New Way Chemistry for Hong Kong A-Level Book 2 1 Rate Equations and Order of Reactions Increasing...
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Transcript of New Way Chemistry for Hong Kong A-Level Book 2 1 Rate Equations and Order of Reactions Increasing...
New Way Chemistry for Hong Kong A-Level Book 2
1
Rate Equations and Rate Equations and Order of ReactionsOrder of Reactions
Increasing concentration of reactants can increase Increasing concentration of reactants can increase the rate of reaction.the rate of reaction.
Is there any mathematical relationship between rate Is there any mathematical relationship between rate of reaction and concentration of reactants?of reaction and concentration of reactants?
YES!!!!!!YES!!!!!!
1414
Initial concentration / mol dm-3 Initial rate / mol dm-3 s-1
NO H2
0.0250 0.100 2.4 10-6
0.0250 0.050 1.2 10-6
0.0125 0.100 0.6 10-6
For the reaction:2NO(g) + 2H2(g) N2(g) + 2H2O (l)
Rate [H2(g)] when [NO(g)] is constant
Rate [NO2(g)]2 when [H2(g)] is constant
Rate [NO(g)]2 [H2(g)]
Rate = k[NO(g)]2 [H2(g)]
New Way Chemistry for Hong Kong A-Level Book 2
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Rate Rate Equations and Equations and
Order of Order of ReactionsReactions
14.14.11
New Way Chemistry for Hong Kong A-Level Book 2
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In general, for the reaction:
mA + nB products
(a rate equation)(a rate equation)
14.1 Rate Equations and Order of Reactions (SB p.25 NB p.8)
Rate Rate EquationEquation
• k = rate constant
• x = order of reaction with respect to reactant A
• y = order of reaction with respect to reactant B
• Overall order of reaction = x + y
Rate =
= k [A]x [B]y
m n
New Way Chemistry for Hong Kong A-Level Book 2
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mA + nB products
14.1 Rate Equations and Order of Reactions (SB p.25)
Rate Rate EquationEquation
Rate = k [A]x [B]y
• x and y are determined experimentally.experimentally.
• x and y may NOTNOT equal to m and n.
• x and y may not be whole numbers!!!
• Rate of reaction is independent of products’ concentration
New Way Chemistry for Hong Kong A-Level Book 2
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14.1 Rate Equations and Order of Reactions (SB p.26)
Order of ReactionsOrder of Reactions
• Usually integers (0, 1, 2, … …)
• If x = 1
first order with respect to reactant A
• If y = 2
second order with respect to reactant B
• The overall reaction is a third order reaction.
Do Class Examples on p. 8
New Way Chemistry for Hong Kong A-Level Book 2
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14.1 Rate Equations and Order of Reactions (SB p.25 NB p. 8)
Rate Rate EquationEquation
• log rate = log k + x log[A] + y log[B]
• If B is kept large excess, its change in concentration will be negligible when compared with A.
log[B] ~ constant
log rate = c’ + x log[A]
when is log rate plotted against log[A]
straight line with slope = x
Rate = k [A]x [B]y
New Way Chemistry for Hong Kong A-Level Book 2
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Zeroth, First anZeroth, First and Second Order d Second Order
ReactionsReactions
14.14.22
For the following reaction:
A product
[A] mol dm-3 Time (s)
1.00 0
0.80 10
0.70 20
0.65 30
0.53 40
: :
: :
How can we find out the order of reaction with respects to [A]?
New Way Chemistry for Hong Kong A-Level Book 2
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Zeroth Order ReactionsZeroth Order Reactions
A products Rate = k [A]0 = k
14.2 Zeroth, First and Second Order Reactions (SB p.27 NB p.10)
e.g.
CH3COCH3(aq) + I2(aq) + H+(aq)
CH3COCH2I(aq) + I-(aq) + 2H+(aq)
Rate = k[CH3COCH3(aq)][H+(aq)][I2(aq)]0
The rate of reaction is independent of [I2]!!!
This means if we keep [CH3COCH3(aq)] and [H+(aq)] constant then measure the initial rate with various [I2(aq)], we will get the same initial rate.
New Way Chemistry for Hong Kong A-Level Book 2
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Zeroth Order ReactionsZeroth Order Reactions14.2 Zeroth, First and Second Order Reactions (SB p.27)
[I2(aq)]
New Way Chemistry for Hong Kong A-Level Book 2
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Zeroth Order ReactionsZeroth Order Reactions14.2 Zeroth, First and Second Order Reactions (SB p.27)
WHY???• The reaction is a multi-step reaction.• Only the slowest step affect the overall reaction rate.
A B
C
D
Iodine is not involved in the slowest step of the reaction.
