rust forming.
Prediction
Reactions occur when the particles in the solutions collide. From past experiments, I can predict that the stronger the concentration of hydrochloric acid, the shorter length of time it will take for the cross to disappear. I make this prediction as I know that stronger concentrations contain more of the Na2S2O3 particles and so increase the probability of collisions occurring and again increase the chance of the reaction occurring faster.
Variables
I understand that there are many factors which can affect the accuracy of my results so I will ensure that I keep all the physical conditions the same for my experiments. My only variable will be the concentration of the sodium thiosulphate. I will be carrying out six separate tests, each with varying concentrations of sodium thiosulphate. They are as follows:
The Experiment
Equipment and apparatus I will need for the experiment: conical flask, a burette, a black cross on white paper, beakers, a stopwatch, safety goggles and chemicals: sodium thiosulphate and dilute hydrochloric acid.
Procedure
I will mix the above concentrations of sodium thiosulphate by measuring out the required amounts of water and sodium thiosulphate and mixing them. I will then put this solution to into a conical flask placed above a piece of black paper with a black cross on it. I will then measure out 5Cm³ and add this to the conical flask and immediately start the stopwatch. I will repeat this for each of the six concentrations.
Below is diagrams showing the mixed solution changing translucent:
Safety
Safety precautions I will take during the experiment will be: wear goggles at all times, pour chemicals into container placed on table, do not inhale any fumes coming out of reactants and always work whilst standing up.
Precision.
Burette - A burette makes our experiment more accurate because each volume is measured accurately so I am adding the same amount each time.
Pipette - I will also use a pipette to add the acid as a pipette is measured in millimetres and it can be easily read
Reliability
My experiment is going to be reliable because I am repeating it 3 times. I will find an average for my results. If there is a large range, it will mean that there is a mistake somewhere.
I will also use the same person to judge wither the X is fully covered, so the same point of view will be held throughout.
I will also observe my results through the top of the glass so the glass doesn’t disillusion the image or affect my judgment.
The last thing I would do to ensure reliability is to do the same experiment on the same day in the same lesson so the temperature doesn’t affect our results.
This is my blank results table
This is the axes of the graph I will use to plot my results
Rate x1000
Concentration (g/dm3)
This is my completed results table
I will plot the following points on a graph
Trend
The trend of the graph shows that the concentration is proportional to the rate of reaction which is shown by the straight line through the origin. This means as the concentration increases the rate of reaction increases.
Conclusion
I think that as the concentration increased the rate of reaction also increased this down to the reactant particles in concentrated solutions. I am going to refer back to my background info to come up with the final conclusion.
I previously stated that in concentrated solutions there are more reactant particles per unit volume and therefore more collisions between the reactant particles, this increases the rate of reaction. So increasing the concentration, it would lead to more collisions and maybe even with greater force. The collisions explains how chemical reactions occur, it states that reactions can only happen when the reactant particles collide, so there will be more chance of fruitful collision.
E.g. if the was double the amount of concentration, it would mean double the amount of reactant particles! So more collisions will take place so the reaction will be over quicker.
I believe my method was the best way of doing the investigation-because I did the experiment twice to gain more accurate results. However if I were to do the experiment again, the changes I would make are as follows: I would us more accurate means of measuring my solutions and by using dry conical flasks for each experiment. I would change these as this could again improve accuracy.
I do not believe that my results were accurate enough, however I do believe that they clearly show a correlation and prove a theory.
