Transcript No Slide Title
THE RATE EQUATION
A guide for A level students
KNOCKHARDY PUBLISHING
KNOCKHARDY PUBLISHING
THE RATE EQUATION
INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.argonet.co.uk/users/hoptonj/sci.htm
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THE RATE EQUATION CONTENTS
Rate changes during a reaction
Graphical methods for determining rate
THE RATE EQUATION
Before you start it would be helpful to…
•
know how the energy changes during a chemical reaction
•
know the basic ideas of Kinetic Theory
•
know the importance of catalysts in industrial chemistry
REVISION
COLLISION THEORY
Collision theory states that...
• • •
particles must COLLIDE before a reaction can take place not all collisions lead to a reaction reactants must possess at least a minimum amount of energy ACTIVATION ENERGY
plus
•
particles must approach each other in a certain relative way - the STERIC EFFECT
REVISION
COLLISION THEORY
Collision theory states that...
• • •
particles must COLLIDE before a reaction can take place not all collisions lead to a reaction reactants must possess at least a minimum amount of energy ACTIVATION ENERGY
plus
•
particles must approach each other in a certain relative way - the STERIC EFFECT According to collision theory, to increase the rate of reaction you therefore need...
more frequent collisions increase particle speed have more particles present or more successful collisions give particles more energy or lower the activation energy
REVISION
INCREASING THE RATE
The following methods may be used to increase the rate of a chemical reaction
•
INCREASE THE SURFACE AREA OF SOLIDS
•
INCREASE TEMPERATURE
•
SHINE LIGHT
•
ADD A CATALYST
•
INCREASE THE PRESSURE OF ANY GASES
•
INCREASE THE CONCENTRATION OF REACTANTS
REVISION
RATE CHANGE DURING A REACTION
Reactions are fastest at the start and get slower as the reactants concentration drops.
In a reaction such as A + 2B ——> C the concentrations might change as shown
Reactants
(
A and B
)
Concentration decreases with time
Product
(
C
)
Concentration increases with time
•
the steeper the curve the faster the rate of the reaction
•
reactions start off quickly because of the greater likelihood of collisions
•
reactions slow down with time as there are fewer reactants to collide B C TIME A
MEASURING THE RATE
Experimental Investigation
• •
the variation in concentration of a reactant or product is followed with time the method depends on the reaction type and the properties of reactants/products e.g.
Extracting a sample from the reaction mixture and analysing it by titration.
- this is often used if an acid is one of the reactants or products Using a colorimeter or UV / visible spectrophotometer.
Measuring the volume of gas evolved.
Measuring the change in conductivity.
More details of these and other methods can be found in suitable text-books.
RATE
MEASURING THE RATE
How much concentration changes with time. It is the equivalent of velocity.
THE SLOPE OF THE GRADIENT OF THE CURVE GETS LESS AS THE REACTION SLOWS DOWN WITH TIME y x gradient = y x TIME
• • •
the rate of change of concentration is found from the slope (gradient) of the curve the slope at the start of the reaction will give the INITIAL RATE the slope gets less (showing the rate is slowing down) as the reaction proceeds
THE RATE EQUATION
Format
links the rate of reaction to the concentration of reactants it can only be found by doing actual experiments it cannot be found by just looking at the equation
the equation...
A + B ——> C + D might have a rate equation like this
r = k [A] [B]
2 r k [ ] rate of reaction rate constant concentration units of conc. / time usually mol dm -3 s -1 units depend on the rate equation units of mol dm -3 Interpretation The above rate equation tells you that the rate of reaction is...
proportional to the concentration of reactant A doubling [A] doubles rate proportional to the square of the concentration of B doubling [B] quadruples (2 2 ) rate
ORDER OF REACTION
Order tells you how much the concentration of a reactant affects the rate Individual order The power to which a concentration is raised in the rate equation Overall order The sum of all the individual orders in the rate equation.
ORDER OF REACTION
Order tells you how much the concentration of a reactant affects the rate Individual order The power to which a concentration is raised in the rate equation Overall order e.g.
