The Influence of Temperature on Reaction Rate in the Reaction between Sodium Thiosulphate and Acid.

Emily Hatton 11MB

July – October 2001

Plan

When sodium thiosulphate (Na S O ) reacts with hydrochloric acid (HCl) sulphur precipitates throughout the solution. It becomes cloudy and eventually you cannot see a black cross on a white piece of paper held underneath the beaker. Sulphur precipitates and doesn’t dissolve as sulphur is insoluble, the sulphur is in suspension.

Sodium Thiosulphate + Hydrochloric Acid  Sodium Chloride + Sulphur + Water + Sulphur Dioxide

Na S O (aq) + 2HCl(aq)  2NaCl(aq) + S(s) + H O(aq)+ SO (g)

I am going to investigate whether changing the temperature at which the solution is heated at will increase the rate of reaction. The rate of reaction is how long it takes for the reactants to turn in to the products of the solution.

A chemical reaction can only occur between particles when they collide. The particles may be atoms, ions or molecules. This is known as the collision theory. There is a minimum amount of energy, which colliding particles need in order to react with each other, if they have less than this minimum energy then they just bounce off of each other and no reaction takes place. The minimum energy is called the activation energy. The faster the particles move, the more energy they have. These fast moving particles are more likely to react when they collide. One way to make sure particles exceed the activation energy level is to heat the particles. By increasing the temperature particles gain more kinetic energy as the applied heat energy is transferred to kinetic energy. This means that they are moving much faster, giving them a greater percentage of particles colliding with each other per second. Also by moving faster a larger proportion of the particles will have acquired the activation energy needed to react.

Another method for speeding up the rate of reaction is by increasing the surface area of the solid. A solid in solution can only react when particles collide with the surface, the bigger the area of the solid surface the more particles will collide with it per second. You can increase the surface area of a solid by turning it in to a powder. A powder has the largest surface area and will have the fastest reaction rate. This is why catalysts are usually in the form of a powder.

The last method of affecting the rate of reaction is the concentration of the solutions. By increasing the concentration there are more particles per dm to react per second meaning you would speed up the rate of reaction. However decreasing the concentration would do the opposite.

Equipment

As my variable is temperature, to do my experiment I will need to use the following equipment: A Bunsen burner to heat the solution, a 50 cm³ and a 5 cm³ measuring cylinder to measure the correct amount of reactants, also a 100 cm³ beaker, 100 cm³ conical flask. I will need to use a tripod, metal gauze with tongs to hold the solution.

Method

To perform my experiment I will:

Mark a black cross on a white piece of paper

Measure 50cm³ of sodium thiosulphate in to a conical flask (I ...

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Equipment

Method

To perform my experiment I will:

Mark a black cross on a white piece of paper

Measure 50cm³ of sodium thiosulphate in to a conical flask (I will use a conical flask so when I swirl it there are no splashes and loss of solutions)
Followed by 5cm³ of dilute hydrochloric acid
Start the stop clock and when the cross disappears stop the stop clock and record the temperature. This will form my first reading (room temperature)
I will then clean the conical flask thoroughly with water and measure another 50cm³ of sodium thiosulphate.
I will heat this to 30ºC and add 5cm³ of dilute hydrochloric acid, start the stop clock and when the cross disappears take the reading.
I will then repeat this method for temperatures 30ºC, 35ºC, 40ºC, 45ºC, 50ºC, 55ºC, 60ºC.

However I will not exceed 60ºC as the precipitation will become to fast for me to measure and more sulphur dioxide gas will be produced, and as a harmful gas I would have to perform the experiment in a fume cupboard.

Safety

I will wear safety goggles as I am to use corrosive and harmful HCl. As a volume of SO gas is produced I will take care not to inhale the gas and to dispose of it appropriately to reduce the risk. SO is an acid gas and I would expect it to turn red on blue litmus paper.

Fair Test

To ensure a fair test I will need to focus on one variable and only change this. A variable that could change the rate of reaction is the concentration of the solution. If the concentration of the HCl or the Na S O was to increase the more particles per dm would give an unfair advantage and speed up the reaction. This is why I need to be precise about my measurements and use the same measuring equipment each time. To reduce any margin for error I will also use the same conical flask, beaker (so all the measurements are equal), also the thermometer and stop clock although the thermometer may be +/- 0.1ºC and the stop clock +/- 0.1 seconds. The black cross on the white piece of paper will also be kept the same so there is the same depth of colour and there is no bias. I will follow the same plan every time and some odd readings may be +/- 1ºC.

