Chemical Changes and Structure National 6 Chemistry
Slide 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.
Slide 3
Controlling the rate Overview Learn to explain how a number of
key factors can influence reaction rate, using the collision
theory.
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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.
Slide 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
Slide 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 Then new chemical bonds
form to make product molecules.
Slide 7
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.
Slide 8
Rates of reaction The rate of reaction can be followed by
measuring changes in Concentration Mass Volume of gas produced
Slide 9
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
Slide 10
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
Slide 11
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
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Effect of surface area
Slide 13
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
Slide 14
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
Slide 15
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)
Slide 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?
Slide 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
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Measuring rate of reaction Two common ways: 1) Measure how fast
the products are formed 2) Measure how fast the reactants are used
up
Slide 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?
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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
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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
Slide 22
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
Slide 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 1 st and 2 nd 25 seconds for each set of
results
Slide 24
Swap results Each group should have a sets of results which can
be used to plot graphs.
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time (s)01020304050607080 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. 0 14 26 36 44 50 50 50 50 0 22 40 48 50 50 50 50 50 Sample
results
Slide 26
Rate over 1 st 25 seconds (cm 3 s -1 ) rate over 2 nd 25seconds
(cm 3 s -1 ) Whole chips (C) 32-0 25-0 =1.3 50-32 50-25 =0.72
Ground chips (G) 45-0 25-0 =1.8 50-45 50-25 =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
Slide 27
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)020406080100120140160 Mass (g) C mass(g) G 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
Slide 28
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) 0.8-2 25-0 =0.05 0.35 -0.8 50-25 =0.018 Ground chips (G) 0.3-2
25-0 =0.068 0.25-0.3 50-25 =1x10 -3 Loss in mass (g) Time (s)
Slide 29
Effect of concentration
Slide 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
Slide 31
Effect of concentration The higher the concentration, the more
particles in a given space, the more chance there is of successful
collisions.
Slide 32
Your challenge is to create a series of solutions that will
change colour in time to music http://www.youtube.com/watch?v=rSAa
iYKF0cs Effect of concentration the chemical clock challenge
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The iodine clock reaction changes from colourless to blue /
black http://www.syngenta.com/country/uk/en/lea rning-zone/science-
lab/experiments/Pages/Chemistry.aspx
http://www.syngenta.com/country/uk/en/lea rning-zone/science-
lab/experiments/Pages/Chemistry.aspx Effect of concentration the
chemical clock challenge
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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
Slide 35
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 -1 + 2S 2 O 3 2- (aa) 2I -
(aq) + S 4 O 6 2- (ag) I 2 (aq) Effect of concentration the
chemical clock challenge
Slide 36
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
Slide 37
Volume of water (cm 3 )Volume of 0.5 mol l-1 KI (aq) (cm 3 )
Time (s)Rate (1/t) O.025.0 5.020.00 10.015.0 10.0 20.05.0 Effect of
concentration the chemical clock challenge
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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
Slide 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
Slide 40
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
Slide 41
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
Slide 42
Slide 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 C 12 H 22 O 11
Slide 44
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
Slide 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
Slide 46
No moles = mass (g) GFM GFM = gram formula mass Effect of
concentration - Cola challenge
Slide 47
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
Slide 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
Slide 49
Effect of temperature
Slide 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.
Slide 51
Effect of temperature -the vanishing cross
Slide 52
Slide 53
Temperature ( 0 C) 1932385160 Reaction time in seconds
10546361812 1/time 0.00950.02170.02780.05560.0833 A 10 o C rise in
temperature will approximately double the rate of the reaction
Effect of temperature -the vanishing cross Results
Slide 54
Temperature and energy
Slide 55
The experiment can be viewed at
http://media.rsc.org/Classic%20Chem%20e
xperiments/CCE-64.pdfhttp://media.rsc.org/Classic%20Chem%20e
xperiments/CCE-64.pdf Effect of temperature -the vanishing
cross
Slide 56
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?
