Investigation into the effect of concentration on the rate of reactions.

Aim

To investigate the effect of changing concentration of hydrochloric acid on the reaction between hydrochloric acid and sodium thiosulphate.

The collision theory states that when reacting particles collide with each other a reaction occurs, if there is sufficient energy to react. The three ways of measuring the speed of a chemical reaction are:

Measuring the volume of gas production.
Measuring the loss of mass.
Timing the formation of precipitate.

The four factors that change the rate of a chemical reaction are temperature, Concentration, surface area and Catalysts. The higher temperature gives particles more energy so there are more collisions. If the concentration is higher there are more particles, which means there are more collisions. A big surface area means more a greater area for contact and increases the amount of collisions. A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. But, they are specific so different reactions need different catalysts. It will lower the amount of energy needed for a successful collision.

Apart from colliding, particles need to have energy to react. Temperature is the only factor that increases the amount of energy particles have when they collide.

The chemical equation between hydrochloric acid and sodium thiosulphate is:

Hydrochloric + Sodium Sodium + Water +Sulphur + Sulphur

Acid Thiosulphate Chloride Dioxide

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Apart from colliding, particles need to have energy to react. Temperature is the only factor that increases the amount of energy particles have when they collide.

The chemical equation between hydrochloric acid and sodium thiosulphate is:

Hydrochloric + Sodium Sodium + Water +Sulphur + Sulphur

Acid Thiosulphate Chloride Dioxide

When two liquids react to give a solid product this solid product is called a precipitate. In the reaction between hydrochloric acid and sodium thiosulphate a precipitate is formed consisting of sulphur. This can be used to measure the rate of an experiment in two ways. A white precipitate will form and you can test it by looking through and deciding when visibility has gone. Another way is to measure the pH because it will increase or decrease as the reaction takes place.

I predict that as the concentration decreases the longer it will take for the reaction to be completed.

Apparatus:

2x clonical flasks

1x beaker

5x 25cm3 Sodium thiosulphate

5x 25,20,10,15,5cm3 Hydrochloric acid

5x20,15,10,5,0cm3 water

1x syringe

1xpaper with cross.

Method

To start off with I will take 25cm3 of thiosulphate and 25cm3 of hydrochloric acid. I will draw a cross on a piece of paper and place it under the beaker. Then I will put the liquids together. I will set the timer and watch the cross until I can no longer see it and at that time I will stop the timer. I will take 5 readings in my experiment recording the concentration of hydrochloric acid and the length of time taken for the precipitate to form. I will make it safe by wearing protective goggles and disposing of and solutions immediately after use. In my experiment I am changing the concentration. I will keep the temperature and surface area the same and will not use a catalyst.

Results

Test no.1

Test no.2

Test no.3

Average

There is a pattern in my graph and I can see that the relationship between the time and concentration is negative.

Conclusion

I have reached the conclusion that there is a connection between the concentration of hydrochloric acid and the resulting reaction rate. They have a negative relationship. The stronger the concentration the less time taken for the precipitate to form.

The reaction rate changes because the higher concentration means more molecules and therefore more collisions, which results in a faster reaction. That rate varies because the concentration affects how fast or slow a reaction takes place.

Evaluating

The method that I used gave accurate results but I would suggest one change. This would be to use the pH method instead. I think that it may have give more accurate results. All of the factors were properly controlled within the method and I had a stable amount of evidence to meet a firm conclusion. Other experiments I could have done might have been measuring the volume of gas production or measuring the loss of mass.