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Scheme of work – Cambridge International AS Level Physical Science (8780)
Ch2 CHEMISTRY
Unit 2: Physical Chemistry
Recommended prior knowledge
Chemistry Unit 1 (Theoretical Chemistry) should have been studied before this unit.
Context
This unit can be studied immediately after Chemistry Unit 1, or after Chemistry Unit 5 (Organic Chemistry). However, it is recommended that it is studied before Unit
3 (Inorganic Chemistry) and Unit 4 (Industrial Processes).
Outline
The unit looks at enthalpy changes of reactions, bond energies and Hess’s law, le Chatelier’s principle and aspects of equilibria in gases and in solution. The
Brønsted-Lowry theory of acids and bases is developed, as are the qualitative aspects of chemical kinetics, including the Boltzmann distribution and catalysis.
Syllabus ref
Learning objectives
Suggested teaching activities
Learning resources
C4(a)
Explain that some chemical
reactions are accompanied by
energy changes, principally in the
form of heat energy; the energy
changes can be exothermic (∆H
negative) or endothermic.
Introduce the ∆H convention: positive for endothermic reactions and
negative for exothermic ones. Some important exothermic processes are
the oxidation of fuels and respiration of glucose + carbohydrates. Some
endothermic ones are the decomposition of CaCO3 and photosynthesis.
Chemistry for Advanced Level (ISBN
9780719586026) 5.1
Advanced Chemistry (ISBN
9780521423328) 42-43
AS Level and A Level Chemistry (ISBN
9780521544719) 5a.1-5a.4
site 4 (enthalpy changes)
site 10 (thermodynamics)
C4(b)
Explain and use the terms:
(i) standard conditions
Explain that ∆Ho is defined at 298K and 100 kPa, per mole of product or
reactant, and producing 1 mol dm–3 solutions (where relevant)
(ii) enthalpy change of reaction,
with particular reference to
formation and combustion
∆Hof, is defined in terms of 1 mol of product; but ∆Hoc in terms of 1 mol of
reactant. State symbols are essential.
Give examples of equations showing these enthalpy changes, e.g.
∆Hof: 2Al(s) + 1½O2(g) → Al2O3(s)
Chemistry for Advanced Level 5.1-5.3,
5.5
Advanced Chemistry 44
AS Level and A Level Chemistry 5a.6
Teaching AS Practical Skills 9
v1 2Y05
Cambridge International AS Level Physical Science (8780)
1
Syllabus ref
Learning objectives
Suggested teaching activities
(iii) bond energy (∆H positive, i.e.
bond breaking)
∆Hoc: C2H6(g) + 3½O2(g) → 2CO2(g) + 3H2O(l)
∆H for this process is positive, i.e. defined in terms of the breaking of 1
mol of bonds. e.g. E(Cl-Cl): ∆Ho for Cl2(g) → 2Cl(g)
E(C-H): ∆Ho for ¼CH4(g) → ¼C(g) + H(g)
Learning resources
C4(c)
Calculate enthalpy changes from
appropriate experimental results,
including the use of the
relationship:
enthalpy change = mc∆T
Suitable experiments include neutralisation of acids with hydroxides or
carbonates (use 1 mol dm–3 or 2 mol dm–3 solutions), powdered metals
(e.g. zinc) with solutions of metal salts (e.g. CuSO4(aq)), dissolving salts
(e.g. NH4NO3 or CaCl2) in water. Expanded polystyrene coffee cups
(supported inside 250 cm3 beakers to avoid tip-up) make useful
insulated calorimeters.
Chemistry for Advanced Level 5.2
Advanced Chemistry 44
AS Level and A Level Chemistry 5a.7
Teaching AS Practical Skills 8
5 (e)
State and apply Hess' Law to
construct simple energy cycles,
and carry out calculations
involving such cycles and
relevant energy terms, with
particular reference to:
(i) determining enthalpy changes
that cannot be found by direct
experiment, e.g. an enthalpy
change of formation from
enthalpy changes of
combustion
Hess’s Law may be defined as follows:
The overall enthalpy change of a reaction is independent of the route
taken providing the initial states and the final states are the same.
Chemistry for Advanced Level 5.4
Advanced Chemistry 45
AS Level and A Level Chemistry 5a.8
(ii) average bond energies
5 (f)
v1 2Y05
Construct, and interpret, a
reaction pathway diagram, in
terms of the enthalpy change of
the reaction, and of the activation
energy (see Section C6).
