Reaction between sodium thiosulphate solution (Na2S2O3) and dilute hydrochloric acid (HCl)

Increasing the surface area of a solid reactant increases the reaction rate.  The H+ ions (acid particles) can only collide with the outer layer of the solid reactant.  It smaller chips of this reactant are used there is a greater surface area exposed to the H+ ions.  With a greater surface area of solid there are more collisions taking place hence the reaction rate is greater.

The rate of reaction increases if you increase the concentration of reactants.  In dilute acids there are not so many H+ ions therefore there is not much chance of an H+ ion colliding with an atom of the other reactant.  But if the acid is more concentrated there are more H+ ions thus there is a greater chance of a successful collision occurring. At the start there are plenty of H+ ions and atoms of the other reactant, but during the reaction they get used up. After a time there are fewer H+ ions and atoms of the other reactant as a result the reaction slows down.

Increasing the pressure of a gas has the same affect on reaction rate as concentration does.  As you squeeze more gas molecules into a given space there is a greater chance of successful collisions.  So we can say that increasing pressure increases the rate of reaction

If you increase the temperature the rate of the reaction will increase.  This is because when temperature is increased the particles have more energy and so they move quicker.  As they travel faster there are more collisions in a given time therefore the reaction get faster as we raise the temperature.  There is a second reason why the rate increases. Some colliding particles don’t bang together hard enough to start a reaction i.e. they don’t have enough energy.  At higher temperature the particles are moving faster consequently they crash together harder and there are more successful collisions.

Catalysts also increase the rate of reaction.  The energy needed to start a reaction is called the activation energy.  A catalyst lowers the activation energy so that a collision needs less energy in order to be successful. Hence more collisions become successful and so the reaction goes faster.

I predict that as the concentration of sodium thiosulphate increases the rate of reaction will increase.  Increasing the concentration of a solution means that there will be more particles per dm3.  The more particles that there are the more will collide per second and so the rate of the reaction increases. The collision theory states that for a reaction to occur particles have to collide with each other.  Not only do particles need to collide but they also need to collide with enough energy otherwise they just bounce off each other harmlessly.  This is called the activation energy and it is needed to break the bonds to get the reaction started.    The rate of a reaction depends on how many successful collisions there are in a given time. In a successful collision bonds are broken (this needs energy) and new bonds are formed (this releases energy).

To make this a fair test I will

• keep the size of the flask the same (so depth of liquid id is the same)
• use the same cross
• measure to the same decimal point i.e. to two decimal places
• use the same total volume of sodium thiosulphate and hydrochloric acid
• use the same standard each time for judging when the x has disappeared
• make sure the measuring cylinders for water, sodium thiosulphate and hydrochloric acid will not be mixed up

I will change the concentration of sodium thiosulphate.  To do this I will keep the total volume of sodium thiosulphate and water the same. When changing the concentrations I will use 50cm3 each time eg. 40cm3 of sodium thiosulphate and 10cm3 of water.

I shall be measuring the time taken for the cross to disappear for a range of concentrations. I will then use my results to calculate the rate of reaction =    1          (1/secs or S-1).  

                 time

To ensure that my results are reliable I will repeat each experiment twice. To guarantee they are accurate I will calculate everything to the same number of decimal places and repeat any strange results that I may find.

All of these precautions will make my final results more reliable and keep anomalies at a minimum thus making the entire investigation more successful.

Apparatus

• Sodium thiosulphate
• Hydrochloric acid
• Water
• 6 Conical Flasks
• 3 Measuring Cylinders
• Filter Paper
• Pencil
• Stopwatch

Diagram

Method

1. Mark the letter X on the filter paper using a pencil
2. Place a conical flask on top of this X
3. Measure 50cm3 of Sodium Thiosulphate using 1 of the measuring cylinders
4. Add to the conical flask
5. Using another measuring cylinder measure 15cm3 of Hydrochloric acid
6. Add to the conical flask, swirl the contents and start the stopwatch immediately
7. Stop the stopwatch when the mixture has turned sufficiently cloudy so that the letter X can no longer be seen
8. Record the time in your results table
9. Repeat the experiment for all of the six concentrations, using the last measuring cylinder to measure out the water
10. Then repeat the whole procedure so that there are two sets of results for each concentration and take the average
11. However if one of the sets of results is entirely different to the other, perform a third experiment to replace the anomalous result.

A pair of goggles will be worn during the experiment in order to protect the eyes.  And as an extra safety measure bags will be put away and stools pushed below benches.

I will record my results in a table, like the one below.