To find out how the concentrations of the reactants affect the rate of reaction
Possible variables that could speed up the rate of a reaction are;
Chosen Variable= Concentration=Independent Variable
Reactions can go at all sorts of different rates e.g. a slow reaction would be the rusting of iron and a really fast reaction is an explosion. The speed of a reaction can be observed either by how quickly the reactants are used or how quickly the products are forming. For this experiment I will be observing how quickly the reactants are forming (how quickly the solution turns milky white), this is because this method is a lot easier to measure. This is called the precipitation method. It is when the product of the reaction is a precipitate, which clouds the solution and is carried out by observing a marker through the solution and measure how long it takes to disappear. Other methods include finding out the change in mass by using a mass balance and working out the volume of gas given off with the use of a gas syringe.
Reaction rates are explained by the Collision Theory, which explains that the rate of reaction depends on how often and how hard the reacting particles collide with each other. The particles have to collide with each other, hard enough in order to react. Therefore more collisions increase the rate of reaction. The factors that affect rate of reaction include temperature, increasing the temperature makes the particles move quicker and collide more. At a higher temperature there will be more particles colliding with enough energy to make the reaction happen. The initial energy is known as the activation energy and it's needed to break the initial bonds. However in this experiment we are not using temperature to change the rate of reaction, to stop this factor interfering with the results I will fill a beaker with water to keep the temperature constant. For this experiment we are we are changing the concentration of solution to change the rate of reaction. If the solution is made more concentrated it means there are more particles of reactant knocking about between the water molecules, which makes collisions between the particles more frequent. In a gas, increasing the pressure makes the molecules more squashed up which causes more collisions. Other factors that change the rate of reaction include size of solid particles (or pressure) and adding a catalyst
The reaction that will occur in the test tube will be between the Hydrochloric acid and the Sodium Thiosulphate to produce Sulphur, Sodium dioxide, Sodium Chloride and water. This is shown in the equation below: -
Sodium Thiosulphate + Hydrochloric Acid Sodium Chloride +Water + Sulphur + Sulphur Dioxide
Na2 S2 O3 (aq) 2HCl (aq) goes to 2NaCl(aq) + H2O (l) + S (s) + SO2 (g)
I predict that as the concentration of the Sodium Thiosulphate increases the rate of reaction will also increase and the time taken for the cross to disappear will decrease. This means that the graph will probably be straight, as the increase in rate of reaction will parallel the increase in concentration, for example the line of best fit should go through 0, because if there is 0 g/l concentration there should logically be 0 rates. The graph showing the rate of reaction will have a positive correlation showing that the higher rate of reaction increases, as the concentration gets higher. This can be justified by relating to the collision theory. If solutions of reacting particles are made more concentrated there are more particles in the volume. Collisions between reacting particles are therefore more likely to occur.
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. Only particles with enough energy can react after colliding. The minimum energy that a particle must have to be able to react...