Sodium Thiosulphate Reaction Rates

Sodium Thiosulphate Reaction

Plan

This investigation is about rates of reaction and what affects them. In my experiment I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction. They react together as shown in the equations below:

Sodium thiosulphate + hydrochloric acid → sodium chloride + sulphur + sulphur dioxide + water

Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

A reaction occurs when particles of the reactants meet and combine; this is called the collision theory. To increase the rate of reaction it is necessary to cause more particles to collide harder/at more speed and make it happen more often. There are several ways to do this and these make up the variables for this experiment.

Two examples of reactions are Rusting and a bomb exploding. Rusting is an example of a very slow chemical reaction between Oxygen and (a liquid) most commonly Water on Iron, this results in a substance called Iron Oxide being formed. A Firework is an example of a very fast reaction, the fuse is lit and then the gunpowder ignites causing an instantaneous explosion.

To ensure that I make the experiment a fair test I will have to ensure that I use: -

The same person to time the experiment.
The same person to watch the cross.
The same type of measuring cylinders (25cm3) and the same type of thermometer.

Equipment

Goggles.
Stopwatch.
Kettle.
Thermometer.
A Black Cross.
2x 400cm3 Beakers.
250cm3 Conical Flask.
2x 25cm3 Measuring Cylinders.

Variables

Possible Variables include: -

Changing the temperature of the acid.
Changing the concentration of the acid. (i.e. 2 molar)
Using a catalyst to increase the rate of reaction.

Out of the three variables I have chosen to change the temperature of the acid. I will be using a range of temperatures starting at 25°c increasing up to 60°c. I will not increase the temperature of the acid above 60°c as this could prove to be dangerous.

Method

This is a preview of the whole essay

Method

I will start by measuring 50cm3 of Sodium Thiosulphate using a measuring cylinder and then transfer it to a large (400cm3) beaker. Then, I will place the beaker on top of a large black cross, which should be clearly visible when looking through the beaker from the top. I will heat up some water using the kettle and then half fill a large (400cm3) beaker. Next I will measure 5cm3 of Hydrochloric Acid (2HCl) into a measuring cylinder and then transfer it into a conical flask. I will Insert the thermometer into the conical flask containing acid and then place it into the beaker of hot water (water bath) holding the conical flask at an angle to ensure the base of the thermometer is completely submerged in acid. Once the temperature of the acid reads 60°c I will remove the conical flask from the water bath and read the thermometer again to ensure that the temperature reading is the same. (If the reading drops by even half a degree this must be recorded to ensure the experiment is a fair test.) I will then add the acid to the Sodium Thiosulphate Solution and start the stopwatch once all the acid has been deposited into the Sodium Thiosulphate. I will carefully watch the cross looking through the solution and stop the stopwatch once I am no longer able to see the cross. I will then empty and wash out the beaker used for the reaction and then dry it to ensure no water is present that could alter the concentration of the Sodium Thiosulphate. I will then repeat the experiment again for the remaining temperatures.

Test Range: - 25°c, 30°c, 35°c, 40°c, 45°c, 50°c and 55°c. [I found that starting with the highest temperature and working my way down to the lowest temperature was more efficient as it meant I only had to heat the water once]

Fair Test: - To ensure my results are accurate I will use the same person each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the Hydrochloric Acid and Thiosulphate will not be mixed up. The amount of Hydrochloric Acid will be 5cm3 each time, and the amount of Sodium Thiosulphate will be 50cm3 each time. All of these precautions will make my final results more reliable and keep anomalies at a minimum.

Key Factors

Factors that will affect my experiment include: -

Concentration of the Hydrochloric Acid & Sodium Thiosulphate as these are both soluble in water.
Temperature of the Acid and Sodium Thiosulphate.
The use of a catalyst to increase the rate of reaction.
The use of different containers in which to hold the experiment as surface area affects the rate of reaction.
Increasing the pressure.

All of these will affect the rate of reaction of my experiment. I will have to control the majority of these by ensuring that I ensure that my apparatus are dry and free of water. I will also try and complete the obtaining of the results on the same day to make sure that the temperature of the Sodium Thiosulphate does not change temperature because of atmospheric temperature. I must also use a the same container for observing the experiment throughout the whole experiment (which unfortunately did not happen) The catalyst factor will not be a controlled variable in my experiment as I will have no catalyst to add to increase the rate of reaction.

Collision Theory: - When the temperature of the substances used is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and therefore react successfully.

Activation Energy: - For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.

Prediction

I predict that for every 10°c rise in temperature of the acid the rate of reaction will double and the time taken will half. This means that the graph drawn up in my analysis will have a positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the temperature is increased. This can be justified by relating to the collision theory. When the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and therefore react successfully. For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.

Preliminary Work

For the preliminary work testing temperature we used 5cm3 of Hydrochloric Acid and 50cm3 of Sodium Thiosulphate.

For the preliminary work testing concentration we used: -

20cm3 of Hydrochloric Acid and 20cm3 of Sodium Thiosulphate for experiment 1 and 15cm3 of Hydrochloric Acid and 25cm3 of Sodium Thiosulphate for experiment 2.

From My Preliminary Work I have found that both changing the concentration of the solutions and also altering the temperature of the solution both affect the time it takes for the reaction to occur and thus the rate of reaction. I will be performing the experiment altering the temperature of the acid, as this would be a hard variable to control. I will perform my main experiment in the same way that I have carried out my preliminary work.