Summary
Summary Chemical Change and Rates IEB
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Summary of the Chemical Change and Rate of Reactions of the grade 11 and grade 12 IEB syllabus.
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0CHEMICAL CHANGE AND RATES
, Definitions
* Heat of Reaction ( DH) → the net of chemical potential
change
energy of the
system
* Exothermic Reactions → reactions which transfer chemical
potential energy into thermal energy
* Endothermic Reaction → reactions which transfer thermal
energy
into chemical
potential energy
Activation Energy → minimum to start a chemical
energy required
*
* Activated complex → a high energy unstable temporary
, ,
transition state between the reactants
and the products .
* Reaction Rate → the in concentration per unit time of
change .
either a reactant or product
*
catalyst → a substance that increases the rate of reaction
but remains
unchanged at the end of the
reactions .
* Key concepts /terminology s
e'
' '
↳ Energy is linked to temperature ST = T temp .
↳ Chemical system consists of reactants / products
Enviornment : consists of chemical system 's Sor
rounding s
,Energy Change in Chemical Reactions
*
During chemical reaction :
↳ Bonds in reactants BREAK
↳ New bonds in product FORM
" '
* Bonds to break → e must be absorbed
'' '
* New bonds form → e is released
'
* Bond → KJ moi
energies
-
.
''
*
stronger bonds = more e' to break
I
H H bond 436 KJ Mol
-
Eg .
- = .
0=0 bond = 496 KJ MOI
.
-
t
I
O H bond 460 KJ MOI
-
-
= .
2h20 t 02 → H2O
bonds 2 (436 ) t 496
Energy to break =
= 1368 KJ
Energy released when
forming bonds = 2 ( 2C 460) ]
=
1884 KJ
Total = I 840 -
1368
''
= 472 KJ of e is released during the reaction
Enthalpy change
*
Enthalpy COH) s potential energy of a substances reactants and
'
products
*
During a reaction energy is exchanged between the system and the environment
resulting
* will NOT change unless the actual
energy of reactants and products changes
* Formula :
Enthalpy = H products -
H reactants
,Exothermic Reactions
"'
is RELEASED when
* More e
forming bonds than absorbed for
breaking "'
bonds .
potential e' of
''
Potential
* e of
''
products = less than chemical reactants
↳ Thermal e' released reaction
during the .
↳ Reactants Products
→ t
energy
* Release heat ; increases the temperature of the reaction mixture
↳ Reaction vessel will be warmer then before the reaction
* OH LO ( Reaction gets warmer )
n n
Graph of Increases
-
÷: v
⇐xeoathaeiromnicfneem.eearaat.int
^ Environment
For
E Os
E
OH SO
-
eggs
s
EE
e
w
od
v pro
-
>
course of Reaction
Endothermic Reactions
E
"
* Less is RELEASED when forming bonds than absorbed for breaking bonds.
* Absorbs heat → decreases temperature of the reaction mixture .
↳ container = cooler than before the reaction took place .
e'
''
potential e'
' '
* Potential of
"'
products = more than of reactants
↳ Thermal e absorbed during the reaction .
↳ Reactants +
Energy → Products
decreases
* OH > O (Reaction gets colder )
Temperature of
'
the Reactions
0£
Environment
energy.es
E.
aneroid
¥ on
+5 OH > O
E Es Graph of
8- of red Endothermic
Reaction
-
course of Reaction
,Activation Energy .
* chemical reactions will not take
place unless the has some min
system
-
e'
''
amount of added to it .
* Two molecules react → must come into connect with sufficient to
overcome the of bonds within the molecules
strength existing ,
* For a collision to be effective ,
the colliding particles must have :
↳ correct orientation
"'
↳ sufficient energy → particle must have kinetic e =/ greater than AC
in order to initiate the bond and formation of the
breaking the
Activated complex
* Successful collision * Unsuccessful coitions
⑤⑦ ①① ①⑦⑤⑤ 5
¥ too →
-
→ ③③
0 A D
③⑦
B
unstable
Activated
* chemical reactions occur when
they are effective collisions between
reactants
exothermic
Reaction
se fence?feed
for
.
