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Enthalpy changes and standard conditions
Chemical reactions involve breaking and making bonds between atoms
These processes involve energy exchanges between the reactants and
the surroundings
Enthalpy ­ the term used to describe the energy content of a system.
It has the symbol H and is measured in KJ

Page 2

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Mean bond enthalpies ­ Enthalpy needed to break a bond averaged over
different molecules. KJmol-1
Formation for one mole- H°r= H°r(bonds broken) - H°f(bonds made)
Bond breaking is endothermic and absorbs energy
Bond making is exothermic and releases energy.
Collision theory
Rate of reaction = increase in concentration/time
When a…

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The shape of the Maxwell-Boltzmann distribution changes as the
temperature is altered.
As the temp increases the energy distribution moves to the right and the
height of the decreases.
The area under the curve is constant as there is always the same amount
of particles.

The purple line is the…

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Increasing the concentration of the reactants will in the majority of cases
increase the rate of reaction, as it increases the probability of the
reactant particles colliding.
It therefore increases the rate because the rate of collision with the
activation energy increases.
Reversible reactions.
This means the reactants with make…

Page 5

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to reduce the pressure the equilibrium will shift to the right to
produce more ethanol to reduce the pressure
When the pressure is increased the reaction will shift to the side which has
less gas molecules.

Effect of heat
CH2CH2 (g) + H2O (g) <-> CH3CH2OH (g) this reaction is…

Page 6

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Some ions are spectators
They take no part in the reaction e.g.
Zn(s) + CuSO4(aq) -> Cu(s) + ZnSO4(aq)
Each zinc atom is oxidised
Zn(s) -> Zn2+(aq) + 2e-
Each copper ion is reduced
Cu2+(aq) + 2e- -> Cu(s)
Notice that the sulphate ions do not appear in these half…

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Down the group the atoms and so the molecules get larger.
The strength of van der waals forces between the molecules
increase, so the molecules are more strongly attracted together.
The most electronegative is fluorine and the least is iodine, so down group
7 the halogens become less electronegative.…

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The electrons are donated more easily, so the halide ions become
stronger reducing agents.
Fluoride ions and sulphuric acid
A fluoride ion is a very weak reducing agent, as it is very reactive and so
gains electrons easily.
The reaction of fluoride ions, in solid sodium fluoride, with concentrated

Page 9

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silver compounds, such as silver carbonate. The acid reacts with the
carbonate ions forming carbon dioxide. You cannot use hydrochloric acid
because it contains Chloride ions, which would give a white precipitate with
silver nitrate solution. Nitric acid contains nitrate ions, but all nitrates are
soluble in water.
To test…

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Reactions with water
Beryllium at the top of group 2 doesn't react with water or steam, even if is
heated until it glows. But the other group 2 elements react with water to
produce the metal hydroxide and hydrogen.
M(s) + 2H2O(l) -> M(OH)2(aq) + H2(g)
The reactions become more…


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