The sum of all the individual orders in the rate equation.
in the rate equation r = k [A] [B] 2 Value(s) and the order with respect to A is the order with respect to B is the overall order is 1 2 3 1st Order 2nd Order 3rd Order need not be whole numbers can be zero if the rate is unaffected by how much of a substance is present
ORDER OF REACTION
Order tells you how much the concentration of a reactant affects the rate Individual order The power to which a concentration is raised in the rate equation Overall order e.g.
The sum of all the individual orders in the rate equation.
in the rate equation r = k [A] [B] 2 Value(s) and the order with respect to A is the order with respect to B is the overall order is 1 2 3 1st Order 2nd Order 3rd Order need not be whole numbers can be zero if the rate is unaffected by how much of a substance is present NOTES The rate equation is derived from experimental evidence not by looking at an equation.
Species appearing in the stoichiometric equation sometimes aren’t in the rate equation.
Substances not in the stoichiometric equation can appear in the rate equation - CATALYSTS
THE RATE EQUATION Experimental determination of order
Method 1 Plot a concentration / time graph qnd calculate the rate (gradient) at points on the curve Plot another graph of the rate (y axis) versus the concentration at that point (x axis) If it gives a straight line, the rate is directly proportional to concentration 1st ORDER .
If the plot is a curve then it must have another order. Try plotting rate v. (conc.)2.
A straight line would mean 2nd ORDER . This method is based on trial and error.
THE RATE EQUATION Experimental determination of order
Method 1 Plot a concentration / time graph qnd calculate the rate (gradient) at points on the curve Plot another graph of the rate (y axis) versus the concentration at that point (x axis) If it gives a straight line, the rate is directly proportional to concentration - 1st ORDER.
If the plot is a curve then it must have another order. Try plotting rate v. (conc.)2.
A straight line would mean 2nd ORDER. This method is based on trial and error.
Method 2 - The initial rates method.
Do a series of experiments (at the same temperature) at different concentrations of a reactant but keeping all others constant. Plot a series of concentration / time graphs and calculate the initial rate (slope of curve at start) for each reaction. From the results calculate the relationship between concentration and rate and hence deduce the rate equation. To find order directly, logarithmic plots are required.
Units
THE RATE CONSTANT (k)
The units of k depend on the overall order of reaction.
e.g. if the rate equation is... rate = k [A] 2 the units of k will be dm 3 mol -1 sec -1 Divide the rate by as many concentrations as appear in the rate equation.
Overall Order units of k 0 mol dm -3 sec -1 1 sec -1 2 dm 3 mol -1 sec -1 3 dm 6 mol -2 sec -1 example in the rate equation r = k [A] in the rate equation r = k [A] [B] 2 k will have units of sec -1 k will have units of dm 6 mol -2 sec -1
RATE EQUATION - SAMPLE CALCULATION
1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 r initial rate of reaction mol dm -3 s -1 [ ] concentration mol dm -3 In an experiment between A and B the initial rate of reaction was found for various starting concentrations of A and B. Calculate...
• • • • •
the individual orders for A and B the overall order of reaction the rate equation the value of the rate constant (k) the units of the rate constant
RATE EQUATION - SAMPLE CALCULATION
1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [B] [A] rate Compare Experiments 1 & 2 same 3 x bigger 3 x bigger
rate
[A] FIRST ORDER with respect to (wrt) A CALCULATING ORDER wrt A Choose any two experiments where...
[A] is changed and, importantly, [B] is KEPT THE SAME See how the change in [A] affects the rate As you can see, tripling [A] has exactly the same effect on the rate so...
THE ORDER WITH RESPECT TO A = 1 (it is FIRST ORDER)
RATE EQUATION - SAMPLE CALCULATION
CALCULATING ORDER wrt B Choose any two experiments where...
[B] is changed and, importantly, [A] is KEPT THE SAME See how a change in [B] affects the rate As you can see, doubling [B] quadruples the rate so...