New Way Chemistry for Hong Kong A-Level Book 2
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Zeroth Order ReactionZeroth Order Reaction
[A] = -kt + [A]0
14.5 Determination of Simple Rate Equations from Integrated Rate Equations (SB p.37)
A product
Rate equation is:
A Plot of [A] against time will give a straight line
][
][ 00
dt ][
][
][
A
A
tkAd
dtkAd
kdt
AdRate
New Way Chemistry for Hong Kong A-Level Book 2
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14.2 Zeroth, First and Second Order Reactions (SB p.28)
First Order ReactionFirst Order Reaction
2H2O2(aq) 2H2O(l) + O2(g)
Rate = k [H2O2(aq)]
• Though the stoichiometric coefficient of H2O2(aq) is 2
• Order of reaction with respect to H2O2
(aq) is 1 (determined experimentally)
• e.g Decomposition of H2O2(aq) to H2O and O2
New Way Chemistry for Hong Kong A-Level Book 2
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First Order ReactionFirst Order Reaction
In A = -k1t + In [A]0
14.5 Determination of Simple Rate Equations from Integrated Rate Equations (SB p.37 NB p. 9))
A product
Rate equation is:
A Plot of ln [A] against time will give a straight line
][
][ 0 1
1
1
0
][
][
][
][
][][
A
A
tdtk
A
Ad
dtkA
Ad
Akdt
AdRate
New Way Chemistry for Hong Kong A-Level Book 2
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Half-life of First Order reactionHalf-life of First Order reaction
14.5 Determination of Simple Rate Equations from Integrated Rate Equations (SB p.38)
The time taken for half of the reactant to be converted to the product is known as the half-life of the reaction.
After time t,
[A] = [A]0
t is called the half life of the reaction.
2
1
New Way Chemistry for Hong Kong A-Level Book 2
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Half-life of First Order ReactionHalf-life of First Order Reaction
14.5 Determination of Simple Rate Equations from Integrated Rate Equations (SB p.38)
ln[A] = -k1t + ln[A]0
k1t = ln[A]0 - ln[A]
][
][ln 0
1 A
Atk
If t = half-life (t0.5), then [A]0 = 2[A]:k1t0.5 = ln 2
15.0
2ln
kt t0.5 = constant
New Way Chemistry for Hong Kong A-Level Book 2
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Half-life of a First Order ReactionHalf-life of a First Order Reaction
14.5 Determination of Simple Rate Equations from Integrated Rate Equations (SB p.39)
For a first order reaction:
• Half-life is a constant
Do Class Examples 1, 3 on p.10
New Way Chemistry for Hong Kong A-Level Book 2
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Second Order ReactionSecond Order Reaction
14.5 Determination of Simple Rate Equations from Integrated Rate Equations (SB p.40 NB p.11)
A product
Rate equation is:
Integrating the above rate equation, obtain:
2]A[2t
[A]- Rate kd
d
0[A]
12[A]
1 tk
A plot of 1/[A] vs time will give a straight line
New Way Chemistry for Hong Kong A-Level Book 2
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SummarySummary
Steps to determine the order of reaction
1. Plot a concentration-time graph
a straight line zeroth order
a curve with constant half life first order
Confirmation: ln[A] vs time straight line
2. Plot a second graph of vs time
a straight line second order
][
1
A
Do Q. 5 on p. 77Do Q. 14 on p. 80Q. 13 (more difficult)Do Q. 3 on NB p.17 Q. 2 on NB p. 21
New Way Chemistry for Hong Kong A-Level Book 2
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Determination of Determination of Simple Rate Simple Rate Equations by Equations by
Graphical MethodGraphical MethodNotes p. 18Notes p. 18
14.4 – 14.514.4 – 14.5
New Way Chemistry for Hong Kong A-Level Book 2
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14.4 Determination of Simple Rate Equations from Differential Rate Equations (SB p.32)
log (rate)-log (concentration) log (rate)-log (concentration) GraphGraph
rate = k[A]n
log (rate) = n log [A] + log k
• Plotting log (rate) against log [A], a straight line is obtained for any orders
• Slope of straight line = Order of reaction (n)
• y-intercept = log k
Do Q. 1 on NB p. 16
New Way Chemistry for Hong Kong A-Level Book 2
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We need to plot two graphs:
(1) [A] vs time to find the rate at different [A]
(by slope to the curve)
(2) log (rate) vs log[A] to find the order and k
New Way Chemistry for Hong Kong A-Level Book 2
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14.4 Determination of Simple Rate Equations from Differential Rate Equations (SB p.36)
(a)Decide which curve in the following graph corresponds to
(i) a zeroth order reaction;
(ii) a first order reaction.
(a) (i)
(3)
(ii)
(2)
New Way Chemistry for Hong Kong A-Level Book 2
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14.4 Determination of Simple Rate Equations from Differential Rate Equations (SB p.33)
Isolation techniqueIsolation technique
When two or more substances react:
A + B products
• Order of reaction with respect to reactant A can be found by
• keeping concentration of B constant
(by using much excess B – isolation technique)
Rate = k[A]x[B]y Rate = k’ [A]x
Do Q. 1, 3 on p. 27Do Q 13 on p.80