Rate of Reaction between Hydrochloric Acid and Sodium Thiosulphate.

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Katherine Allen

Rate of Reaction between Hydrochloric Acid and Sodium Thiosulphate

Aim

The aim of this investigation is to find out how the concentration of a solution affects the rate of reaction. I am going to investigate how the concentration of sodium thiosulphate affects the time it takes to react with hydrochloric acid.

Introduction

When hydrochloric acid and sodium thiosulphate solution react, one product of the reaction is sulphur. The sulphur is produced and appears slowly, eventually resulting in the whole mixture becoming opaque.

The word equation for this reaction is:

sodium thiosulphate

+

hydrochloric acid

-->

sodium chloride

+

sulphur dioxide

+

sulphur

+

water

The chemical equation is:

Na2S2O3 + 2HCl --> 2NaCl + SO2 + S + H2O

There are several factors affecting the rate of reaction. These include the surface area, the temperature, the concentration and catalysts.

Surface area

The larger the surface area, the more collisions can occur. The reaction is therefore faster.

This is not a factor that I need to worry about, considering it is liquids that I will be reacting.

Temperature

In most reactions, bonds in reactant molecules must first be broken before bond in product molecules can form. The minimum energy required to break the bonds is called the activation energy. If molecules hit each other slowly, they may not have enough energy to react. Heating will mean more there is more energy to be converted into kinetic energy, allowing the molecules to move faster, so more collisions are likely to result in a reaction. A thermometer will therefore be kept near to where the experiment is being carried out, to monitor the temperature and to ensure no change has occurred.

Catalysts

A catalyst speed up a reaction without being used up itself. It weakens the bonds in the reactant molecules, so less energy is needed in a collision to make the molecules react. More molecules will therefore have the energy required.

No catalysts will be added at any point throughout the experiment.
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Concentration

It is the concentration that I will be investigating and is not therefore a variable that needs to remain constant.

Prediction

I predict that as the concentration of the sodium thiosulphate increases, the time taken for the cross to disappear will decrease. Furthermore, I predict that this will be inversely proportional, so as the concentration of the sodium thiosulphate is doubled, the time taken for the cross to disappear will be halved. For example, if the time taken for 40cm³ of sodium thiosulphate to react with the hydrochloric acid, making the liquid opaque was ...

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