Effect of concentration on rate of reaction experiment sodium thiosulphate and hydrochloric acid

21st June 2018 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 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.

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Aim: To investigate how the rate of reaction between Sodium Thiosulphate and Hydrochloric acid is affected by changing the concentration.

Background:

THE REACTION: when Sodium Thiosulphate reacts with hydrochloric acid sulphur is produced. The sulphur forms in very small particles and causes the solution to cloud over and turn a yellow colour. This causes the cross to fade and eventually disappear.

Sodium Thiosulphate + Hydrochloric acid »» Sulphur + Sodium Chloride + Sulphur Dioxide + Water

NA2S2O3 + 2HCL »» S + 2NaCl + SO2 + H2O

(aq) + (aq) »» (s) + (aq) + (g) + (l)

PREDICTION: As the concentration of Sodium Thiosulphate increases the length of time for cross to disappear decreases (inverse). This is because the increase of concentration of Sodium Thiosulphate will increase the rate of reaction between Hydrochloric acid and sodium Thiosulphate particles.

SCIENTIFIC REASONS FOR PREDICTION: the results from preliminary experiments support the prediction made.

From the results you can see that there is a directly proportional relationship between the concentration and the rate of reaction. If you increase the concentration then the rate of reaction will also increase.

This experiment is testing how the rate of reaction is affected when concentration is changed. The theory is said that increasing the concentration can increase the rate of reaction by increasing the rate of molecular collisions. The phenomenon behind all of this is the collision theory and how it plays a big role in this investigation. The higher the concentration the less time/faster it will take for the system to turn into equilibrium, and if concentration id decreased, time taken for the solution to go cloudy increases.

Hypothesis: The higher the concentration the faster the rate of reaction will be and the time taken to reach equilibrium will decrease. A more diluted concentration will have a longer rate of reaction and a longer time to reach equilibrium.

Method:

Gathered all the apparatus needed for the experiment.

Using a weight balance we measure out 8g of Sodium thiosulphate, that we added too 200cm³ of water. We mixed the solution until all the crystals were dissolved.

Then you pour 50 cm³, 40 cm³, 30 cm³, 20 cm³, and 10 cm³ of the solution into five identical conical flasks. Then you add water to the other conical flasks so that the total volume in each flask in 50 cm³. Make sure to label the flasks so you know which one has so much concentration.

Once that’s done, you must now take a beaker and add 35 cm³ of concentrated Hydrochloric acid to 65 cm³ of water to make a diluted solution.

Now take a piece of paper and draw a black cross on it, and then place one of the flasks on the paper (do one flask at a time). Using a measuring cylinder measure out 5 cm³ of the hydrochloric solution, and add this to the flask. Immediately stir the flask and start the stop watch. One person should do this part.

As soon as you can’t see the cross any more stop the stopwatch, and record the results in a table. Repeat this with all the flasks.

Results:

The rate of reaction is measured by dividing 1 by the time taken for the reaction to take place.

Number of moles of sulphur used: n= m/M

n= 8/32 = 0.25 mols

Discussion:

You can see from the graph that as concentration increases, the time taken for the solution to go cloudy decreases. So the stronger the concentration the faster the rate of reaction is. As the concentration of sodium Thiosulphate decrease the time taken

for the cross to disappear increases, this is an inverse relationship.When equilibrium was reached the solutions turned a yellow color, the stronger the concentration was the higher the turbidity was. When equilibrium was reached SO2 gas and water were released. The more concentrated solution has more molecules, which more collision will occur. So therefore the rate of reaction should depend on how frequently the molecules collide, so more molecules have greater collisions and the reaction happens faster as more products are made in a shorter time. All related to the collision theory.

What we saw what happened was exactly what we expected from the experiment. Our predictions were accurate.

Evaluation:

The method we used was fairly accurate, our results weren’t perfect but they were good enough for us to see what happens during the experiment. So overall the results proved the hypothesis and I was able to draw graphs with a line of best fit. In our experiment we keep the HCL a constant, and also keeping the volume of the solution was important to get more accurate results. The results were fairly reliable under our conditions. They could be a bit off from bad measuring, unclean equipment and the timing.

Conclusion:

When the concentration of Sodium thiosulphate was increased the rate of reaction increased and the time taken to reach equilibrium decreased, so therefore the rate of reaction is directly proportional to the concentration.

Bibliography:

azete.com/view/48253 6 September 2009

woodrow.org/teachers/ci/1986/exp19.html 9 September 2009

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