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.
At a lower temperature the particles have less energy therefore they move more slowly and have less chance of colliding. At a high temperature the particles move more vigorously and have more chance of colliding and reacting.
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. The particles are closer together and have more chance of coming into contact. There is more room in the sealed container so the particles have less chance of coming into contact.
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.
For a chemical reaction to take place, some bonds in the reactants must be broken. The colliding particles must have enough energy to break these bonds. This minimum amount of energy is called the activation energy. Only the very fastest moving particles have enough energy to break bonds.
In gases, liquids and in a solution, the particles move at a range of speeds. Some are moving very slowly and others are moving very fast. To react, particles must collide with enough energy and in the correct orientation for bonds to be broken.
Prediction
I predict that the higher the concentration of Sodium Thiosulphate solution the faster it will react with the hydrochloric acid. Firstly for a reaction to take place the particles must collide and secondly collide with enough energy to break and make new bonds. When the sodium thiosulphate is concentrated there are more particles in a given volume, therefore there is a greater chance of the particles successfully colliding and reacting with the hydrochloric acid. The more collisions between particles in a given time, the faster the reaction will take place.
Below is the relevant equation: -
Sodium thiosulphate + hydrochloric acid sulphur + sulphur dioxide + sodium
chloride + water.
Na2S2O3 (aq) + 2HCl (aq) S (s) + S2O (g) + 2NaCl (aq) + H2O (L)
Preliminary
I drew a black cross using a black biro on a piece of supplied paper, and collected a stop watch, a conical flask, and three measuring cylinders. Throughout my experiment, I would be having a total concentration of 0.05dm3 , so as to ensure a fair test. In each experiment, there would be 10cm3 of 2 molar of HCl, as this is a fixed variable. I would have different amounts of 20g/dm3 Sodium Thiosulphate, and to make sure the concentration totalled 0.05dm3 , varying amounts of H2O. I mixed the different liquids with different volumes, and placed the conical flask containing them over the black cross. Looking directly above the cross, with my goggles on, I counted the time it took for the cross to disappear due to the opaque solution. My results are on the following page.
