Chemical Changes and Structure

1. Chemical Changes and Structure This unit studies how chemist can control the rate of chemical reactions, and the enthalpy changes that take place. It studies trends in the periodic table and investigates the relationship between the arrangement of elements in the periodic table and their bonding, structure and properties. The unit also considers polar covalent bonds in the context of the bonding continuum, followed before studying intermolecular forces.

2. Chemical changes and Structure From previous work you should know and understand the following: Collision theory Atomic structure Electron orbital's or energy levels Valency Covalent and ionic bonding Physical properties of metals.

3. Controlling the rate Overview Learn to explain how a number of key factors can influence reaction rate, using the collision theory.

4. a) Collision theory Learning intention Learn how chemists control reaction rates by careful consideration of the influence of concentration, pressure, temperature, surface area and collision geometry.

5. Rates of Reaction Reactions happen at different rates. Industry needs to control reaction rates to increase production and get a good return for the investment Rates may need to be controlled for safety, or to keep the rate of production within the limit of the plant

6. Collision Theory For a chemical reaction to occur, reactant molecules must collide. The collision must provide enough energy to break the bonds in the reactant molecules. The collision must also occur with the correct geometry. Then new chemical bonds form to make product molecules.

7. Progress of a Reaction Reactions can be followed by measuring changes in concentration, mass and volume of reactants and products. A. Where is the reaction the quickest? B. Why does the graph level off? Rate No more products formed. C. Why does the graph curve? A The concentration of the reactants decrease with time. time

8. Rates of reaction The rate of reaction can be followed by measuring changes in Concentration Mass Volume of gas produced

9. Measuring reaction rates Products Change in mass (g) Reactants time (s) change in mass of product or reactant Average rate of reaction = time interval Unitsg s-1

10. Measuring reaction rates Products Change in volume (cm3) Reactants time (s) Average rate of reaction = change in volume of product or reactant in time for the change to occur Units cm3s-1

11. Measuring reaction rates Products Change in concentration (mol l-1) Reactants Time (s) Average rate of reaction = change in concentration of product or reactant time interval Units mol l-1 s-1

12. Effect of surface area

13. Effect of surface area 4X4= 16 cm2 2x2 = 4 cm2 24X8= 192 cm2 16x6=96 cm2 4X6= 24 cm2 Particle size, the smaller the particles, the greater the surface area, the greater the chance of successful collisions.

14. Rate and Particle Size Higher Chemistry Eric Alan and John Harris Only the particles on the surface of a solid can be involved in a collision Crushing a solid increases the surface area more particles are available for collision therefore increased rate of the reaction

15. Effect of surface area Hydrochloric acid reacts with marble chips (calcium carbonate) 2HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O (l)

16. How can we follow the reaction? A gas is produced. What will happen to the gas if there is no lid on the container? What will happen to the mass? How can we follow the rate?

17. What to do You are going to follow the rate of the reaction by Measuring the volume of gases produced over time Measuring the loss of mass over time

18. Measuring rate of reaction Two common ways: 1) Measure how fast the products are formed 2) Measure how fast the reactants are used up

19. How can we follow the reaction? If we use a container fitted with a delivery tube we could measure the amount of gas produced. How?

20. What to do - Group 1 Measure 25 cm3 of 2 mol l-1HCl into a conical flask fitted with a stopper and a delivery tube Set up an inverted measuring cylinder of water to collect the gas Add 2g marble chips to the acid Measure the volume of gas every 10 seconds Repeat with 2g crushed marble chips

21. What to do - Group 1 Record your results in a the table. Plot a graph of volume vs time using the same axes for both sets of data rate = change in volume ( the unit is cm3 s-1) time interval Calculate the rate for the 1st and 2nd 25 seconds for each set of results

22. What to do – Group 2 Weigh out 2 g marble chips Measure 25 cm3 1 mol l-1HCl into a conical flask Place on balance and zero it Add 2g marble chips to the boat. Now add it to the acid and take mass readings every 10 seconds Repeat using crushed chips

23. What to do – Group 2 Record your results in a the table. Plot a graph of volume vs time using the same axes for both sets of data rate = change in mass ( the unit is g s-1) time interval Calculate the rate for the 1st and 2nd 25 seconds for each set of results

24. Swap results Each group should have a sets of results which can be used to plot graphs.

25. Method 1 results Plot the results on a graph with time on the x axis and volume on the y. Use the same set of axes for both sets of results. 0 14 26 36 44 50 50 50 50 0 22 40 48 50 50 50 50 50 Sample results

26. Volume of gas cm3 Time (s) Work out the rate of reaction over the first 25 seconds and the second 25 seconds using the formula rate = change in volume = _____________ cm3 s-1 time interval

27. Method 2 results • Plot the results on a graph with time on the x axis and mass on the y. Use the same set of axes for both sets of results. 2.0 1.4 1.0 0.7 0.5 0.35 0.3 0.25 0.25 2.0 0.7 0.4 0.3 0.25 0.25 0.25 0.25 0.25 Sample results

28. Loss in mass (g) Time (s) Work out the rate of reaction over the first 25 seconds and the second 25 seconds using the formula rate = change in mass The answer will have the units g s-1 time interval

29. Effect of concentration

30. Rate and Concentration for a reaction to take place the particles must collide Increasing the concentration of a solution increases the number of particles in the same volume. Therefore more chance of collision i.e. increased rate of the reaction

31. Effect of concentration The higher the concentration, the more particles in a given space, the more chance there is of successful collisions.

