TOPIC 01
Energetics
Energetics is the study of energy changes that occur during chemical reactions.
Enthalpy Change
The enthalpy change of a reaction is the heat energy change when reactants are converted into products under specified conditions.
Exothermic vs Endothermic
Energy is released to the surroundings and ΔH < 0.
Endothermic:Energy is absorbed from the surroundings and ΔH > 0.
Calorimetry
Calorimetry can be used to determine the energy transferred during a reaction.
- q = energy transferred
- m = mass of solution
- c = specific heat capacity
- ΔT = temperature change
WORKED EXAMPLE
A reaction heats 100 g of water by 15 °C. Take the specific heat capacity of water as 4.18 J g⁻¹ K⁻¹.
Therefore, 6.27 kJ of energy is transferred to the water.
Bond Enthalpy
Bond breaking requires energy, while bond formation releases energy.
Hess's Law
Hess's Law states that the total enthalpy change for a reaction is independent of the route taken between reactants and products.
TOPIC 02
Intermolecular Forces
Intermolecular forces act between molecules and influence physical properties such as boiling point and melting point.
London Forces
London forces arise from temporary fluctuations in electron density and occur between all atoms and molecules.
Larger molecules generally have stronger London forces because they contain more electrons and have more easily polarised electron clouds.
Permanent Dipole-Dipole Forces
These occur between polar molecules because of attractions between permanent partial charges.
Hydrogen Bonding
Hydrogen bonding is a particularly strong intermolecular force involving hydrogen bonded to a highly electronegative atom, such as nitrogen, oxygen or fluorine.
Stronger intermolecular forces generally require more energy to overcome, leading to higher boiling temperatures.
TOPIC 03
Redox Chemistry
Redox reactions involve the transfer of electrons.
Oxidation and Reduction
Oxidation Is Loss of electrons. Reduction Is Gain of electrons.
Oxidation Numbers
- Oxidation number increases → oxidation.
- Oxidation number decreases → reduction.
Half Equations
Half equations show the electron transfer occurring during oxidation or reduction.
TOPIC 04
Group 1 — The Alkali Metals
Group 1 contains lithium, sodium, potassium and other alkali metals.
Reactivity
Reactivity increases down the group because the outer electron is further from the nucleus and is more easily removed.
Reaction with Water
Group 1 metals react with water to produce an alkaline solution and hydrogen gas.
Reaction with Oxygen
Group 1 metals react with oxygen to form oxides and, for some heavier members, other oxygen-containing compounds.
TOPIC 05
Group 2 — The Alkaline Earth Metals
Group 2 contains beryllium, magnesium, calcium, strontium, barium and radium.
Reactivity
Reactivity generally increases down the group because the outer electrons become easier to remove.
Reaction with Water
The reactions generally become more vigorous down the group.
Solubility Trends
Group 2 hydroxide solubility generally increases down the group.
Group 2 sulfate solubility generally decreases down the group.
TOPIC 06
Group 7 — The Halogens
Group 7 elements are reactive non-metals that exist as diatomic molecules.
Reactivity
Reactivity decreases down the group because the outer shell is further from the nucleus and attraction for an incoming electron becomes weaker.
Displacement Reactions
A more reactive halogen can displace a less reactive halide ion from solution.
Halide Ion Tests
Aqueous silver nitrate can be used to distinguish chloride, bromide and iodide ions.
- Chloride → white precipitate.
- Bromide → cream precipitate.
- Iodide → yellow precipitate.
TOPIC 07
Introduction to Kinetics
Kinetics concerns the rate at which chemical reactions occur.
Collision Theory
For a reaction to occur, particles must collide with sufficient energy and with the appropriate orientation.
Activation Energy
Activation energy is the minimum energy required for a successful reaction.
A catalyst provides an alternative reaction pathway with lower activation energy.
Factors Affecting Rate
- Temperature
- Concentration
- Pressure for gaseous reactions
- Surface area of a solid
- Presence of a catalyst
Maxwell-Boltzmann Distribution
Increasing temperature gives particles greater average kinetic energy and increases the proportion of particles with energy greater than the activation energy.
