AQA A-level Chemistry 'Unit 5 (CHEM5)' Revision

Comprehensive revision notes for the AQA's specification of Chemistry Unit 5 (CHEM5).

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Explain two ways of improving the reliability of results obtained from titration of a sample of
water from a polluted river
Repeat titration until concordant results, average concordant results
Analyse several samples from different areas of the river
Thermodynamics
Explain why the enthalpy of vaporisation of HF is greater than the enthalpy of vaporisation
of NH3
Both compounds have hydrogen bonding
F atom has a smaller size than N atom
F atom has a greater electronegativity than N atom
HF is more polar than NH3
HF has stronger hydrogen bonding than NH3
Enthalpy Change H
Remember to practice constructing Born-Haber cycles and calculating experimental values from
Born-Haber cycles by adding upward arrows and subtracting downward arrows
State the definition of the standard enthalpy of formation Hf
Enthalpy change when one mole of a substance is formed from its elements
With all reactants and products in their standard states
Under standard conditions, temperature of 298K and pressure of 100kPa
State the definition of the enthalpy of atomisation Hat
Enthalpy change when one mole of gaseous atoms is formed from its element
With the element in its standard state
State the definition of the first ionisation enthalpy Hie1 and second ionisation enthalpy Hie2
First ionisation enthalpy is the enthalpy change when one mole of electrons is
removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
Second ionisation enthalpy is the enthalpy change when one mole of electrons is
removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

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State the definition of the first electron affinity Hea1 and second electron affinity Hea2
First electron affinity is the enthalpy change when one mole of electrons is added to
one mole of gaseous atoms to form one mole of gaseous 1- ions
Second electron affinity is the enthalpy change when one mole of electrons is added
to one mole of gaseous 1- ions to form one mole of gaseous 2- ions
State the definition of the enthalpy of lattice formation Hlf and enthalpy of lattice…read more

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State the definition and equation for the first electron affinity Hea1 of Cl and second
electron affinity Hea2 of S
First electron affinity is the enthalpy change when one mole of electrons is added to
one mole of gaseous Cl atoms to form one mole of gaseous Cl- ions
Cl(g) + e- Cl-(g)
Second electron affinity is the enthalpy change when one mole of electrons is added
to one mole of gaseous S- ions to form one mole of gaseous S2- ions
S-(g) + e-…read more

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Explain why the mean bond dissociation enthalpy of Cl2 is greater than the mean bond
dissociation enthalpy of Br2
Bond is shorter, bonding pair of electrons is close to nucleus
Stronger attraction between bonding pair of electrons and nucleus
Explain why the second ionisation enthalpy is greater than the first ionisation enthalpy
Electron removed is closer to nucleus
Electron removed from a positive species, stronger attraction between electron
removed and nucleus
Explain why the enthalpy of lattice dissociation of CaF2 is greater than the enthalpy…read more

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Explain why the enthalpy of hydration of Al3+ is greater than the enthalpy of hydration of Na+
Al3+ ion has a greater charge, greater charge density than Na+ ion
Stronger electrostatic forces of attraction between positively-charged Al3+ ions and
the electron-rich O- of water, stronger ionic bonds between positively-charged Al3+
ions and the electron-rich O- of water
More energy required to break the stronger ionic bonds between positively-charged
Al3+ ions and the electron-rich O- of water
State the definition of the mean bond dissociation enthalpy…read more

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State two ways of influencing the theoretical lattice enthalpy calculated using the perfect
ionic model and state two ways of increasing the difference between the theoretical lattice
enthalpy and the experimental lattice enthalpy
Ionic radius, distance between ions
Ionic charge, charge density
Increase covalent character, increase polarisation
Positive ion has a large size, negative ion has a small size
Both ions have a high charge, high charge density
State and explain the type of bonding in LiF
Perfect ionic model assumes only electrostatic attraction between…read more

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Entropy Change H
Remember to practice calculating the temperature when a reaction becomes feasible by using the
third formula
State the units of S in the formula
S is in JK-1mol-1
Explain why entropy is zero when the temperature is 0K and why entropy increases when
the temperature increases
Particles are not moving, stationary
No disorder, maximum order
Particles start moving, vibrating
More disorder
Explain why boron has a greater entropy than carbon and nitrogen has a greater entropy
than mercury
Boron is a liquid,…read more

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Free-Energy Change G
State the units of G, H, T and S in the formulae
G is in kJmol-1
H is in kJmol-1
T is in K
S is in kJK-1mol-1
Explain how G, H, T and S in the formula follow for a straight line
G is y, G is a variable
H is c, H is a constant
T is x, T is a variable
-S is m, -S is the gradient, -S is a constant
A graph of G against T is…read more

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State the equation for the reaction between N2 and O2 to produce NO and explain why G
remains approximately constant at all temperatures
N2 + O2 NO2
No change in number of gas species, 2 moles of gas to 2 moles of gas
Entropy change S is very small, disorder remains approximately constant
Free-energy change G remains approximately constant at all temperatures
State the equation for the reaction that occurs when CH3OH combusts completely in oxygen
and explain why the reaction will occur at any…read more

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State the equation for the reaction between NO and O2 to produce NO2 and given that the
reaction is exothermic, explain why the reaction will not occur in an internal combustion
engine
2NO + O2 2NO2
Enthalpy change H is negative, H < 0
Decrease in number of species, 3 moles of species to 2 moles of species
Entropy change S is negative, S < 0
Free-energy change G is positive at high temperatures, internal combustion engine
is at high temperatures above the critical temperature,…read more

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