3. R1 Determine the effect of conc on rate of rxn Answer

10 Name: ___________________________( ) Class: _______ Date: __________ Rate of Reaction Skills: MMO: Manipulation, measurement, and observation PDO: Presentation of data and observations ACE: Analysis, conclusions and evaluation P: Planning Experiment R1: Concentration and Speed of Reaction (Modified from 5074/03/O/N/15) Aim: To investigate the effect of concentration on the speed of reaction Apparatus and chemicals 250 cm3 conical flask Burette 25 cm3 measuring cylinder Distilled water 10 cm3 measuring cylinder P: Aqueous solution of ammonium peroxodisulfate, (NH4)2S2O8 Stop watch Q: Aqueous solution containing iodide ions, I-, and another substance White tile Background When P and Q are mixed together, they react to produce iodine. S2O82- + 2I- → 2SO42- + I2 The addition of starch allows the iodine to be detected as the solution turns blue-black. Procedure 1. Put Q into a burette and use it to measure 10 cm 3 of Q into a conical flask. To Q in the flask, add 2 cm3 of starch solution using a 10 cm3 measuring cylinder. Place the flask and its contents on a white tile. 2. Measure 25 cm3 of P as accurately as possible, using a 25 cm3 measuring cylinder. Pour this volume of P rapidly into the mixture of Q and starch solution in the flask. Start the stopwatch immediately and swirl the contents of the flask, then leave the mixture to stand. When the mixture first turns blue -black, stop the stopwatch. Record the time to the nearest second in column E of the table. 3. Empty the conical flask and rinse it thoroughly with water. 4. Repeat the procedure described in step s 1 to 3 but in step 2 use the different volumes of P and water given in columns C and D of the table. For example in experiment 2, measure 20 cm3 of P as accurately as possible, using a measuring cylinder and then add water to P until the total volume of liquid in the measuring cylinder is 25 cm3. A B C D E Experiment no. Volume of Q / cm3 Volume of P / cm3 Volume of water / cm3 Time taken for blue-black colour to appear / s 1 10 25 0 17 2 10 20 5 23 3 10 15 10 35 4 10 10 15 56 5 10 5 20 115

11 (a) Plot a graph of time taken for the blue-black colour to appear (column E) against volume of P (column C) on the grid below. Draw a line of best fit.

12 (a) Compare the concentration of ammonium peroxodisulfate in experiment 2 and its concentration in experiment 4. The concentration of ammonium peroxodisulfate in experiment 2 is doubled / twice the concentration in experiment 4. (b) Explain, in terms of collisions between reacting particles, why the time taken for the blue-black colour to appear increases as the volume of P decreases. As the volume of P decreases, the concentration of P decreases. The number of particles per unit volume of solution decreases. As such, the particles collide less frequently, and the frequency of effective collisions decreases, causing the time taken to be longer. (c) Explain one disadvantage of using a beaker instead of a conical flask. A beaker has a wider mouth compared to a conical flask. As such, w hen the mixture in a beaker is swirled, spillage of the mixture could occur. (d) Jeremy suggested that for every 10 °C rise in temperature the time taken for the blue -black colour to appear will be halved. Given the same solutions P and Q, describe how you would investigate this suggestion. You can assume that all the apparatus and reagents normally found in a school laboratory are available. You should include the measurements you would take and explain how you would use your results to confirm the suggestion. Note: Use 10 cm3 of P with 15 cm3 of water and 10 cm3 of Q in all the experiments. Suggested steps: 1. Measure 10 cm3 of the same concentration of Q using a measuring cylinder and pour it into a conical flask. 2. Add 2 cm3 of starch solution to the conical flask and swirl the contents of the flask. 3. In a separate measuring cylinder, measure 5 cm 3 of P and 20 cm 3 of water and pour it into a beaker. Measure the temperature of the mixture at room temperature and record this value as x °C. 4. Pour the mixture of P and water to the mixture of Q and starch in the conical flask. Start the stopwatch immediately and swirl the contents of the flask, then leave the mixture to stand. When the mixture first turns blue-black, stop the stopwatch and record the time taken to the nearest second. 5. Repeat Step 3 by placing the beaker contain ing the same volume of P and water in a water bath heated to (x + 10 °C). Add this mixture to Q and starch and record the time taken. 6. The experiment is repeated with three more temperatures, (x + 20 °C), (x + 30 °C) and (x + 40 °C) and the time for the blue-black to appear was recorded. 7. Compare the time taken to see if the timing is halved for every 10 °C rise in temperature to confirm his hypothesis.