Chemical Changes and Structure National 6 Chemistry

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.

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

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

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

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 Then new chemical bonds form to make product molecules.

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

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

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

Measuring reaction rates Average rate of reaction = change in volume of product or reactant in time for the change to occur Units cm 3 s -1 time (s) Change in volume (cm 3 ) Products Reactants

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

Effect of surface area

Particle size, the smaller the particles, the greater the surface area, the greater the chance of successful collisions. 4 X 4= 16 cm 2 16x6=96 cm 2 2x2 = 4 cm 2 24 X 8= 192 cm 2 4 X 6= 24 cm 2

Rate and Particle Size 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 Higher Chemistry Eric Alan and John Harris

Effect of surface area Hydrochloric acid reacts with marble chips (calcium carbonate) 2HCl(aq) + CaCO 3 (s) CaCl 2 (aq) + CO 2 (g) + H 2 O (l)

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?

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

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

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?

What to do - Group 1 Measure 25 cm 3 of 2 mol l -1 HCl 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

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 cm 3 s-1) time interval Calculate the rate for the 1 st and 2 nd 25 seconds for each set of results

What to do – Group 2 Weigh out 2 g marble chips Measure 25 cm 3 1 mol l -1 HCl 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

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 1 st and 2 nd 25 seconds for each set of results

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

time (s) volume (cm 3 ) C volume (cm 3 ) G Method A 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 Sample results

Rate over 1 st 25 seconds (cm 3 s -1 ) rate over 2 nd 25seconds (cm 3 s -1 ) Whole chips (C) = =0.72 Ground chips (G) = =0.2 Work out the rate of reaction over the first 25 seconds and the second 25 seconds using the formula rate = change in volume = _____________ cm 3 s -1 time interval Time (s) Volume of gas cm 3

Method B 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. time (s) Mass (g) C mass(g) G Sample results

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 Rate over 1 st 25 seconds (g s -1 ) rate over 2 nd 25seconds (g s -1 ) Whole chips (C) = =0.018 Ground chips (G) = =1x10 -3 Loss in mass (g) Time (s)

Effect of concentration

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

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

Your challenge is to create a series of solutions that will change colour in time to music iYKF0cs Effect of concentration –the chemical clock challenge

The iodine clock reaction changes from colourless to blue / black rning-zone/science- lab/experiments/Pages/Chemistry.aspx rning-zone/science- lab/experiments/Pages/Chemistry.aspx Effect of concentration –the chemical clock challenge

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

2I - (aq) + H 2 O 2 (aq) + 2H + (aq)  2H 2 O (l) + I 2 (aq) 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. t being a measure of how long it takes for the blue/black colour to form. (when excess I 2 forms) Relative Rate = 1 t Units s S 2 O 3 2- (aa)  2I - (aq) + S 4 O 6 2- (ag) I 2 (aq) Effect of concentration –the chemical clock challenge

1) Using syringes measure out 10cm 3 sulphuric acid 0.1moll -1 10cm 3 sodium thiosulphate 0.005moll -1 1cm 3 starch solution 25cm 3 potassium iodide solution 0.1mol l -1 Into a dry 100cm 3 beaker 2) Measure out 5cm 3 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 cm 3 of potassium iodide solution and 5cm 3 of water with, then using repeated dilutions Effect of concentration –the chemical clock challenge

Volume of water (cm 3 )Volume of 0.5 mol l-1 KI (aq) (cm 3 ) Time (s)Rate (1/t) O Effect of concentration –the chemical clock challenge

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

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

Use the relative concentration to help you work out the volume of water and KI(aq) needed to make up 100 cm 3 of the required concentration. 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. Effect of concentration –the chemical clock challenge

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 C 12 H 22 O 11

Work out the mass of sucrose required to make up 100cm 3 of sucrose solution of the same concentration as cola, assuming there are 24g in 330cm 3 Make up your solution. What is the concentration of this solution? Effect of concentration - Cola challenge

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

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

Sucrose is a non-reducing sugar – it does not react with Bendicts unless it is first hydrolysed. Boil 10 cm 3 sugar solution with 5cm 3 1mol/l HCl. Then neutralise the solution with 5cm 3 1mol/l NaOH. Allow this solution to cool to room temperature. Repeat with the lemonade solution. Add 2cm 3 of Benedicts to each sample. Prepare a beaker of boiling water. Effect of concentration - Cola challenge

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

Effect of temperature

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.

Effect of temperature -the vanishing cross

Temperature ( 0 C) Reaction time in seconds /time A 10 o C rise in temperature will approximately double the rate of the reaction Effect of temperature -the vanishing cross Results

Temperature and energy

The experiment can be viewed at xperiments/CCE-64.pdfhttp://media.rsc.org/Classic%20Chem%20e xperiments/CCE-64.pdf Effect of temperature -the vanishing cross

Rate measurement and temperature. Oxalic acid/permanganate reaction 5(COOH) 2 (aq) + 6H + (aa) + 2MnO 4 2- (aq)  2Mn 2+ (aq) + 10 CO 2 (aq) 8H 2 O (l) 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. What colour change takes place?