What Happens When Hydrochloric Acid and Sodium Thiosulphate React?

by Lucy Bell-Young

When hydrochloric acid (HCl) and sodium thiosulphate (Na2S2O3) are combined, an interesting reaction takes place and the colourless solution turns opaque. But why does this happen, and how can we use this opacity to determine the rate of reaction?

The Reactants

The chemicals used in this experiment are both extremely important in their own areas. In case you missed our previous posts, hydrochloric acid is a strong acid that plays an important role in a range of industries. From regenerating cation exchange resins to neutralising the pH of swimming pools, it is a workhorse chemical that is used in nearly every industry.

Sodium thiosulphate is a chemical that has been classified by the World Health Organisation as one of the most effective and safe medicines needed in the health system. An efflorescent compound that appears as a colourless pentahydrate, sodium thiosulphate is used as a medication for things like cyanide poisoning and pityriasis versicolour.

While these compounds have crucial impacts in their separate applications, when they come together they provide a perfect example of how the rate of a reaction increases, decreases and how it can be measured.

What is the Rate of a Reaction?

A reaction happens when particles collide, resulting in the reactants getting consumed and new products getting formed. Therefore, in order for a reaction to be successful, the collisions have to have sufficient energy. The greater the number of particles, the more energy these collisions will create. This means that the concentration of the reactants directly affects the energy of a reaction.

With this in mind, the rate of a reaction can be defined as an increase or decrease of concentration in any one of the reactants or final product.

As the concentration of a reactant increases, for example, the number of reacting molecules increases. This means that there is a greater number of collisions which leads to a quicker reaction time and a larger rate of reaction.

Therefore, although there is an inverse relationship between concentration and the rate of a reaction, it is a relationship that is directly proportional. This concept is best demonstrated by the reaction between hydrochloric acid and sodium thiosulphate.

The rate of reaction between hydrochloric acid & sodium thiosulphate
When hydrochloric acid and sodium thiosulphate react, the solution turns cloudy. You can measure the rate of the reaction by altering the concentration of sodium thiosulphate and measuring the time it takes for the solution to turn fully opaque.

The Reaction

When sodium thiosulphate is added to a solution of hydrochloric acid, an insoluble precipitate of sulphur (S) is formed. Sulphur dioxide (SO2) and water (H2O) are also formed, but it is the solid sulphur that has the biggest impact here.

The sulphur is a colloid in this reaction, staying in suspension and eventually blocking the light from reaching the solution. This transforms the solution from being colourless to being milky and entirely opaque. This happens because of the precipitates of elemental sulphur that are being formed, which are insoluble and eventually cloud the water. You can see this by drawing an X on a piece of paper, placing it under your beaker and watching as it begins to disappear.

If the concentration of sodium thiosulphate is high, the solution will cloud fairly quickly (generally between 15-30 seconds). If the concentration of sodium thiosulphate is low, then it will take longer for the reaction to occur. This is how you can measure the rate of the reaction.

Measuring the Rate of Reaction

The rate of the reaction can be studied by measuring the opaqueness of the solution against the time taken for it to change. Changing the concentration of sodium thiosulphate will change the time it takes for a certain amount of sulphur to form and, therefore, how long it takes for the solution to turn cloudy.

You can lower the concentration of sodium thiosulphate by diluting it with distilled water. This will reduce the number of NasS2O3 particles which ultimately means fewer collisions. The sulphur precipitates will then appear at a lower rate. This means a longer reaction time and smaller reaction rate.

Comparatively, a reaction that uses a very low concentration of sodium thiosulphate may take up to 5 minutes for the solution to become fully opaque.

A woman looking at test tubes in a lab


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