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CHANGING THE SPEED OF A REACTION
Four factors which can have an effect on the rate of a reaction are –
• TEMPERATURE:
Increasing the temperature will increase the reaction rate.
• CONCENTRATION: This applies mainly to solutions. Increasing the concentration of the solution will increase the reaction rate.
• SURFACE AREA: This applies to solids where smaller pieces have a larger surface area than larger pieces. Increasing the surface area of the solid will increase the reaction rate.
• CATALYST: A catalyst is a substance which increases the rate of a reaction but is itself not used up.
REMEMBER…
When studying the effect of a variable on the rate of a
reaction, only ONE variable should be altered at a time.
All other variables must remain CONSTANT.
COLLISION THEORY
Before a reaction can take place, the reactant particles must
collide with enough energy to form products.
The colliding particles could be ATOMS, MOLECULES or IONS.
The effect of changing a variable on the rate of a reaction can be explained in terms of “collision
theory”.
MOThe r
how
how
Eg. Thydromoni
1.
In thi
from
ONITOrate of a
fast the
fast the
he reactochloricitored in
is exper
the con
ORINGa reactio
e reacta
e produc
tion betc acid prn 2 diffe
riment t
ntainer
G THEon can b
nts are
cts are m
tween croduceserent wa
he LOSS
is noted
E SPEEbe studi
used up
made.
calcium carbonays ‐
S of MA
d at regu
ED OFed by m
p, or
carbonan dioxide
SS of ca
ular tim
F A REmeasurin
ate and e gas wh
arbon di
e interv
EACTIng eithe
hich can
ioxide
vals.
ION r –
n be
2.
This noted
time, thd at reg
he VOLUgular tim
UME of me interv
carbonvals.
dioxide PRODUUCED is
ANA
1.
Egma
ALYSI
By loo
g. The gade in 3
ING R
oking a
graph b3 expe
Ranaste
fastwof
RATES
at grap
below sriment
Rememalysing eeper tter the hen theff, the r
S OF R
phs.
shows ts
mber whgraphsthe linereactioe line lereaction
REACT
the am
hen s – the e, the on and evels n has
TION
ount off produ
uct
Looking at graphs ( continued ) –
a. Compare experiment 1 with experiment 2. 1. Which experiment is faster? 2. How does the graph show this? 3. What could be the reasons for the difference?
b. Now compare experiment 1 with experiment 3.
1. Why do these experiments level off at different “quantity of product produced “?
c. Which line could represent the reaction between – 1. 2g of Magnesium pieces and excess 2M Hydrochloric Acid?
2. 2g of Magnesium powder and excess 2M Hydrochloric Acid?
3. 1g of Magnesium pieces and excess 2M Hydrochloric Acid?
ANALYSING RATES OF REACTION
2. By calculation.
Since the rate of a reaction is changing all the time it is only possible to calculate the AVERAGE rate during any time interval.
AVERAGE RATE = CHANGE IN QUANTITY
CHANGE IN TIME
The prod
Comtime
a.
Av
b.
Av
graphduced
mpare te perio
Betwe
verage
Betwe
verage
showduring
the aveods
een tim
e rate =
een tim
e rate =
s the vg a rea
erage
me 0s
= 40cm
me 20s
= 22cm
volumection o
rate o
and 20
m3 – 20
s and 4
m3 – 20
e of Hyover a
f the r
0s
0s = 2
40s
0s = 1
ydrogeperiod
eactio
2cm3 s
.1cm3
en d of tim
on duri
s‐1
s‐1
me.
ng 2
It is clear from this that the reaction is faster during the first 20s, something which can also be deduced from the graph by comparing how steep the line is during the 2 time periods.
WHEN DOING THESE CALCULATIONS, MAKE SURE YOUR ANSWER
CONTAINS THE CORRECT UNITS!
EXO
REA
1.
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