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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Page 1

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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…

Page 2

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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…

Page 3

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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)…

Page 4

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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…

Page 5

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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+…

Page 6

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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…

Page 7

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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…

Page 8

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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…

Page 9

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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…

Page 10

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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…

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