Rates and Equilibria

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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 work to determine the reaction
rate leads to the rate equation, which, in turn, leads to the reaction mechanism
Take care with units; marks are often (see page 12)
given for correct units in addition to the
change in concentration c
correct value. The units of rate rate of reaction 5 _____________________, or ___
concentration , giving mol dm23 s21. time t
are ___________
Note that positive indices are always
written first.
Rate equations
Rate equations show the relationship between the rate and the powers to which the
reactant concentrations are raised. If two reactants, A and B, react together then
measurements of reaction rates with different concentrations of A and B (see below)
could give the rate equation
rate 5 k[A]x[B]y
In this case:
· k is the rate constant, which is constant at a particular temperature
· [A] and [B] are the concentrations of substances A and B
· x is the order with respect to A, and y is the order for B
· The overall order of reaction is the sum of the individual orders, x 1 y.
Worked Example
Studies of some reactions gave the following rate equations:
a rate 5 k[A][B]2 b rate 5 k[A]0[B]3
c rate 5 k[A]2[B]2 d rate 5 k[A][B]
Examiner tip Which reactions have the same overall order, and which rate is independent of
the concentration of one reactant?
Since any value raised to the power
0 is simply 1, you can omit zero order Reactions a, b are both third order overall. Reaction b is zero order with
reactants when writing rate equations. respect to reactant A, and so its rate is not affected by varying the amount of
reactant A. We can write rate 5 k[B]3
Measuring reaction rates
Rate k[A]2 ­ this produces Rate k[A] ­ this
Rates of reaction are measured by monitoring the
a curve which is a parabola. gives a straight line. rate of change of an observable property.
· Colorimetry measures the intensity of a colour
in a reaction mixture with time, such as in the
oxidation of iodide ions to give brown iodine.
In clock reactions the reaction is timed until a
Rate k[A]0 k 1
sudden colour change happens when a certain
­ the rate does not amount of product is formed.
depend on [A]. · Mass change is used when a gas is produced.
For example, when calcium carbonate reacts
with acids to release carbon dioxide the mass of
the flask decreases.
· Volume change is an alternative to mass
changes for gases. For example, magnesium
reacts with acids to release hydrogen, which can
be collected in a syringe.
[A] · Titrimetric analysis uses titrations to measure
How the reaction rate varies for the concentration of A for zero, first and changing concentrations of a reactant
second order reactions or product ­ for example, the fall in acid
concentration during esterification.
8 A2 Chemistry Revision Guide

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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 curve is found by drawing a tangent. The gradient of the tangent Zero, first and second order reactions
to the concentration­time graph at a particular time gives the rate at that moment.…read more

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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 reactant particles (molecules or ions) before products can form.
Natural gas (methane) and
air react exothermically in
a Bunsen flame.…read more

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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
temperature changes the value of k because the proportion of molecules that have activation energy or you may lose a
the required energy (greater than the activation energy) is increased and the colliding mark.…read more

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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 tip is called the rate-determining step.
Kinetic measurements establish the order of the reaction for each species.…read more

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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 and it must be a two-step reaction.
Kinetics of the reaction between propanone
and iodine in acid solution
· Hydrogen ions catalyse this reaction.…read more

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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
Rate of reaction Understand these terms: rate of reaction, rate equation,
order of reaction, rate constant, half-life
Describe one experimental method to obtain rate data,
choosing from colorimetry, mass change or gas volume 4.…read more

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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 of B/mol dm23
1 0.0500 0.100 1.00 1024
2 0.0500 0.200 3.92 1024
3 0.1000 0.100 1.95 1024
a i Calculate, showing your working, the orders of reaction with respect to A and to B.…read more

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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.…read more

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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 of the addition from the burette.…read more


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