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Enthalpy changes
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Slide 1:
Diapositive
Chemistry
Secondary Education
Age 12,13
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Slide 1 - Diapositive
Entalpia
total chemical energy inside a substance/heat content
Slide 2 - Diapositive
Enthalpy change
happens when chemical reactions take place, changes in chemical energy take place
ΔH (Δ= change; H = enthalpy)
An enthalpy change can be positive or negative
Slide 3 - Diapositive
Exo vs Endothermic Reactions
Slide 4 - Diapositive
Exo vs Endothermic Reactions
Slide 5 - Diapositive
Reaction pathway diagrams
A diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
Transition state
= stage during the reaction when chemical bonds are
partially
broken and formed.
Slide 6 - Diapositive
Reaction pathway diagrams
The
transition state
is very
unstable
– it cannot be isolated and is higher in energy than the reactants and products.
The
activation energy (Ea)
is needed to
reach the transition state
.
Slide 7 - Diapositive
Reaction pathway diagrams
Activation energy
=
‘the
minimum
amount of energy needed for reactant molecules to have a successful collision and start the reaction.
Slide 8 - Diapositive
Exothermic Reactions
Slide 9 - Diapositive
Endothermic Reactions
Slide 10 - Diapositive
Sample
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
Slide 11 - Diapositive
Sample
Slide 12 - Diapositive
Standard Enthalpy Change
To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under
standard conditions
:
Slide 13 - Diapositive
Standard Enthalpy Change
Slide 14 - Diapositive
Sample
Since two moles of water molecules are formed in the question above, the energy released is:
ΔHr
Ꝋ
= 2 mol x (-286 kJ mol
-1
)
= - 572 kJ mol
-1
Slide 15 - Diapositive
Sample
Since two moles of Fe2O3 (s) are formed the total change in enthalpy for the reaction above is:
ΔHf
Ꝋ
= 2 x (-824.2 kJ mol
-1
)
= - 1648 kJ mol
-1
Slide 16 - Diapositive
Sample
Answer 1: ΔHr
Ꝋ
Answer 2: ΔHf
Ꝋ
as one mole of CO
2
is formed from its elements in standard state and ΔHc
Ꝋ
as one mole of carbon is burnt in oxygen
Answer 3: ΔHneut
Ꝋ
as one mole of water is formed from the reaction of an acid and alkali
Slide 17 - Diapositive
Type two things you learned today.
Slide 18 - Question ouverte
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