Practical work could include finding the ∆Ho for MgSO4 + 7H2O(l) →
MgSO4.7H2O(s) by dissolving each in water, and measuring the
respective temperature changes; or the ∆Ho for CaCO3(s) → CaO(s) +
CO2(g) by dissolving each solid in HCl(aq). Numerical practice with
cycles involving combustions of alkenes/alkenes; C(s), H2(g) and
hydrocarbons or alcohols. Practice with signs of ∆H and reactions
involving different numbers of moles.
Practice with cycles involving combustions and decompositions (e.g.
H2O2). Emphasise that bond energies are average values, so they are
not exact in specific cases. So, enthalpy changes calculated using
average bond energies are likely to be less reliable than values
calculated using data specific to the substances involved.
Use reaction pathway (enthalpy profile) diagrams to explain the meaning
of endo- and exo-thermicity. Practice with profile diagrams for a variety
of reactions. (Diagrams should include a y-axis labelled enthalpy, an xaxis labelled reaction progress, the reactants and products, together with
their state symbols. An arrow up from the reactants line to the peak of
the activation ‘hump’ should be labelled Ea(F). For a reversible reaction,
Cambridge AS Level Physical Science (8780)
Chemistry for Advanced Level 5.1
Advanced Chemistry 42-43
AS Level and A Level Chemistry 5a.15a.4
site 4 (enthalpy changes)
site 10 (thermodynamics)
2
Syllabus ref
Learning objectives
Suggested teaching activities
Learning resources
an arrow up from the products line to the peak of the activation ‘hump’
should be labelled Ea(R). A vertical arrow from an extended reactants
line to the products line should be labelled with ∆H. I.e. the ∆H arrow
would be shown going up for an endothermic reaction, and vice versa.)
C5(a)
Explain, in terms of rates of the
forward and reverse reactions,
what is meant by a reversible
reaction and dynamic equilibrium
A reversible reaction can take place in either direction. A dynamic
equilibrium is characterised by the following:
• it must occur in a closed system.
• forward and backward reactions occur at the same rate.
• concentrations of both reactants and products remain constant.
Chemistry for Advanced Level 9.1,
9.22
Advanced Chemistry 52
AS Level and A Level Chemistry 7a.1
Teaching AS Practical Skills 12
site 4 (equilibria)
site 6 Le Chatelier)
site 8 (chemical equilibrium)
C5(b)
State Le Chatelier's principle and
apply it to deduce qualitatively
(from appropriate information) the
effects of changes in
temperature, concentration or
pressure, on a system at
equilibrium
Emphasise that the principle states that it is the position of equilibrium
that changes to oppose (not cancel) an applied constraint such as a
change in temperature or concentration or pressure.
Practical examples could include: BiOCl + HCl; Cr2O72– + OH–;
indicators + H+/OH–; CrO42–/Cr2O72– + H+/OH–; NO2/N2O4 in a syringe
in a beaker of warm water, or squeezed. There are cd-roms that allow
Haber process simulations.
Chemistry for Advanced Level 9.3-9.5
Advanced Chemistry 52-53
AS Level and A Level Chemistry 7a.2
Teaching AS Practical Skills 12
C5(c)
Show understanding of, and
apply the Brønsted-Lowry theory
of acids and bases
Acids as proton donors, bases as proton acceptors. The idea of
competition between bases for protons. Use of the terminology A1 – B1
and A2 – B2 to define acid-base conjugate pairs. Examples such as; HCl
+ H2O; NH3 + HCl; NH3 + H2O; HNO3 + H2SO4
Chemistry for Advanced Level 6.1-6.3
Advanced Chemistry 74
AS Level and A Level Chemistry 7a.8
site 9 (acids and bases)
C5(d)
Explain qualitatively the
differences in behaviour between
strong and weak acids and bases
in terms of the extent of
dissociation
Examples could include NH3(aq), NaOH(aq), CH3CO2H(aq), HCl(aq).
The concept of pH as a measure of the degree of acidity/basicity of a
solution but formal instruction into the calculation of pH is not required.
The notion that, e.g. for acidic solutions, a dilute solution of a strong acid
could have the same pH value as a more concentrated solution of a
weaker acid. Conductivity measurement for equimolar solutions could
reinforce the idea of relative extents of dissociation.
