In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification.
I’m giving you my notes from each lesson.
You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months.
This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least.
That written in italics is for the higher tier paper only.
C3 Chemical Reactions
C3.2 Exo and Endothermic reactions
C3.1c use the names and symbols of common elements from a supplied periodic
table to write formulae and balanced chemical equations where appropriate for
the first 20 elements, Groups 1, 7, and 0 and other common elements included
within the specification.
C3.2a distinguish between endothermic and exothermic reactions on the basis
of the temperature change of the surroundings.
In an exothermic reaction, the vessel holding the reaction gets hotter
because the reaction releases heat energy to the surroundings.
In an endothermic reaction, the vessel holding the reaction gets cooler
because the reaction takes in heat energy from the surroundings that in this
case would be your hand if you were holding the cup.
Here we
describe ammonium nitrate dissolving in water endothermically.
C3.2b draw and label a reaction profile for an exothermic and an
endothermic reaction.
Here are the reaction profiles for the above reactions:
An exothermic reaction profile:
C3.2c explain activation energy as the energy needed for a reaction to
occur
The activation energy (Ea) is the minimum energy the reactants particles
need to change into the products on collision.
If the
reaction transfers energy to the surroundings then the reaction level of energy
falls.
An
endothermic reaction profile
Notice here how the products lie at a higher level of energy than the
reactants.
Higher tier
C3.2d calculate energy changes in a chemical reaction
by considering bond making and bond breaking energies
Bond energies tell us the energy required to break a
particular chemical bond.
Some useful bond energies:
Chemical bond
|
Bond energy (kJ.mol—1)
|
C—H
|
+435
|
C=O
|
+805
|
O=O
|
+498
|
O—H
|
+464
|
Here’s a way of using bond energies to calculate the
energy change when methane burns in oxygen:
Reactants
|
Products
|
||
Methane and oxygen
 |
 |
||
Bonds broken
|
Energy required
(kJ.mol—1)
|
Bonds formed
|
Energy released
(kJ.mol—1)
|
4 C—H
|
4 × 435 = 870
|
2 C=O
|
2 × 805 = 1610
|
2 O=O
|
2 × 498 = 996
|
4 O—H
|
4 × 464 = 928
|
Total (kJ.mol—1)
|
+1866
|
Total (kJ.mol—1)
|
+2538
|
Energy change on reaction = +1866
— +2538 = —672 kJ.mol—1
|
|||
Note that the energy change is negative since in an
exothermic reaction the reaction transfers energy to the surroundings so its
energy content goes down.
Here is the reaction profile for the combustion of
methane showing how it is exothermic.
More energy is released forming new bonds than is
required to break the bonds in the reactants.
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