8.1 rate of reaction

8/8/2019 8.1 Rate of Reaction

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1. The rate of reaction is the change of

concentration of a reactant or a product per unit time.

2. Two ways to measure the rate of reaction:

a) Rate of reaction = rate of increase of the

concentration or quantity of the product b) Rate of reaction = rate of decrease of the

concentration or quantity of the reactant.

changetime

changeionconcentrat Rate

!



3. In the reaction A B:

where d[A]=change in concentration of reactant A,

d[B]=change in concentration of product B,dt=change in time [ ] = concentration

d t

Bd rate ][!

d t Ad rateor ][ !

reactantproduct



4. Consider A + 2B 3C:

Since 1 mol of A reacts with 2 mol of B to

form 3 mol of C:

a) The rate of reduction of B is twice that of A

b) The rate of formation of C is 3 times the rate

of reduction of A

d t

C d

d t

d

d t

d

rate][

3

1][

2

1][!!!



5. Generally, for reaction aA + bB cC + dD:

eg: N2(g) + 3H2(g) 2NH3(g)

d t

Bd

bd t

Ad

ad t

Dd

d d t

d

cr ate

][1][1][1][1!!!!

d t

NH d

d t

H d

d t

N d rate

][

2

1][

3

1][ 322!!!



Average rate, instantaneous rate & initial rate

Average rate = the change in concentration of a

substance over a fixed interval time.

eg: In the decomposition of N2O5in CCl

4solution to

form O2, the volume of O2 formed is collected at

regular intervals. A graph of volume against time isshown.



Instantaneous rate = rate of the reaction at a

specific time.

It is given by the gradient / slope of the tangent

to the curve at the specified time.

The larger the gradient, the higher the

instantaneous rate.

Instantaneous rate with

respect to O2

at 10th second

13

! scmb

aab



When the tangent to the curve at t=5s, 10s, 15s,20s & 25s & plotted.

It is found that the tangents to the curve becomesmaller & smaller.

Therefore, the rate of the reaction decreases asthe reaction proceeds.

When tangent is zero (at t = 25s), the rate iszero, meaning reaction has stopped because one

or all of the reactants have been used up.



Instantaneous rate can also be obtained from

the gradient of the tangent of the graph of

concentration of the reactant against time.

The tangent to the curve decreases with time.

[reactant]



In a few reactions, the graphs of the concentration

of the reactants or products against time are

straight lines.

This means that the gradient is a constant value.

For such reaction, the rate is constant throughout

the reaction.



During a reaction, the concentration of a reactant

or a product cannot be determine accurately. Thus, it is preferable to measure the initial rates.

The initial rate is the instantaneous rate of the

reaction at the start of the exp., when t = 0.



In the reaction A + B C ,

to find out how the concentration of B affects the

rate of reaction, two exp are set up using the same

volume of A & B but different concentrations of

B.

The concentration of A is kept constant so that

any change in the rate of the reaction is due to the

change in the concentration of B.



By comparing the values of the initial rate of

reaction for both the exp., the rate of the

reaction can be determined.

The advantage of this method is that the

concentrations of the reactants at the start of the

reaction are known.



Determining Reaction Rate

There are a few methods:1. By collecting & measuring the quantity of a

gas produced over a period of time.

2. By measuring the change of theconcentration of a substance over a period of

time in volumetric analysis using titration

method.

3. By measuring the time taken for a certain

amount of substance to be formed or reacted

using colour change.



1. Rate as quantity of gas produced over time

For a reaction involves the production of a gas,the rate of reaction can be measured by

measuring the change of the volume or the

pressure of the gas produced over a period of

time.

Volume of a gas can be measured using a syringe

of displacement of water in a burette.

Pressure of a gas can be measured using a

manometer.

Rate of reaction is calculated from graph plotted.



2. Rate as a change of concentration

of a substance over time

For reaction in solution, the rate of reaction can be

measured by measuring the change of concentration of a

reactant or a product using titration method.

In this method, a known volume of reaction mixture iswithdrawn using a pipette at regular intervals & rapidly

added to a large volume of ice-cold water or excess

quenching agent to stop the reaction.

The concentration of one of the reactant or product isdetermined by suitable titration.

Rate of reaction is calculated from a graph plotted.



3. Rate as quantity of product formed over time

(indicated by colour change)

The rate of reaction is inversely proportional to the time taken.

eg: oxidation of iodide ions by hydrogen peroxide producing

iodine: H2O2(aq) + 2H+

(aq) + 2I (aq) 2H2O(l) + I2(aq)

The reactants are added to a known quantity of thiosulphate ions,S2O3

2- & starch solution.

The iodine formed in the reaction will first react with S2O32- :

I2(aq) + S2O32-

(aq) 2IØ(aq) + S4O6

2-(aq)

When all the S2O32- ions have reacted, further formation of iodine

will form a dark blue colour with the starch solution.

The shorter the time taken for the blue solution to form, the faster the iodine is produced & the higher the rate of reaction.

T r ate

1 w

8.1 rate of reaction

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