TOPIC 08
Introduction to Equilibria
A reversible reaction can reach a state of dynamic equilibrium.
At equilibrium, concentrations of reactants and products remain constant, although both forward and reverse reactions continue.
Le Chatelier's Principle
When a system at equilibrium is disturbed, the equilibrium position shifts in the direction that opposes the change.
Temperature
Increasing temperature favours the endothermic direction. Decreasing temperature favours the exothermic direction.
Pressure
For gaseous equilibria, increasing pressure favours the side with fewer moles of gas.
Catalysts
A catalyst speeds up both forward and reverse reactions equally. It does not change the equilibrium position.
TOPIC 09
Alcohols
Alcohols contain the hydroxyl functional group, −OH.
Classification
- Primary alcohols
- Secondary alcohols
- Tertiary alcohols
Combustion
Alcohols can undergo complete combustion to form carbon dioxide and water.
Oxidation
Primary alcohols can be oxidised first to aldehydes and then to carboxylic acids.
Secondary alcohols can be oxidised to ketones. Tertiary alcohols are resistant to oxidation under typical laboratory conditions.
Dehydration
Alcohols can undergo dehydration to form alkenes.
TOPIC 10
Halogenoalkanes
Halogenoalkanes contain a halogen atom bonded to a carbon atom.
X may be F, Cl, Br or I.
Nucleophilic Substitution
Halogenoalkanes can react with nucleophiles in substitution reactions.
Reaction with Aqueous Hydroxide
Aqueous hydroxide ions can replace the halogen atom to produce an alcohol.
Reaction with Cyanide
Cyanide ions can replace the halogen and increase the carbon chain by one carbon atom.
Reaction with Ammonia
Halogenoalkanes can react with ammonia to form amines.
Hydrolysis
Iodoalkanes generally hydrolyse more readily than bromoalkanes, which hydrolyse more readily than chloroalkanes.
TOPIC 11
Organic Spectra
Spectroscopic techniques provide information that can be used to identify organic compounds.
Infrared Spectroscopy
Infrared spectroscopy can be used to identify functional groups from characteristic absorption regions.
- O−H absorption → broad absorption at higher wavenumbers.
- C=O absorption → strong absorption around 1700 cm⁻¹.
- C−H bonds → absorptions in the appropriate C−H region.
Mass Spectrometry
Mass spectrometry can provide information about molecular mass and fragmentation.
The molecular ion peak can provide the relative molecular mass of an organic compound.
Using Spectra Together
Different analytical techniques can be combined to identify an unknown organic compound.
PRACTICE
Unit 2 Mixed Practice
Try each question before opening the answer.
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q = mcΔT
8360 = 200 × 4.18 × ΔT
ΔT = 10.0 K
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Larger molecules generally have more electrons and are more polarizable, producing stronger London forces. More energy is therefore required to separate the molecules.
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The outer electron is further from the nucleus and increasingly shielded by inner electron shells. The attraction between the nucleus and outer electron decreases, so the electron is lost more easily.
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Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
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Particles have greater average kinetic energy. A greater proportion of particles have energy greater than the activation energy and successful collisions occur more frequently.
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The equilibrium shifts towards the side with fewer moles of gas, provided the two sides contain different numbers of gaseous moles.
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A primary alcohol can be oxidised to an aldehyde and then further oxidised to a carboxylic acid.
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CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
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Bromide ions, Br⁻.
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A catalyst provides an alternative pathway with lower activation energy. It speeds up both the forward and reverse reactions and does not change the equilibrium position.
QUICK REVIEW
Unit 2 Key Ideas
q = mcΔT
ΔH = Σ bonds broken − Σ bonds formed
Oxidation = loss of electrons. Reduction = gain of electrons.
Group 1 reactivity increases down the group.
Group 7 reactivity decreases down the group.
At dynamic equilibrium, forward rate = reverse rate.
Primary alcohol → aldehyde → carboxylic acid.
R−X + OH⁻ → R−OH + X⁻