THE ORDER WITH RESPECT TO B = 2 It is SECOND ORDER 1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [A] [B] rate Compare Experiments 1 & 3 same 2 x bigger 4 x bigger
rate
[B] 2 SECOND ORDER wrt B
RATE EQUATION - SAMPLE CALCULATION
1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [B] [A] rate Compare Experiments 1 & 2 same 3 x bigger 3 x bigger
rate
[A] FIRST ORDER with respect to (wrt) A 1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [A] [B] rate Compare Experiments 1 & 3 same 2 x bigger 4 x bigger
rate
[B] 2 SECOND ORDER wrt B OVERALL ORDER = THE SUM OF THE INDIVIDUAL ORDERS = 1 + 2 = 3
RATE EQUATION - SAMPLE CALCULATION
1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [B] [A] rate Compare Experiments 1 & 2 same 3 x bigger 3 x bigger
rate
[A] FIRST ORDER with respect to (wrt) A 1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [A] [B] rate Compare Experiments 1 & 3 same 2 x bigger 4 x bigger
rate
[B] 2 SECOND ORDER wrt B
rate = k [A] [B] 2 By combining the two proportionality relationships you can construct the overall rate equation
RATE EQUATION - SAMPLE CALCULATION
1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [B] [A] rate Compare Experiments 1 & 2 same 3 x bigger 3 x bigger
rate
[A] FIRST ORDER with respect to (wrt) A 1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [A] [B] rate Compare Experiments 1 & 3 same 2 x bigger 4 x bigger
rate
[B] 2 SECOND ORDER wrt B
rate = k [A] [B] 2
re-arranging
k = rate [A] [B] 2
Chose one experiment (e.g. Expt. 3) and substitute its values into the rate equation
k = 8 (0.5) (2) 2 = 4 dm 6 mol -2 sec -1
RATE EQUATION - SAMPLE CALCULATION
SUMMARY 1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [B] [A] rate Compare Experiments 1 & 2 same 3 x bigger 3 x bigger
rate
[A] FIRST ORDER with respect to (wrt) A 1 2 3 [A] 0.5
1.5
0.5
[B] 1 1 2 Initial rate (r) 2 6 8 [A] [B] rate Compare Experiments 1 & 3 same 2 x bigger 4 x bigger
rate
[B] 2 SECOND ORDER wrt B
rate = k [A] [B] 2
re-arranging
k = rate [A] [B] 2
Chose one experiment (e.g. Expt. 3) and substitute its values into the rate equation
k = 8 (0.5) (2) 2 = 4 dm 6 mol -2 sec -1
No 1
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [A] / mol dm -3 0.25
0.25
0.50
[B] / mol dm -3 0.25
0.50
0.25
Rate / mol dm -3 s -1 4 8 8 CALCULATE THE ORDER WITH RESPECT TO A THE ORDER WITH RESPECT TO B THE OVERALL ORDER OF REACTION THE FORMAT OF THE RATE EQUATION THE VALUE AND UNITS OF THE RATE CONSTANT ANSWER ON NEXT PAGE
No 1
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [A] / mol dm -3 0.25
0.25
0.50
[B] / mol dm -3 0.25
0.50
0.25
Rate / mol dm -3 s -1 4 8 8 ANSWER Expts 1&2 Explanation: [A] is constant Therefore [B] is doubled rate
[B] Rate is doubled 1st order wrt B What was done to [B] had exactly the same effect on the rate
No 1 Expts 1&2 Explanation: Expts 1&3 Explanation:
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [A] / mol dm -3 0.25
0.25
0.50
[B] / mol dm -3 0.25
0.50
0.25
Rate / mol dm -3 s -1 4 8 8 ANSWER [A] is constant Therefore [B] is doubled rate
[B] Rate is doubled 1st order wrt B What was done to [B] had exactly the same effect on the rate [B] is constant Therefore [A] is doubled rate
[A] Rate is doubled 1st order wrt A What was done to [A] had exactly the same effect on the rate
No 1
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [A] / mol dm -3 0.25
0.25
0.50
[B] / mol dm -3 0.25
0.50
0.25
Rate / mol dm -3 s -1 4 8 8 ANSWER Expts 1&2 Explanation: Expts 1&3 Explanation: [A] is constant Therefore [B] is doubled rate
[B] Rate is doubled 1st order wrt B What was done to [B] had exactly the same effect on the rate [B] is constant Therefore [A] is doubled rate
[A] Rate is doubled 1st order wrt A What was done to [A] had exactly the same effect on the rate
Rate equation is
r = k[A][B]