Overall I am going to take 8 results ranging between one at room temperature (between 18ºC and 23ºC) and seven every 5ºC between 30ºC and 60ºC making them accurate by recording them to 1dp. I will repeat my investigation three times to increase the reliability of my results and allow me to take an accurate range, mean and median. Any odd results will be noted, explained and repeated.

Prediction

I expect my results to show that by increasing the temperature every 5ºC the time in which the cross disappears to decrease. As I explained before by increasing the temperature, the sodium thiosulphate particles and the hydrochloric acid particles will gain more heat energy, which is transferred to kinetic energy. Therefore colliding (reacting) more frequently and with more speed.

If I were to plot my results on a graph I would expect the line to increase as the temperature increases but to decrease like the time. Therefore inverse proportionality to occur. As a general guide at some point I expect the rate of reaction to double therefore the gradient of the plot become twice as steep.

Trial Run

In my trial run I only investigated the extremes of temperature (23ºC and 60ºC). My results were as follows:

Showing that my plan is accurate and works, also that prediction is correct. I need to make sure that I am precise with my measurements and I need to heat the thiosulphate to + 1ºC of the interval temperature as when I add the HCl it cools it down.

Obtaining Evidence

Analysing Evidence

Looking at my graph I can see that as the temperature increases the time needed for the cross to disappear decreases meaning a faster reaction occurs. I can also see that the results are formed in a curve and not in a straight line, this shows that the millions of particles heated at the same time are not directionally proportional to a small increase in temperature.

My table also shows clearly that the time for the cross to disappear decreases as the temperature increases. From my results I have come to the conclusion that if the temperature of a solution is raised the time for the reaction to occur decreases. The cross disappeared more rapidly as the temperature rose and I think this was due to the increase of energy between the particles and an increase in energy between collisions that successfully passed the energy barrier. This released the sulphur quicker and in larger doses, which turned the solution cloudy thus making the cross invisible.

At the lower temperatures the time for the cross to disappear was less because the particles did not have as much energy as they did with the higher temperatures. At these temperatures the particles are colliding with much more energy thus the reaction that releases sulphur works and now releases more sulphur at a much quicker rate thus increasing the rate at which the cross disappears.

My original prediction was that if you increase the temperature of a reaction, you decrease the time it takes to occur. And from looking back at my results I can see that this hypothesis was correct as the time for the cross to disappear decreased as the temperature rose. My conclusion matches my prediction very well overall and my results clearly show this where at room temperature (23˚C), the time for the cross to disappear was 88/89 seconds and at 60˚C the time for the cross to disappear was 10 seconds (average) a difference of 78/9 seconds. The particles were moving around with more energy; enough to break the energy barrier and for a reaction to occur as the temperature rose.

My results could be developed further by working out the rate of reaction. I can do this by plotting the reciprocal. I will plot the reciprocal using only the average results. Because the same amount of sulphur was precipitated throughout the experiment I will calculate the rate of reaction like this:

(Rate of reaction ) 1 .

Time for cross to disappear

However if the amount of sulphur was to vary I would calculate the rate of reaction like this:

Reaction Rate = amount of sulphur precipitated

Time for cross to disappear

The results obtained from this table support my prediction further by showing that as the time for the cross to disappear decreases the rate of the reaction increases meaning gets faster.

Looking at my reciprocal graph I can see that as you increase the temperature that the solution is heated at the rate of reaction increases proving my prediction. Between 55˚C and 60˚C the graph steepens and the average difference of rate of reaction is 0.024 s-¹.

Evaluating Evidence

All of my results fit the pattern except one, the 35˚C result from the second set of readings, I think this was because I didn’t measure the sodium thiosulphate solution accurately enough meaning I added to much thiosulphate in the 35˚C solution thus the cross took longer to disappear.

I think that my experimental method gave very accurate results. I could have improved my experiment by using a data logger because the data logger takes readings every second whereas I was only taking readings when the cross disappeared. Apart from this the method I used was very successful and because I stuck to plan there were no results that could not be explained. The results obtained by this experiment are reliable as I have a graph, table and rate of reaction table all supporting the same prediction and results. Although I could expand these further by plotting the reciprocal on a graph using all the readings, not just the averages and also taking further readings. Also repeating any odd results.

I feel that I can make a firm conclusion with my results as the background information matches the prediction also. With more time and money I could extend the investigation by investigating more thorough temperatures and looking at the concentration of the solution too.