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OCR AS Chemistry Notes


ENTHALPY
All compounds have a certain amount of energy called enthalpy
During a chemical reaction, energy is either lost or gained from the surroundings
This is represented by H (enthalpy change)
o Units are always in KJmol-1

EXOTHERMIC

Energy released into surroundings
Examples:
o Neutralisation
o…

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Bond making is exothermic
o `BMX'




Normal Calorimeter

Water evaporates causing heat loss
Energy is lost as heat to surroundings
Incomplete combustion may occur causing energy loss
Water must be stirred constantly to maintain uniform temperature




Bomb Calorimeter

Fuel burns using oxygen to ensure complete combustion
Water is well insulated…

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OCR AS Chemistry Notes


HESS'S CYCLES
STANDARD CONDITIONS:
o 1 atmosphere pressure (100 kPa)
o 25oC (-298K)
o Concentration of 1 moldm-3


Hr o
o Enthalpy change that accompanies a reaction
o In the molar quantities expressed in a chemical equation
o Under standard conditions
o All reactants and products…

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USING STANDARD ENTHALPHY OF COMBUSTION




Hf o (C4H10) = (4 x Hc o [C]) + (5 x Hc o [H2]) ­ (5 x Hc o
[C4H10])
= (4 x -394) + (5 x -286) ­ (-2877)
= -129 KJmol-1
USING STANDARD ENTHALPHY OF FORMATION

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Hr o (CaCO3) = ­(Hf o [CaCO3]) + (Hf o [CaO]) + (Hf o [CO2])

= ­(-1096) + (-602) + (-296)
= +100 KJmol-1




EQUILIBRIA
DYNAMIC EQUILIBRIUM
A reversible reaction is in equilibrium when both the forward and backward reactions take place at the same time
and at the same…

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If more B is added, equilibrium shifts to right (in direction that uses up B)
If C is removed from the mixture, equilibrium shifts right (to make some more)
3. CHANGING PRESSURE
o Apply to gaseous reactions only
o Increase of pressure favours reaction which produces fewer molecules
o Decrease…

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Still produces acceptable yield
o Pressure of 200 atmospheres
High pressure must still be used
Not so high that workforce / environment put in danger
o Iron catalyst is used
Speeds up rate of reaction
Allows equilibrium to be established faster and lower temperatures to be used
Less energy used…

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1. TEMPERATURE
o Molecules have more energy
o Molecules move more
o Molecules are more likely to collide
Higher chance of successful collisions
o More molecules have energy of activation
More successful collisions
2. PRESSURE (WITH GASES)
o Same amount of molecules in smaller volume
o Molecules more likely to…

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Catalysts lead to greater profitability
o Products made more quickly and easily
o Lower fuel costs and reduce waste

BIOLOGICAL CATALYSTS
Biocatalysis = when the catalyst is an enzyme
Lock and key mechanism
o Enzyme and substrate shape are complementary to each other
Enzymes operate under mild conditions
o Low…

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