Cambridge IGCSE Chemistry (0620) Syllabus

Describe metallic bonding and use it to explain electrical conductivity and malleability.

Metallic bonding is the attraction between a lattice of positive metal ions and a 'sea' of delocalised electrons. The mobile electrons carry charge (conducts), and layers of ions can slide over one another without breaking the bonding (malleable/ductile).

State the formulae of these common ions: ammonium, hydroxide, nitrate, carbonate, sulfate.

Ammonium $\ce{NH4+}$, hydroxide $\ce{OH-}$, nitrate $\ce{NO3-}$, carbonate $\ce{CO3^2-}$, sulfate $\ce{SO4^2-}$.

Write the balanced equation for the complete combustion of methane, including state symbols.

$\ce{CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)}$.

What is the formula of aluminium oxide and how is it deduced from ion charges?

$\ce{Al2O3}$. Aluminium forms $\ce{Al^3+}$ and oxide is $\ce{O^2-}$; balancing charges (lowest common multiple 6) gives two $\ce{Al^3+}$ and three $\ce{O^2-}$.

Define the mole and state the value of the Avogadro constant.

A mole is the amount of substance containing the same number of particles as there are atoms in 12 g of carbon-12. The Avogadro constant is $6.02 \times 10^{23}$ particles per mole.

State the relationship between moles, mass and molar mass.

$n = \dfrac{m}{M}$, where $n$ = number of moles, $m$ = mass in grams, and $M$ = molar mass in $\text{g/mol}$.

State the molar volume of a gas at room temperature and pressure (r.t.p.) and the formula linking moles and gas volume.

At r.t.p. one mole of any gas occupies $24\,\text{dm}^3$ ($24000\,\text{cm}^3$). $n = \dfrac{V}{24}$ where $V$ is volume in $\text{dm}^3$.

How is concentration in $\text{mol/dm}^3$ calculated from moles and volume?

$c = \dfrac{n}{V}$, where $c$ is concentration in $\text{mol/dm}^3$, $n$ is moles, and $V$ is volume in $\text{dm}^3$. To convert $\text{cm}^3$ to $\text{dm}^3$, divide by 1000.

Define percentage yield and percentage purity.

$\%\text{ yield} = \dfrac{\text{actual yield}}{\text{theoretical yield}} \times 100$. $\%\text{ purity} = \dfrac{\text{mass of pure product}}{\text{total mass of sample}} \times 100$.

List four factors that increase the rate of a chemical reaction.

Increasing temperature, increasing concentration (or pressure for gases), increasing surface area of a solid (smaller pieces), and adding a catalyst.

Explain in terms of collision theory why increasing temperature speeds up a reaction.

Higher temperature gives particles more kinetic energy, so they move faster and collide more frequently, and a greater proportion of collisions have energy equal to or above the activation energy, so successful collisions increase.

What is a catalyst, and how does it increase reaction rate?

A catalyst is a substance that increases the rate of a reaction without being chemically changed or used up. It works by providing an alternative reaction pathway with a lower activation energy.

What is meant by a reversible reaction, and what is dynamic equilibrium?

A reversible reaction can proceed in both forward and backward directions (shown by $\rightleftharpoons$). Dynamic equilibrium occurs in a closed system when the forward and backward reactions occur at equal rates, so concentrations of reactants and products stay constant.

State Le Chatelier's principle.

If a change in conditions (concentration, pressure or temperature) is made to a system at equilibrium, the position of equilibrium shifts to oppose (partially counteract) that change.

Give the equation and conditions for the Haber process.

$\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$. Conditions: about $450\,^{\circ}\text{C}$, $200\,\text{atm}$ pressure, and an iron catalyst.

Give the equation and conditions for the Contact process step that makes sulfur trioxide.

$\ce{2SO2(g) + O2(g) <=> 2SO3(g)}$. Conditions: about $450\,^{\circ}\text{C}$, $1{-}2\,\text{atm}$ pressure, and a vanadium(V) oxide ($\ce{V2O5}$) catalyst. Used in making sulfuric acid.

Define exothermic and endothermic reactions in terms of energy and surroundings.

Exothermic reactions release energy (heat) to the surroundings, so $\Delta H$ is negative and temperature rises. Endothermic reactions absorb energy from the surroundings, so $\Delta H$ is positive and temperature falls.

On a reaction profile (energy level diagram), what do activation energy and $\Delta H$ represent?

Activation energy ($E_a$) is the minimum energy needed for reactants to react — the energy barrier from reactants to the peak. $\Delta H$ is the overall enthalpy change — the energy difference between reactants and products.

