Temperature
By altering the temperature of a reaction you can affect or control the rate at which it occurs. If we look at temperature, we know that the motion of the particles will increase because they have gained more energy. This will lead to an increase in the number of collision increasing the rate of the reaction.
Pressure
In gaseous reactions, changing the pressure will alter the rate at which it occurs. Increasing the pressure will increase the density of the number of particles per unit volume leading to more collisions at a faster rate as the particles have less room to move so more chance of coming into contact with each other and reacting.
Surface area
When solids are reacted, the surface area that is exposed to the reactant will greatly affect the rate of the reaction. If two equal weights of solid, one in the form of a fine powder and one in the form of one large lump are added to equal amounts of solution the powder will dissolve quicker because more surface area is exposed to the solution. This can be easily understood when looking at sugar. One large lump of sugar will dissolve much slower than caster sugar because more surface area is exposed to the reactant. In addition, the large lump is much harder to break down that the finer caster sugar.
Catalysts
Theses are chemicals, which are added in a reaction to increase the rate, which it occurs at. Yet they are not actually converted or consumed in the reaction. Specific catalysts are used for each type of reaction. They may often achieve the same product of yield as the normal reactants but with a lower temperature or pressure. Industry finds catalysts very useful and makes good use of them. Lower temperatures and pressure will always be favourable, economical, and safer.
I think the most important factor that affects the rate of the reaction is the concentration because an increase or decrease in the concentration of the reactant will have a huge affect on the rate at which it occurs. I also think that the temperature is another important factor to control.
Investigating the rate of the reaction between Sodium Thiosulphate and hydrochloric acid.
Aim; The aim of this investigation is to see how the concentrations of reactants effect the rate at which a reaction occurs.
The reaction between Sodium Thiosulphate and hydrochloric acid can be used to investigate how the concentration of reactants used can effect the rate at which a reaction occurs. When the two chemicals react, they form a yellow precipitate turning a transparent solution into an opaque one.
By using a beaker with a marked piece of paper positioned underneath it we can react the two chemicals of different concentrations and time the duration of the reaction. The pattern (in this case a cross) will disappear after a specific quantity of Sulphur suspension has formed.
Prediction:
I think that increasing the concentration of the reactants will increase the rate at which the reaction occurs because I know that the greater the concentration the greater the number of reactant species available. Increasing the concentration will therefor increase the collision frequency. Increasing the collision frequency will therefor increase the chance the particles have of reacting. I think that doubling the concentration of the reactants will double the rate at which the reaction occurs this can be explained when looking at the concentrations more closely. If the concentration of hydrochloric acid is doubled and the Sodium Thiosulphate is kept at the same concentration then there are twice the number of Hydrochloric acid molecules in the solution when compared with the Sodium Thiosulpahte. This means the ratio of Hydrochloric acid to Sodium Thiosulphate is 2:1, which means there is a one In two chance of the coming into contact with the Sodium Thiosulphate particles and reacting. I think the Speed of the reaction will be proportional to the concentration.
(Information obtained from GCSE Text book by B.EARL AND L.D.R Willford, AirCom Education CDRom and Compton’s Interactive Encyclopaedia)
Pre test:
To ensure that the correct range of the concentration of the reactant is used in the experiment I will complete a pre test to check. I will do this by measuring out the Sodium Thiosulphate to concentrations of 20%, 40%, 60%, 80% and 100% and adding them separately to 100% concentrations of hydrochloric acid and time how long it takes for the solution to change from transparent to opaque.
I will then change the experiment by changing the concentration of the Hydrochloric acid to the concentrations mentioned above and add the to a 100% concentration of Sodium Thiosulphate.
Pre Test results.
Concentration ( % )
Time taken (sec) for cross to disappear
These results tell me that the different concentrations of Sodium thiosulphate I have decided to use should be fine for the experiment. The results are in line with my prediction although as the concentrations of Hydrochloric acid were changed it seemed it made little or no difference to the results. This could be due to a number of factors, which will need to be controlled during the experiment to ensure reliable results are gained. This anonymous result could have been caused by inaccurate timing, reading of the time or not rinsing out the beaker thoroughly enough (leaving some of the reactants from the previous test could give inaccurate results because it would affect the concentrations of the new one). These results may have been due to the concentration of Hydrochloric acid being too low in the first place. At 0.4m p/litre. To prevent this from happening in the actual experiment I will use 1M P/ litre of Hydrochloric acid. All of the above will need to be considered when doing the actual experiment.
Factors to control
To ensure that reliable results are gained form this experiment there are a number of factors which need to be controlled, these include:
∙ Only changing the concentration of the reactant to the required measurements there for I will need to …
∙ Keep the temperature of the substances the same. Changing the temperature could effect the rate at which the reaction occurs. Increasing the temperature increases the kinetic energy the molecules have, therefor increasing the chance of collisions between the particles. Decreasing the temperature would have the opposite effect.
∙ The volume of the container needs to be kept the same. Increasing or decreasing would effect the rate at which the particles collide and react with each other. This is because the change in the volume of the container results in a change of the area the particles have to move.
∙ Use the same marked piece of paper, which is positioned under the beaker because the shading of the cross may effect the rate at which it disappears. Darker crosses may be a lot easier to see through the solution than lighter shaded ones.
∙ For each concentration, I will need to take the results three times and get an average of the time taken (in seconds) for the cross to disappear. This will give me more reliable results.
Equipment
2x 25cm3 measuring 1x White tile (with cross drawn)
cylinders
1x 50cm3 glass beaker The chemicals used for experiment
1x Stop clock
Method:
∙ On white certamic tile draw a large cross in the centre and place a 50cm3 beaker on top of it.
∙ Measure out the different concentrations of Hydrochloric acid starting with 25cm3 to 5cm3(see table). With this, add the amount of water needed to create a total solution volume of 25cm3 e.g. if 15cm3 are measured out add 10cm3 of water etc.
∙ In turn, add the solutions to a beaker of 25cm3 of Sodium Thiosulphate that has been placed on the piece of paper with a cross on it. In-between tests ensure the beaker is thoroughly rinsed and dried to remove all reactants from the previous tests.
∙ Record the time (in seconds) it takes the solution to turn opaque and the cross to disappear.
∙ Repeat the experiment this time changing the concentration of the Sodium Thiosulphate making sure the total volume of the solution equals 25cm3 and add it to 100% concentration Hydrochloric acid.
∙ Repeat the whole experiment 3 times and record the average time (in seconds) it takes for the cross to disappear at each concentration of Hydrochloric acid and Sodium Thiosulphate.
The Results Table
