Energy - Module 3 UNIT 2 Chemsitry AS OCR

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Key notes on Module 3 of Unit 2 for OCR AS Chemistry
Enthalpy is the heat content that is stored in a chemical system
Although we cannot directly measure enthalpy, we are able to measure the change in enthalpy,
whether that be directly or indirectly (this depends on circumstances later explained)
Energy is conserved ­ it cannot be destroyed, and so heat that is lost from a chemical system is
equal to the heat gained to the chemical system and vice versa
In a chemical reaction, heat is exchanged with surroundings
Exothermic reactions are where the enthalpy of the reactants is greater than the enthalpy of the
products (i.e. more energy is needed to form the bonds than break the bonds) which means that
heat is lost to the surroundings. Heat increase to surroundings.
Endothermic reactions are where the enthalpy of the products is greater than the enthalpy of the
reactants (i.e. more energy is needed to break the bonds that is released when bonds are
formed) and so energy is taken into the chemical system from their surroundings. Heat decrease
to surroundings
Oxidation of fuels is an example of an exothermic reaction e.g. CH4 + 2O2 -> CO2 + 2H2O
Respiration is the most important exothermic reaction for life, and is needed by organisms to
convert sugars into carbon dioxide and water to release energy. Without this chemical reaction,
there would be no life
Photosynthesis is the most important endothermic reaction for life, where carbon dioxide and
water are converted into glucose and oxygen. Without photosynthesis, there would be no life
The thermal decomposition of limestone is also an example of an endothermic reaction, where
the calcium carbonate it contains decomposes by heat to make calcium oxide
Enthalpy profile diagrams are used to compare the enthalpy of the products with the enthalpy of
the reactants
o Exothermic ­ high to low
o Endothermic ­ low to high
Activation energy is the minimum amount of energy required to start a reaction by the breaking
of bonds (remember, for bonds to break, energy needs to be taken into a chemical system)
Standard conditions are used by all chemists because the enthalpy change varies depending on
conditions, so it's important that reactions carried out by scientists are done in the same way (i.e.
under the same conditions). The standard conditions are:
o 101 kPa (1 atmosphere)
o 298K ( 25 degrees Celsius)
o 1.0 moldm-3
Standard state is the physical state of a substance under the standard conditions of:
o 101 kPa (1 atmosphere)
o 298K ( 25 degrees Celsius)
The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in
its molar quantities as stated by its chemical equation, with all reactants and products in their
standard states under standard conditions
The standard enthalpy change of combustion is the enthalpy change that takes place when one
mole of a substance reacts completely with oxygen under standard conditions with all reactants
and products in their standard states

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Standard enthalpy change of formation of a compound is the enthalpy change that takes place
when one mole of a compound is formed from its constituent elements in their standard states
under standard conditions
o The enthalpy change of formation of an element is defined as 0 kJ mol-1
Q = MCT
o M = the mass of the surroundings that are involved in the heat exchange
o C = the specific heat capacity of the surroundings e.g. for water this is 4.…read more

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More than one reaction taking place
Enthalpy changes can be measured indirectly using enthalpy change of combustion ­ A = B ­ C
Enthalpy changes can be measured indirectly using enthalpy change of formation ­ A = C ­ B
An enthalpy cycle is a diagram that shows alternative routes between reactants and products
which allows the indirect determination of an enthalpy cycle change from other known enthalpy
changes using Hess' law
The rate of reaction is the change in the concentration of a reactant…read more

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Enzymes allow pure products to be created without any side reactions
The Boltzmann distribution is the distribution of energies of molecules at a particular
temperature, often shown as a graph
o Molecules have different energies within a reaction
o Important features of the distribution include:
The area under the curve is equal to the total number of molecules within the
sample, and the area does not change with conditions
There are no molecules with zero energy and so the curve starts at the origin
There…read more

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Catalyst presence ­ this has no affect on the position of the equilibrium because it speeds up the
rate of the forward and reverse reactions and actually increases the rate that the equilibrium is
established
The Haber Process is the process that produces ammonia by reacting together nitrogen from
distillation and hydrogen from reacting together methane and water
o the reaction is in equilibrium and is overall exothermic
o there are a greater number of moles of gas on the reactant side than the product…read more

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