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4 Unit 4: General principles of chemistry I ­ Rates, equilibria and further organic chemistry


Rate of reaction
A study of reaction rates (chemical kinetics) allows us to understand exactly how
reactions work. You cannot deduce a reaction mechanism simply by examining the
Examiner tip ordinary balanced chemical equation. Experimental…

Page 2

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Topic 1: How fast? ­ Rates 4

Graphs and tangents
Examiner tip
The simplest type of relationship between variables is the linear one of general form
y 5 mx 1 c, where m is the slope or gradient and c the intercept on the y-axis.
· The gradient of a…

Page 3

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4 Unit 4: General principles of chemistry I ­ Rates, equilibria and further organic chemistry


Activation energy and catalysis
When reactant molecules collide, it may result in a chemical reaction. There is an
Worked Example energy requirement before this can happen. The activation energy is the minimum
energy needed by…

Page 4

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Topic 1: How fast? ­ Rates 4

Effect of temperature on rates
When the temperature increases, the rate of a reaction increases too because the rate Watch out!
constant increases.
The rate constant k is only a constant for a particular temperature. Changing the Remember to give the units of…

Page 5

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4 Unit 4: General principles of chemistry I ­ Rates, equilibria and further organic chemistry


Mechanisms and rate-determining steps
Reactions do not just happen whenever all the relevant molecules collide at once ­
they happen in steps. The slowest step controls how fast the overall reaction occurs ­ it
Examiner…

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Topic 1: How fast? ­ Rates 4
The mechanism must be consistent with the evidence:
· if the reaction is second order overall it must involve two different species
· if the reaction is first order overall (only one species in the rate equation) then this is
the rate-determining step…

Page 7

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4 Unit 4: General principles of chemistry I ­ Rates, equilibria and further organic chemistry


Topic 1: How fast? Rates checklist
By the end of this topic you should be able to:

Revision spread Checkpoints Specification Revised Practice
section exam
questions
Rate of reaction Understand these terms: rate of reaction,…

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Topic 1: How fast? ­ Rates 4


Build Better Answers


1 Two gases A and B react according to the equation: A(g) 1 2B(g) AB2(g)
A series of kinetics experiments performed at constant temperature gave the following results:

Experiment Initial concentration Initial concentration Initial rate/mol dm23 s21
of A/mol dm23…

Page 9

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4 Unit 4: General principles of chemistry I ­ Rates, equilibria and further organic chemistry




Build Better Answers

2 In aqueous solution, propanone and bromine react as follows:
CH3COCH3(aq) 1 Br2(aq) CH3COCH2Br(aq) 1 HBr(aq)
This reaction is zero order with respect to bromine. What can we deduce from this? Choose…

Page 10

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Topic 1: How fast? ­ Rates 4

Practice exam questions
1 In acid solution, ethanedioic acid (oxalic acid) is oxidized by the addition of an
aqueous solution of potassium manganate(VII). During a titration, it was noticed
that the rate at which the manganate(VII) decolorised accelerated significantly just
after the start…

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