32. Your challenge is to create a series of solutions that will change colour in time to music http://www.youtube.com/watch?v=rSAaiYKF0cs Effect of concentration –the chemical clock challenge

33. The iodine clock reaction changes from colourless to blue/black http://www.syngenta.com/country/uk/en/learning-zone/science-lab/experiments/Pages/Chemistry.aspx Effect of concentration –the chemical clock challenge

34. You will carry out the reaction using a series of dilutions of the iodide solution. This will be diluted by replacing some of the volume with water. Effect of concentration –the chemical clock challenge

35. Units s-1 1 Relative Rate = t Effect of concentration –the chemical clock challenge 2I-(aq) + H2O2(aq) + 2H+(aq) 2H2O (l) + I2(aq) I2(aq) + 2S2O32-(aa)2I-(aq) + S4O62-(ag) The reaction mixture stays colourless as the iodine molecules are converted back to iodide molecules by the thiosulphate ions. Once all the thiosulphate ions have been used, a blue black colour appears suddenly as iodine reacts with starch. tbeing a measure of how long it takes for the blue/black colour to form. (when excess I2 forms)

36. 1) Using syringes measure out 10cm3 sulphuric acid 0.1moll-1 10cm3 sodium thiosulphate 0.005moll-1 1cm3 starch solution 25cm3 potassium iodide solution 0.1mol l-1 Into a dry 100cm3 beaker 2) Measure out 5cm3 of hydrogen peroxide 0.1moll-1 into a syringe. Add it to the mixture as quickly as possible and start the timer. 3) Stop the clock when the mixture suddenly turns dark blue. 4) Repeat, using 20 cm3 of potassium iodide solution and 5cm3 of water with, then usingrepeated dilutions Effect of concentration –the chemical clock challenge

37. Effect of concentration –the chemical clock challenge

38. RESULTS - Plot a graph showing the volume of potassium iodide x axis and the rate of reaction on the y axis. Effect of concentration –the chemical clock challenge

39. Listen to the song and identify points where you want to have a colour change come in Time them accurately. Allocate times to each group. Look at your results and check that these are times you can achieve Calculate the rate that each time requires (rate = 1/t) Read off the required concentration from your graph Effect of concentration –the chemical clock challenge

40. Use the relative concentration to help you work out the volume of water and KI(aq) needed to make up 100 cm3 of the required concentration. Good luck! Effect of concentration –the chemical clock challenge

41. Effect of concentration –the chemical clock challenge • State the; • Aim of the experiment. • Method, which variables to control and change. • What to measure and how. • How to record your results. • What graph to draw. • Make a conclusion. • Evaluate.

43. Effect of concentration - Cola challenge Your task is to make up a solution containing the same concentration of sugar as a can of coke which contains 24g of sucrose C12 H22 O11

44. Work out the mass of sucrose required to make up 100cm3 of sucrose solution of the same concentration as cola, assuming there are 24g in 330cm3 Make up your solution. What is the concentration of this solution? Effect of concentration - Cola challenge

45. Concentration (c ) is measured in moles per litre ( mol l-1) no moles = C x V 1000 To calculate the concentration you need to work out the number of mol of sugar present Effect of concentration - Cola challenge

46. No moles = mass (g) GFM GFM = gram formula mass Effect of concentration - Cola challenge

47. Sucrose is a non-reducing sugar – it does not react with Bendicts unless it is first hydrolysed. Boil 10 cm3 sugar solution with 5cm3 1mol/l HCl. Then neutralise the solution with 5cm3 1mol/l NaOH. Allow this solution to cool to room temperature. Repeat with the lemonade solution. Add 2cm3 of Benedicts to each sample. Prepare a beaker of boiling water. Effect of concentration - Cola challenge

48. Add the test tubes to the boiling water If you have made up the solutions correctly, all your solutions should take the same time to change colour. Effect of concentration - Cola challenge

49. Effect of temperature

50. Effect of temperature -the vanishing cross Sodium thiosulfate solution is reacted with acid. A precipitate of sulfur forms. The time taken for a certain amount of sulfur to form is used to indicate the rate of the reaction.