No 1
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [A] / mol dm -3 0.25
0.25
0.50
[B] / mol dm -3 0.25
0.50
0.25
Rate / mol dm -3 s -1 4 8 8 ANSWER Expts 1&2 Explanation: Expts 1&3 Explanation: [A] is constant Therefore [B] is doubled rate
[B] Rate is doubled 1st order wrt B What was done to [B] had exactly the same effect on the rate [B] is constant Therefore [A] is doubled rate
[A] Rate is doubled 1st order wrt A What was done to [A] had exactly the same effect on the rate
Rate equation is
r = k[A][B]
Value of k
Substitute numbers from Exp 1 to get value of k k = rate / [A][B] = 4 / 0.25 x 0.25
= 64
No 1
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [A] / mol dm -3 0.25
0.25
0.50
[B] / mol dm -3 0.25
0.50
0.25
Rate / mol dm -3 s -1 4 8 8 ANSWER Expts 1&2 Explanation: Expts 1&3 Explanation: [A] is constant Therefore [B] is doubled rate
[B] Rate is doubled 1st order wrt B What was done to [B] had exactly the same effect on the rate [B] is constant Therefore [A] is doubled rate
[A] Rate is doubled 1st order wrt A What was done to [A] had exactly the same effect on the rate
Rate equation is
r = k[A][B]
Value of k Units of k
Substitute numbers from Exp 1 to get value of k k = rate / [A][B] = 4 / 0.25 x 0.25
rate / conc x conc = dm 3 mol -1 s -1 = 64
No 2
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [C] / mol dm -3 0.40
0.20
0.40
[D] / mol dm -3 0.40
0.40
1.20
Rate / mol dm -3 s -1 0.16
0.04
1.44
CALCULATE THE ORDER WITH RESPECT TO C THE ORDER WITH RESPECT TO D THE OVERALL ORDER OF REACTION THE FORMAT OF THE RATE EQUATION THE VALUE AND UNITS OF THE RATE CONSTANT ANSWER ON NEXT PAGE
No 2 Expts 1&3 Explanation:
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [C] / mol dm -3 0.40
0.20
0.40
[D] / mol dm -3 0.40
0.40
1.20
Rate / mol dm -3 s -1 0.16
0.04
1.44
ANSWER [C] is constant Therefore [D] is tripled rate
[D] 2 Rate is 9 x bigger 2nd order wrt D Squaring what was done to D affected the rate (3 2 = 9)
No 2 Expts 1&3 Explanation: Expts 1&2 Explanation:
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [C] / mol dm -3 0.40
0.20
0.40
[D] / mol dm -3 0.40
0.40
1.20
Rate / mol dm -3 s -1 0.16
0.04
1.44
ANSWER [C] is constant Therefore [D] is tripled rate
[D] 2 Rate is 9 x bigger 2nd order wrt D Squaring what was done to D affected the rate (3 2 = 9) [D] is constant Therefore [A] is halved rate
[C] 2 One half squared = one quarter Rate is quartered 2nd order wrt C
No 2
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [C] / mol dm -3 0.40
0.20
0.40
[D] / mol dm -3 0.40
0.40
1.20
Rate / mol dm -3 s -1 0.16
0.04
1.44
ANSWER Expts 1&3 Explanation: Expts 1&2 Explanation: [C] is constant Therefore [D] is tripled rate
[D] 2 Rate is 9 x bigger 2nd order wrt D Squaring what was done to D affected the rate (3 2 = 9) [D] is constant Therefore [A] is halved rate
[C] 2 One half squared = one quarter
Rate equation is
r = k[C] 2 [D] 2 Rate is quartered 2nd order wrt C
No 2
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [C] / mol dm -3 0.40
0.20
0.40
[D] / mol dm -3 0.40
0.40
1.20
Rate / mol dm -3 s -1 0.16
0.04
1.44
ANSWER Expts 1&3 Explanation: [C] is constant Therefore [D] is tripled rate
[D] 2 Rate is 9 x bigger 2nd order wrt D Squaring what was done to D affected the rate (3 2 = 9) Expts 1&2 Explanation: [D] is constant Therefore [A] is halved rate
[C] 2 One half squared = one quarter
Rate equation is
r = k[C] 2 [D] 2
Value of k Units of k
Substitute numbers from Exp 2 to get value of k k = rate / [C] 2 [D] 2 = 0.04 / 0.2
2 x 0.4
2 = 6.25
rate / conc 2 x conc 2 = dm 9 mol -3 s -1 Rate is quartered 2nd order wrt C
No 3
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [E] / mol dm -3 0.40
0.80
0.80
[F] / mol dm -3 0.40
0.80
1.20
Rate / mol dm -3 s -1 0.16
0.32
0.32
