Foundation Physical and
Inorganic Chemistry
The energy of reactions
Contents
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Energetics
Kinetics
Equilibria
Redox Reactions
Group VII, The Halogens
Energetics
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Enthalpy Change (ΔH )
Calorimetry
Hess’s Law
Calculation of Bond Enthalpies
Using Bond Enthalpies
Enthalpy change (ΔH )
• All chemical reaction involve a transfer of energy. Chemists call this
energy change as the enthalpy change of the reaction.
• Exothermic reactions have a negative enthalpy change, that is they
transfer energy to their surroundings.
• Endothermic reactions have a positive enthalpy change, that is they
take in energy from their surroundings.
• The standard enthalpy change of combustion is the enthalpy change
when 1 mole of a compound is burnt completely in oxygen under
standard conditions (298K and 100kPa), all reactants and products
being in their standard state.
• The standard enthalpy change of formation is the enthalpy change
when 1 mole of a compound is formed from its elements under
standard conditions (298K and 100kPa), all reactants and products
being in their standard state.
Calorimetry
• The enthalpy change can be calculated from the
temperature change in a reaction using the equation:
– q = mcΔT
• q is the enthalpy change (J), m is the mass (g) c is the specific heat
capacity J g-1K-1, ΔT is the temperature change in K.
• Example: excess of magnesium was added to 100cm3 of
2M CuSO4 solution. The temperature increased by 45K.
The specific heat capacity of water is 4.2 J g-1K-1
• Find q using the above equation
– q = 100 x 4.2 x 45 =18810J
• Find the amount in moles that reacted
– 2X100/1000 =0.2
• Scale the enthalpy change for one mole of CuSO4
– 18810/0.2 = 94050J =94 kJ mol-1
Hess’s Law
• Hess’s law states that, if a reaction can take place by
more than one route and the initial and final conditions
are the same for each route.
route 1
ΔHf
• N2(g) + 3H2(g)
route 2
ΔH1
2N(g) + 6H(g)
• ΔHf = ΔH1 + ΔH2
2NH3 (g)
ΔH2
Calculation of Bond Enthalpies
• Bond enthalpies are a measure of the energy
required to break a bond. They are always
negative.
• The values quoted are usually average values,
as the strength of a bond between two atoms is
dependant on the molecule.
• They are difficult to measure directly they are
usually calculated from data enthalpy changes
of combustion of several compounds.
Using Bond Enthalpies
route 1
• N2(g) + 3H2(g)
ΔH1
ΔHf
route 2
2NH3 (g)
ΔH2
2N(g) + 6H(g)
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Bond enthalpies can be used to calculate the enthalpy change for a
reaction.
ΔHf = ΔH1 + ΔH2
– ΔH1 = the enthalpy for breaking bonds
– ΔH2 = the enthalpy for making bonds
For this reaction
– ΔHf = 3xE(H–H) + E(N) – 6xE(N–H)
• Where E(X–X) represents the bond enthalpy for XX
Kinetics
• Collision Theory
• Maxwell-Boltzmann Distribution
Collision Theory
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Reactions can only occur when reacting particles collide with enough
energy. This energy is called the activation energy. Most collisions do not
result in reaction.
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Increasing the temperature of the reaction increases the speed at which
particles move. There are more collision and the collisions have a higher
energy. This increases the number of effective collisions. Therefore the rate
of reaction is increased
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Increasing the concentration of reactants increase the number of particles
available to react. More collisions occur. Therefore the rate of reaction is
increased. Decreasing the size of particles increases the surface area. If
there is a larger surface available for particles to collide with there will be
more collisions. Therefore the rate of reaction is increased.
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Catalyst provide a different route for the reaction to happen. This route has
a lower activation energy. This means that more of the collisions will be
effective. The rate of reaction is increased
Maxwell-Boltzmann Distribution
•Below is a Maxwell-Boltzman distribution curve. It shows the distribution of
molecular energies in a gas.
•As the temperature increases the peak of the line moves lower and to the right.
(From the blue to the red on the picture.)
•The area under the graph represents the total number of particles and stays the
same whatever the temperature.
These particles do not have
enough energy to react
Number
of
particles
with
Energy E
Ea
•Ea is the activation energy, only collisions occurring
above this energy result in reaction.
•Increasing the temperature increases the number
of collisions occurring above the activation energy.
