Chemistry In Action

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  • Created on: 18-05-11 10:25
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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
We measure Enthalpy changes, H
Enthalpy change ­ the heat energy change measured under conditions of
constant pressure
Enthalpy changes are calculated by H= H of products- H of reactants.
Exothermic ­reactions release heat energy, Delta H is negative
Endothermic- reactions absorb heat energy, Delta H is positive
Standard conditions ­ enthalpy change is dependent on temp and
pressure and amount of substance used
Standard enthalpy changes are H°
The standard pressure is 100Kpa
The substances are in their standard states at 298K
The value is given by kJ/per mole
Standard enthalpy of Formation H°f - the enthalpy change when one
mole of a compound is formed from its elements in their standard states.
The H°f of an element is zero.
Standard enthalpy of Combustion H°C- the enthalpy change when one
mole of a substance is completely burned in oxygen under standard
Calculations enthalpy of reaction
The enthalpy changes of chemical reactions can be found using a
It insulates the reaction from its surroundings, the reaction is carried out in
the calorimeter and the change in temperature of the calorimeter is
Dissolving, Neutralization, formation and combustion reactions can use this
Heat= mass of substance*specific heat capacity*temperature change.
Specific heat capacity is the heat required to raise the temp of 1g of a
substance by 1K. For water it is 4.18JK-1g-1
Energy cycle diagrams
The law of conservation of energy- Energy cannot be created or
destroyed only changed from one from to another.
Hess' Law- If a reaction can occur by more than one route, the overall
enthalpy change is independent of the route taken.
H°f= H°f(products) - H°f (reactants).
H°c= H°c(reactants) - H°c (products)
Bond enthalpies- the standard enthalpy change associated with breaking
one mole of bonds in a gaseous substance into individual gaseous atoms.
The values are always positive as breaking bonds requires energy.
The value for making a bond is the same with an opposite sign.

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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.…read more

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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 effect of increasing the temperature.
It increases the energy with which the particles collide so that more of
these collisions possess the activation energy.
Effect of catalysts.…read more

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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 the products and the products will
make the reactants.
Dynamic equilibrium
In a reversible reaction, when the concentration of the reactants and
products are constant, then the reaction is at equilibrium.…read more

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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 exothermic, so it
gives off heat.
If the temperature was increased then
the forward reaction is exothermic so the backward reaction is
The temp is raised so it will want to cool down so the endothermic
reaction is favoured as it absorbs heat.
So the equilibrium shifts to the left.…read more

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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 equations, they
don't change during the reaction, so they are spectator ions.
Oxidation states
the number of electrons needed to be gained or lost to make a
neutral atom
Rules of an oxidation state
1.…read more

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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. Electonegativity is the power
of an atom to withdraw electron density in a covalent bond.
Down group 7:
Successive atoms have more occupied energy levels.…read more

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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
sulphuric acid is not a redox reaction it is an acid base reaction
NaF + H2SO4 -> NaHSO4 + HF.…read more

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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 the colour add ammonia solution as the solubility of the silver
halides decreases in the order AgCl> AgBr> AgI.
Precipitate Addition of dilute Addition of
ammonia solution concentrated
ammonia solution.…read more

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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 vigorous as you go down the group.
Burns vigorously with a bright white flame.
Reacts slowly with water.
The Mg(OH)2 produced is sparingly soluble. Just enough dissolves to
produce an alkaline solution.…read more


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