CALCULATE THE ORDER WITH RESPECT TO E THE ORDER WITH RESPECT TO F THE OVERALL ORDER OF REACTION THE FORMAT OF THE RATE EQUATION THE VALUE AND UNITS OF THE RATE CONSTANT ANSWER ON NEXT PAGE
No 3
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [E] / mol dm -3 0.40
0.80
0.80
[F] / mol dm -3 0.40
0.80
1.20
Rate / mol dm -3 s -1 0.16
0.32
0.32
Expts 2&3 Explanation: ANSWER [E] is constant [F] is x 1.5
Rate is UNAFFECTED Concentration of [F] has no effect on the rate Rate unchanged ZERO order wrt F
No 3 Expts 2&3 Explanation: Expts 1&2 Explanation:
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [E] / mol dm -3 0.40
0.80
0.80
[F] / mol dm -3 0.40
0.80
1.20
Rate / mol dm -3 s -1 0.16
0.32
0.32
ANSWER [E] is constant [F] is x 1.5
Rate is UNAFFECTED Concentration of [F] has no effect on the rate Rate unchanged ZERO order wrt F [E] is doubled Therefore [F] is doubled rate
[E] 2 Rate is doubled 2nd order wrt E Although both concentrations have been doubled, we know [F] has no effect. The change must be all due to [E]
No 3
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [E] / mol dm -3 0.40
0.80
0.80
[F] / mol dm -3 0.40
0.80
1.20
Rate / mol dm -3 s -1 0.16
0.32
0.32
ANSWER Expts 2&3 Explanation: Expts 1&2 Explanation: [E] is constant [F] is x 1.5
Rate is UNAFFECTED Concentration of [F] has no effect on the rate Rate unchanged ZERO order wrt F [E] is doubled Therefore [F] is doubled rate
[E] 2 Rate is doubled 2nd order wrt E Although both concentrations have been doubled, we know [F] has no effect. The change must be all due to [E]
Rate equation is
r = k[E]
No 3
RATE EQUATION QUESTIONS
Expt 1 Expt 2 Expt 3 [E] / mol dm -3 0.40
0.80
0.80
[F] / mol dm -3 0.40
0.80
1.20
Rate / mol dm -3 s -1 0.16
0.32
0.32
ANSWER Expts 2&3 Explanation: Expts 1&2 Explanation: [E] is constant [F] is x 1.5
Rate is UNAFFECTED Concentration of [F] has no effect on the rate Rate unchanged ZERO order wrt F [E] is doubled Therefore [F] is doubled rate
[E] 2 Rate is doubled 2nd order wrt E Although both concentrations have been doubled, we know [F] has no effect. The change must be all due to [E]
Rate equation is
r = k[E]
Value of k Units of k
Substitute numbers from Exp 1 to get value of k k = rate / [E] = 0.16 / 0.4
rate / conc = s -1 = 0.40
GRAPHICAL DETERMINATION OF RATE
The variation in rate can be investigated by measuring the change in concentration of one of the reactants or products, plotting a graph and then finding the gradients of the curve at different concentrations.
In the reaction… A(aq) + B(aq) ——> C(aq) + D(aq) the concentration of B was measured every 200 minutes.
The reaction is obviously very slow!
RATE CALCULATION The rate of reaction at any moment can be found from the gradient of the tangent at that point. The steeper the gradient, the faster the rate of reaction Place a rule on the outside of the curve and draw a line as shown on the graph.
y x gradient = y / x
GRAPHICAL DETERMINATION OF RATE
The variation in rate can be investigated by measuring the change in concentration of one reactants or product, plotting a graph and then finding the gradients of tangents to the curve at different concentrations.
concentration = 1.20 mol dm -3 gradient = - 1.60 mol dm -3 rate 3 1520 min = - 1.05 x 10 -3 mol dm The rate is negative because the reaction is slowing down RATE CALCULATION The rate of reaction at any moment can be found from the gradient of the tangent at that point. The steeper the gradient, the faster the rate of reaction Place a rule on the outside of the curve and draw a line as shown on the graph.
y x gradient = y / x
GRAPHICAL DETERMINATION OF RATE
The variation in rate can be investigated by measuring the change in concentration of one of the reactants or products, plotting a graph and then finding the gradients of the curve at different concentrations.