•The change from blue to red is only an increase in
temperature of 10°C, however the number of
particles that have enough energy to react has
increased significantly
These particle have enough
Energy to react
Energy E
Equilibria
• The Dynamic Nature of Equilibria
• Le Chatelier’s Principle.
• The Haber Process
The Dynamic Nature of Equilibria
• Many reactions are reversible. That is they can
proceed in both directions at the same time.
• An equilibrium can be established if a reaction
occurs in a closed system
• Reactions do not stop at equilibrium, but the
concentration of products and reactants remains
constant because the reaction is proceeding at
the same rate in both directions.
• This does not mean that there are equal
amounts of products and reactant though.
Le Chatelier’s Principle
• Le Chatelier’s principle states that:
– The position of the equilibrium of a system changes to
minimise the effect of any imposed change in conditions.
• This means:
– Increasing the temperature favours the endothermic reaction
– Increasing the pressure in a gaseous reaction favours the side of
the reaction which has the fewest number of gaseous molecules,
because this will educe the pressure.
– However adding a catalyst does not effect the position of the
equilibrium.
The Haber Process
• N2(g) + 3H2(g) 2NH3(g) ΔH=-92kJmol-1
• Increasing the temperature will move the equilibrium in
favour of the reactants, in the endothermic direction.
• Increasing the pressure will favour the products because
this side has the fewer number of gaseous molecules.
• Lowering the temperature will favour the formation of the
products, but will mean that reaction will happen slowly.
A compromise temperature is used.
• Too high a pressure would have high energy costs and
expensive thicker walled pipes and reactor so a
compromise pressure is used.
• A finely divided iron catalyst is used to speed up the
reaction, and the unconverted reactants are recycledd
Redox Reactions
• Oxidation and Reduction
• Oxidation States
• Redox Equations.
Oxidation and Reduction
• The term redox is used for the
simultaneous processes of oxidation and
reduction.
• Oxidation is the loss of electrons.
• Reduction ids the gain of electrons.
• Any reaction which involves reduction also
involves oxidation.
• An oxidising agent removes electrons, a
reducing agent donates electrons.
Oxidation States
• Oxidation states show how oxidised or reduced an
element is within a compound or ion.
• If the oxidation state increases the substance is oxidised
• If the oxidation state decreases the substance is
reduced.
• Rules for oxidation numbers:
– For atoms in element the oxidation state is always 0
– For atoms in simple ions the oxidation state is the same as the
charge
– In compounds the total of all the oxidation states is 0
– In polyatomic ions the total of all the oxidation states is the same
as the charge on the ion
Redox Equations
• Sometimes it is easier to show what is being
oxidised and reduced by writing half equations.
• E.g Ca +Cl2  CaCl2
• This can be written as:
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– and
Ca  Ca2+ + 2eCl2 + 2e-  2Cl-
• Remember only one element loses or gains
electrons.
• The equations must balance for atoms and
charge.
The Halogens
• Trends
• Key facts
Trends
Property
Trend down Explanation
group
Electronegativity
decreases
because size of atoms increase
Boiling point
increases
because temporary dipole induced
dipole interactions increase
Oxidising ability
decreases
because the ability to accept an
electron decreases down the group
Reducing ability of increases
halide
because the ability to donate an
electron from the halide increases
down the group
Key Facts
• All halogens form diatomic molecules
• A halogen will displace a halogen which is below it from
its salts because reactivity decreases down the group. ie
Chlorine will displace bromine.
• The reaction of silver nitrate with the halide is a test for
halides: AgCl –white, AgBr –cream, and AgI –yellow.
• Silver halides are used in photography as they darken in
the presence of light.
• Chlorine is used extensively for disinfection.
Summary
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Energetics
– All chemical changes involve the transfer of energy. The overall enthalpy transfer
during a chemical change is independent of the route taken.
Kinetics
– The rate of reaction is effect by factors that effect the overall collision rate. We
can use the Maxwell-Boltzman distribution to show the change in distribution of
molecular kinetic energy with increasing temperature.
Equilibria
– Many reactions are reversible and in a closed system reach an equilibrium. The
position of the equilibrium of a system changes to minimise the effect of any
imposed change in conditions. The nature of equilibria means that sometimes
compromises need to be made in industrial processes.
Redox Reactions
– These are reactions which involve simultaneous oxidation and reduction. We use
the concept of oxidation number to indicate how oxidised or reduced elements
are.
Group VII, The Halogens
– The halogens have many similar properties. They are all reactive and this
reactivity decreases down the group. Chorine is used as a disinfectant.