OCR AS Chemistry (F322.3) - Energy Revision Notes

These revision notes are for Energy - the third module of F322 (Organic Chemistry) from OCR AS Chemistry A.

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OCR AS Chemistry Notes
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
Energy released into surroundings
o Neutralisation
o Combustion
o Respiration
Energy is absorbed from the environment
o Thermal decomposition
o Photosynthesis
Calculating Enthalpy Changes
Using calorimetry
25cm3 of 1 moldm-3 of HCl + 25cm3 of 1.2 moldm-3 of NaOH
Temperature increases by 6.7oC
1. Energy released (J ) = m×c×T
= mass of solution ×specific heat capacity ×temperature change
o Specific heat capacity always = 4.2
So: 50 ×4.2 ×6.7 = 1407J
2. Calculate number of moles of the limiting reagent
o Limiting reagent = reagent with the least number of moles
c ×v = 1 ×25 = 0.025 mol
So : n = 1000 1000
3. Energy released
Number of moles
1407 = 56280 J /mol
= 0.025
But, is exothermic so: ­56,280 J/mol or ­56.28 KJ/mol
Bond breaking is endothermic

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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 with air layer to reduce heat loss to
Stirrer circulates water to maintain a uniform temperature
Water is in a closed system, so it can't evaporate
Electronic ignition makes the process easier…read more

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OCR AS Chemistry Notes
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 being in their standard state
Hc o
o Enthalpy change that takes place
o When one mole of a substance
o Reacts completely with oxygen
o Under standard conditions
o All reactants and products being…read more

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

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Hr o (CaCO3) = ­(Hf o [CaCO3]) + (Hf o [CaO]) + (Hf o [CO2])
= ­(-1096) + (-602) + (-296)
= +100 KJmol-1
A reversible reaction is in equilibrium when both the forward and backward reactions take place at the same time
and at the same rate
Can only be established in closed system
o Nothing can be added or removed from reacting mixture
Can only be established under constant conditions of temperature and pressure
No overall change occurs
o Concentrations…read more

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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)
o Apply to gaseous reactions only
o Increase of pressure favours reaction which produces fewer molecules
o Decrease of pressure favours reaction that produces more molecules
E.g.…read more

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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 to generate heat, reducing costs
Actual compromise conditions only convert 15% of nitrogen and hydrogen to ammonia
o Ammonia is liquefied and removed
o Unreacted gases are passed through reactor again
o Eventually all nitrogen…read more

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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
o Same amount of molecules in smaller volume
o Molecules more likely to collide
Higher chance of successful collisions
o More molecules in smaller volume
o Molecules more likely to collide
Higher chance of successful collisions
4.…read more

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Catalysts lead to greater profitability
o Products made more quickly and easily
o Lower fuel costs and reduce waste
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 temperatures
o Atmospheric pressure
o Optimum pH value
Often used in synthesis of organic chemicals
Enzymes in industry has many benefits:
o Lower temperatures and pressures can be used
o Often allow reaction which forms…read more

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