Collison
,
O>
endothermic
needed for
µ
Reaction
*
effective
*
collision
so
,Rate of Reaction
* How fast a reaction takes place
↳ Many factors affect this
* COLLISION THEORY :
order for two molecules
↳ In to react
chemically they ,
must come into contact
with sufficient É to break existing bonds within the molecules and come
into contact with the correct orientation .
↳ This results in an EFFECTIVE COLLISION
* Rate of reaction = amount of effective collisions per unit time
* In a reaction :
reactants are used up and products are formed
* calculate Rate of Reaction
amount of reactant used OR product formed
Aug . Rate =
time taken
* ROR is NOT constant
* Instantaneous Rate ROR in the reaction
at
any given point
=
ROR at A amount
Eg .
=
time
*
Avg Rate avg ROR over a period
.
= . of time .
Amount A- Am out B
Eg Avg ROR between A and B
.
.
=
Time A- Time B
B
☒ gradient becomes zero ;
••
shows the reaction
has stopped .
gradient decreases with time
f- as the reaction slows
A
••
steep gradient = Fast ROR at
t the start
, Factors Affecting Rate of Reactions
① Surface Area (solids )
* SA → increases rate
Increasing of reaction
* More of the solid is exposed
* More contact points for effective collisions which increases the amount of effective
,
collisions per unit time .
↳ powder has larger SA than granules
5
±
&
£
§
£
er
Time CS)
② concentration ( solutions)
* → increases
Increasing concentration rate of reaction
* More confined area
particles in a
* More reactants unit volume allows for more effective collisions per
per ,
unit time .
l M 011dm 3
Ea
N
§ 0.5 Moll dm 3
I
¥ u
I
¥¥
8 E
-
o
I >
er
energy
Time CS)
* Rate of reaction decreases over time as the reaction is used up and their
concentration decreases
,③ Pressure caas)
* Increasing pressure → increases rate of reaction
* More
particles per unit volume
* More effective collisions
per unit time .
* Increase in pressure = decrease in volume
5
±
&
£
§
1-
±
Time CS)
④ Temperature
* Increase in temp → increases rate of reaction
.
" '
*
higher temp . =
greater avg . kinetic e of the particles
↳ particles moving faster have more collisions per unit time
''
↳ more particles with e' greater or equal to the Ea , more likely to react
upon colliding
* More effective collisions unit time
per .
5
±
&
£
§
1-
±
Time CS)
,⑤ Addition of a
catalyst
rate of reaction
*
Adding a catalyst → increases
* Function :
provides an alternative
path for the reaction which has a lower
,
activation
energy
* More
particles have Ea greater equal to the Ea and will have sufficient
'
or . .
energy to react .
More effective collisions time
*
per unit
I
5
±
&
£
§
£
er
Time CS)
NB ! ! !
Catalyst has no effect on OH
⑥ Nature of Substance
* substances have different
physical and chemical properties that affect the
the rate of reaction
↳ electronegativity / molecular structure
reactive
*
Generally ,
the more a substance , the more effective collisions
time
per .
5
±
&
£
§
£
er
Time CS)
, Effect Questions
and what effect it has on the reaction
↳ state Factor being changed
Explain the effect of the factor collision
↳ using theory
↳ state there will be more / less effective collisions per unit time
↳ State whether the ROK will increase S1 decrease
Examples :
Question 2.6
pg 29.
-
Effect of Temperature
At
higher temperature the average kinetic energy of the
•
particles is
higher
More molecules have
energy equal greater to the
•
or
activation
energy
•
More effective collisions per unit time
•
The rate of reaction increases
Question 6. d) 24 -
Effect of a
catalyst
pg .
catalyst decreases the activation of the reaction
energy
-
More
particles will have
energy greater or equal to the
-
activation
energy
.
There will be more effective collisions per unit time
.
The rate of reaction will increase .
Question 5. d) 24 Effect of concentration
pg
-
.
•
The concentration of the acid is increased
•
There are more particles per unit volume that have
sufficient to react
energy .
•
There will be more effective collisions per unit time
.
The rate of reaction increases
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