The gradients of tangents at several other concentrations are calculated.
Notice how the gradient gets less as the reaction proceeds, showing that the reaction is slowing down.
The tangent at the start of the reaction is used to calculate the initial rate of the reaction.
RATE CALCULATION The rate of reaction at any moment can be found from the gradient of the tangent at that point. The steeper the gradient, the faster the rate of reaction Place a rule on the outside of the curve and draw a line as shown on the graph.
y x gradient = y / x
FIRST ORDER REACTIONS AND HALF LIFE
One characteristic of a FIRST ORDER REACTION is that it is similar to radioactive decay. It has a half-life that is independent of the concentration.
It should take the same time to drop to one half of the original concentration as it does to drop from one half to one quarter of the original. The concentration of a reactant falls as the reaction proceeds
FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes
FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes 2 to 1 in a further 17 minutes
FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes 2 to 1 in a further 17 minutes 1 to 0.5 in a further 17 minutes
FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes 2 to 1 in a further 17 minutes 1 to 0.5 in a further 17 minutes
FIRST ORDER REACTIONS AND HALF LIFE
A useful relationship k t ½ where t ½ = log e = 0.693
2 = the half life Half life = 17 minutes k t ½ = 0.693
k = 0.693
t ½ k = 0.693 = 0.041 min -1 17
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
PLOTTING RATE AGAINST CONCENTRATION
CONCENTRATION / mol dm -3
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
PLOTTING RATE AGAINST CONCENTRATION
ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.
CONCENTRATION / mol dm -3
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
PLOTTING RATE AGAINST CONCENTRATION
ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.
FIRST ORDER – the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction.
CONCENTRATION / mol dm -3
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
PLOTTING RATE AGAINST CONCENTRATION
SECOND ORDER – the rate is proportional to the square of the concentration. You get an upwardly sloping curve.
ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.
FIRST ORDER – the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction.
CONCENTRATION / mol dm -3
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
PLOTTING RATE AGAINST CONCENTRATION
SECOND ORDER – the rate is proportional to the square of the concentration. You get an upwardly sloping curve.
ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.
FIRST ORDER – the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction.
CONCENTRATION / mol dm -3
ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.
PLOTTING RATE AGAINST TIME
ZERO ORDER A straight line showing a constant decline in concentration.
FIRST ORDER A slightly sloping curve which drops with a constant half-life.
SECOND ORDER The curve declines steeply at first then levels out.
TIME / s
ORDER OF REACTION GRAPHICAL DETERMINATION
Calculate the rate of reaction at 1.0, 0.75, 0.5 and 0.25 mol dm -3 Plot a graph of rate v [A] Calculate the time it takes for [A] to go from...
1.00 to 0.50 mol dm -3 0.50 to 0.25 mol dm -3 Deduce from the graph that the order wrt A is 1 Calculate the value and units of the rate constant, k
RATE DETERMINING STEP
Many reactions consist of a series of separate stages.
Each step has its own rate and rate constant.
The overall rate of a multi-step process is governed by the slowest step
(like a production line where overall output can be held up by a slow worker).
This step is known as the
RATE DETERMINING STEP
.
If there is more than one step, the rate equation may not contain all the reactants in its format.
RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone react in the presence of acid The rate equation is...
CH 3 COCH 3 + I 2 r = k [CH 3 COCH 3 ] [H + ] CH 3 COCH 2 I + HI Why do H + ions appear in the rate equation?
Why does I 2 not appear in the rate equation?
RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone react in the presence of acid The rate equation is...
CH 3 COCH 3 + I 2 r = k [CH 3 COCH 3 ] [H + ] CH 3 COCH 2 I + HI Why do H + ions appear in the rate equation?
Why does I 2 not appear in the rate equation? The reaction is catalysed by acid [H + ] affects the rate but is unchanged overall The rate determining step doesn’t involve I 2
RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE
Iodine and propanone react in the presence of acid The rate equation is...