Chemistry for Advanced Level 6.4
Advanced Chemistry 75
AS Level and A Level Chemistry 7a.9
C6(a)
Explain and use the terms: rate of
reaction; activation energy;
Define rate in terms of change of concentration per unit time; introduce
but do not expand upon the units of rate, mol dm–3 s–1. Explain that other
Chemistry for Advanced Level 8.1, 8.2
Advanced Chemistry 77,79
v1 2Y05
Cambridge AS Level Physical Science (8780)
3
Syllabus ref
Learning objectives
Suggested teaching activities
Learning resources
catalysis
properties may be proportional to concentrations (e.g. vol. of gas
evolved, or mass of reaction mixture remaining). Define catalysis in
terms of providing an alternative route of lower activation energy, and
define activation energy as the minimum energy reactants need in order
to undergo reaction (NOT the energy needed to start a reaction).
site 4 (kinetics)
site 8 (reaction kinetics)
site 10 (reaction kinetics)
C6(b)
Explain qualitatively, in terms of
collisions, the effect of
concentration changes on the
rate of a reaction
The greater the concentration, the greater the collision rate as particles
are closer together. Practical examples could include acid + marble
chips (gas syringe or top pan balance); thiosulfate + acid; acid + metal
(gas syringe).
Chemistry for Advanced Level 8.3
Advanced Chemistry 78
Teaching AS Practical Skills 14
C6(c)
Show understanding, including
reference to the Boltzmann
distribution, of what is meant by
the term activation energy
Plotting some Maxwell-Boltzmann data would emphasise the shape of
the curve. A realistic curve has, at room temperature, the most common
molecular energy at about 8 kJ mol–1, whereas a typical Eact is over
100 kJ mol–1. Emphasise the asymmetry of the curve, starting at E = 0
(where it is assumed that no particles have zero energy) and having a
long asymptotic high-energy tail.
Chemistry for Advanced Level 8.4
Advanced Chemistry 78
AS Level and A Level Chemistry 8a.3
C6(d)
Explain qualitatively, in terms of
the significance of changes to the
Boltzmann distribution compared
to changes in collision frequency,
the effect of temperature change
on the rate of a reaction
For the same number of molecules, the higher temperature curve should
have its maximum (modal value) at a higher energy, and the high-energy
tail should stretch further to the right. However, since the two curves
enclose the same area under them, the maximum of the higher
temperature curve will have a lower y-axis value.
It is a ‘rule of thumb’ that increasing the temperature of a reaction
mixture by 10 °C doubles the rate of a reaction. The increase in the
average velocity, and hence collision frequency, of the molecules will
account for less than 2% of this rate increase. A small rise in
temperature will cause there to be a much larger number/proportion of
molecules with E ≥ Eact. So, a much larger number/proportion of
collisions will result in molecules reacting together. I.e. a small increase
in temperature produces a large increase in reaction rate.
Chemistry for Advanced Level 8.5
Advanced Chemistry 78
AS Level and A Level Chemistry 8a.4
Teaching AS Practical Skills 15
C6(e)
(i) explain that, in the presence
of a catalyst, a reaction has a
different mechanism, i.e. one
of lower activation energy
Adding a ‘catalysed activation hump’ to an energy profile helps clarify
this point (see Section C5(f)).
Heterogeneous catalysts often work by weakening the bonds in the
reactants by the process of adsorption; for example, Pt in the car
exhaust catalytic converter. (see Section C13(i)). Homogeneous
catalysts either enhance the electro- or nucleo-philicities of reagents.
Chemistry for Advanced Level 8.6,
19.7
Advanced Chemistry 78, 81, 105
AS Level and A Level Chemistry 8a.58a.6
Teaching AS Practical Skills 16
v1 2Y05
Cambridge AS Level Physical Science (8780)
4
Syllabus ref
v1 2Y05
Learning objectives
Suggested teaching activities
Learning resources
(ii) Interpret this catalytic effect in
terms of the Boltzmann
distribution
Placing Eact(catalysed) at a lower energy than Eact on the distribution
curve shows the larger number of molecules (those to the right of the
Eact value) that can now react.
AS Level and A Level Chemistry 8a.7
Cambridge AS Level Physical Science (8780)
5