19.1-Two rates experiments

Core Practicals

• It is important in this core practical to use appropriate apparatus to make and record a range of measurements accurately, including time, temperature and volume.

  • This includes the safe use of apparatus, and monitoring chemical changes.

  • This outlines one way to carry out the practical. Eye protection must be worn.

Aims

To investigate the effect of changing the temperature on the rate of a reaction.

• Sodium thiosulfate solution reacts with dilute hydrochloric acid:

  • sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur

  • Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

• The sulfur forms a cloudy yellow-white precipitate during the reaction.

• The time taken for this to achieve a given cloudiness provides a way to measure the reaction time.

Method

1. Using a measuring cylinder, add 50 cm3 of dilute sodium thiosulfate solution to a conical flask.

2. Place the conical flask on a piece of paper with a black cross drawn on it.

3. Using a different measuring cylinder, add 10 cm3 of dilute hydrochloric acid to the conical flask. Immediately swirl the flask to mix its contents, and start a stop clock.

4. Look down through the reaction mixture. When the cross can no longer be seen, record the time on the stop clock.

5. Measure and record the temperature of the reaction mixture, and clean the apparatus as directed by a teacher.

6. Repeat steps 1 to 5 with different starting temperatures of sodium thiosulfate solution.

Results

• Record the results in a table.

• This table gives some example results.

19.2-Finding reaction rates from graphs

Representing Rates of Reaction

• Graphs can represent rates of reaction.

• We can draw a graph of product (or reactant) against time in order to represent the rate of reaction.

• The gradient of the graph will be the rate of reaction.

  • The steeper the gradient, the faster the rate of reaction.

• Mean rate of reaction can be calculated.

• If we work out the overall change in y value (i.e. product formed or reactants used up) then divide by the total time taken for the reaction, we can calculate the mean rate of reaction.

Using Graph to make calculation

1.Identify the values on the graph.

• Identify the final volume of gas made from the graph, and the final time taken to make this volume.

• This is shown by the lines in red. The final volume = 70 cm3 and the time taken to make this volume is 80 seconds.

2. Write out the equation. Now, we can write out the equation to calculate a value for the rate of reaction.

• Mean rate of reaction = overall change in volume/change in time

3. Substitute in the numbers.

• Now that we have identified the correct equation, we can substitute in our numbers.

Mean rate of reaction = (70)/(80)

Practice Questions:

• After every how many degrees we find increase in temperature when the rate of reaction is doubled?

  • 10 degrees

• What signifies a faster rate of reaction

  • Steeper gradient on a graph

• How many conical flasks are required for the experiment?

  • 5 conical flasks

• What is the concentration of Hydrochloric acid used in the experiment?

  • 1M

• What quantity of conical flask must be taken for this experiment?

  • 250mL