CH 3 COCH 3 + I 2 r = k [CH 3 COCH 3 ] [H + ] CH 3 COCH 2 I + HI Why do H + ions appear in the rate equation?
Why does I 2 not appear in the rate equation? The reaction is catalysed by acid [H + ] affects the rate but is unchanged overall The rate determining step doesn’t involve I 2 The slowest step of any multi-step reaction is known as the rate determining step and it is the species involved in this step that are found in the overall rate equation.
Catalysts appear in the rate equation because they affect the rate but they do not appear in the stoichiometric equation because they remain chemically unchanged
RATE DETERMINING STEP
HYDROLYSIS OF HALOALKANES Haloalkanes (general formula RX) are hydrolysed by hydroxide ion to give alcohols.
RX + OH ROH + X With many haloalkanes the rate equation is...
This is because both the RX and OH must collide for a reaction to take place in
ONE STEP r = k [RX][OH ] SECOND ORDER
RATE DETERMINING STEP
HYDROLYSIS OF HALOALKANES Haloalkanes (general formula RX) are hydrolysed by hydroxide ion to give alcohols.
RX + OH ROH + X With many haloalkanes the rate equation is...
This is because both the RX and OH must collide for a reaction to take place in
ONE STEP r = k [RX][OH ] SECOND ORDER but with others it only depends on [RX]...
r = k [RX] FIRST ORDER -
The reaction has taken place in
TWO STEPS
...
- the first involves breaking an R-X bond the second step involves the two ions joining i)
RX
ii)
R + + OH R + + X ROH
Slow Fast
The first step is slower as it involves bond breaking and energy has to be put in.
The first order mechanism is favoured by tertiary haloalkanes because the hydroxide ion is hindered in its approach by alkyl groups if the mechanism involves the hydroxide ion and haloalkane colliding
.
RATE DETERMINING STEP
The reaction
H 2 O 2 + 2H 3 O + + 2I¯
Step 1 Step 2 Step 3
H 2 O 2 + I¯ IO¯ + H 3 O + HIO + H 3 O + + I¯ I 2 + 4H 2 O IO¯ + H 2 O HIO + H I 2 2 O + 2H 2 O The rate determining step is STEP 1 as it is the slowest
takes place in 3 steps
SLOW FAST FAST
RATE DETERMINING STEP
The reaction
H 2 O 2 + 2H 3 O + + 2I¯
Step 1 Step 2 Step 3
H 2 O 2 + I¯ IO¯ + H 3 O + HIO + H 3 O + + I¯ I 2 + 4H 2 O IO¯ + H 2 O HIO + H I 2 2 O + 2H 2 O The rate determining step is STEP 1 as it is the slowest
takes place in 3 steps
SLOW FAST FAST
The reaction Step 1 Step 2
2N 2 N O 2 5 O 5 NO 2 + NO 3 4NO 2 + O 2 NO 2 + NO 3
takes place in 3 steps
NO + NO 2 + O 2 SLOW FAST
Step 3
NO + NO 3 2NO 2
from another Step 1
FAST The rate determining step is STEP 1
rate = k [N
2
O
5
]
OTHER TOPICS
Autocatalysis A small number of reactions appear to speed up, rather than slow down, for a time. This is because one of the products is acting as a catalyst and as more product is formed the reaction gets faster. One of the best known examples is the catalytic properties of Mn 2+ (aq) on the decomposition of MnO 4 ¯(aq). You will notice it in a titration of KMnO 4 with either hydrogen peroxide or ethanedioic (oxalic) acid.
Molecularity
e.g.
The number of individual particles of the reacting species taking part in the rate determining step of a reaction A + 2B C + D molecularity is 3
-
one A
and
two B’s
need to collide
A 2B
however has a
molecularity of 1
- only
one A
is involved
REVISION CHECK
What should you be able to do?
Explain
how the rate changes during a chemical reaction
Recall and understand
the terms... rate equation, individual order and overall order
Interpret and calculate
data to derive a rate equation
Construct
graphs to find the order of reaction
Understand
the importance of the rate determining step
CAN YOU DO ALL OF THESE?
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WELL DONE!
Try some past paper questions
THE RATE EQUATION
The End
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