Explain bond breaking and bond making in terms of energy.

Bond breaking is endothermic (absorbs energy); bond making is exothermic (releases energy). If energy released forming bonds exceeds energy absorbed breaking bonds, the reaction is overall exothermic.

Define oxidation and reduction in terms of oxygen, hydrogen and electrons.

Oxidation is gain of oxygen, loss of hydrogen, or loss of electrons. Reduction is loss of oxygen, gain of hydrogen, or gain of electrons. Remembered as OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons).

What is an oxidising agent and a reducing agent?

An oxidising agent oxidises another species and is itself reduced (it gains electrons). A reducing agent reduces another species and is itself oxidised (it loses electrons).

How can acidified potassium manganate(VII) and potassium iodide be used to test for oxidising/reducing agents?

A reducing agent turns acidified potassium manganate(VII) from purple to colourless. An oxidising agent turns potassium iodide solution from colourless to brown (iodine, $\ce{I2}$, is released).

Distinguish between electrolysis behaviour at the cathode and anode.

During electrolysis, positive ions (cations) move to the cathode (negative electrode) and are reduced (gain electrons); negative ions (anions) move to the anode (positive electrode) and are oxidised (lose electrons).

Write the electrode half-equations for the electrolysis of molten lead(II) bromide.

Cathode: $\ce{Pb^2+ + 2e- -> Pb}$. Anode: $\ce{2Br- -> Br2 + 2e-}$.

In the electrolysis of concentrated aqueous sodium chloride (brine), what is produced at each electrode?

Cathode: hydrogen gas ($\ce{H2}$). Anode: chlorine gas ($\ce{Cl2}$). The remaining solution contains sodium hydroxide ($\ce{NaOH}$).

What is electroplating and why is it done?

Electroplating is coating an object with a thin layer of metal using electrolysis, with the object as the cathode and the plating metal as the anode in a solution of the metal's ions. It is done to improve appearance and to resist corrosion.

Define an acid, a base and an alkali (proton/ion definitions).

An acid is a proton ($\ce{H+}$) donor that produces $\ce{H+}$ ions in aqueous solution. A base is a proton acceptor (often a metal oxide/hydroxide). An alkali is a soluble base that produces $\ce{OH-}$ ions in aqueous solution.

Give the colours of litmus, thymolphthalein, and methyl orange in acid and alkali.

Litmus: red in acid, blue in alkali. Thymolphthalein: colourless in acid, blue in alkali. Methyl orange: red in acid, yellow in alkali.

State the general products of: acid + metal, acid + base, and acid + carbonate.

Acid + metal $\to$ salt + hydrogen. Acid + base (neutralisation) $\to$ salt + water. Acid + carbonate $\to$ salt + water + carbon dioxide.

Write the reactivity series for these metals from most to least reactive: K, Na, Ca, Mg, Al, Zn, Fe, Cu, Ag, Au.

Potassium > Sodium > Calcium > Magnesium > Aluminium > (Carbon) > Zinc > Iron > (Hydrogen) > Copper > Silver > Gold.

Name the raw materials and main reactions in the blast furnace extraction of iron.

Raw materials: iron ore (haematite, $\ce{Fe2O3}$), coke, limestone and hot air. Key reactions: $\ce{C + O2 -> CO2}$; $\ce{CO2 + C -> 2CO}$; $\ce{Fe2O3 + 3CO -> 2Fe + 3CO2}$. Limestone forms slag: $\ce{CaCO3 -> CaO + CO2}$ then $\ce{CaO + SiO2 -> CaSiO3}$.

What two conditions are needed for iron to rust, and name two methods of rust prevention?

Rusting needs both oxygen (air) and water. Prevention: barrier methods (painting, greasing, plating) and sacrificial protection (e.g. galvanising with zinc, or attaching a more reactive metal such as zinc/magnesium).

Give the chemical tests for these gases: hydrogen, oxygen, carbon dioxide, ammonia, chlorine.

Hydrogen: lighted splint gives a 'squeaky pop'. Oxygen: relights a glowing splint. Carbon dioxide: turns limewater milky/cloudy. Ammonia: turns damp red litmus blue. Chlorine: bleaches damp litmus paper white.

Describe the flame test colours for lithium, sodium, potassium, calcium and copper(II) ions.

Lithium $\ce{Li+}$: red. Sodium $\ce{Na+}$: yellow. Potassium $\ce{K+}$: lilac. Calcium $\ce{Ca^2+}$: orange-red. Copper(II) $\ce{Cu^2+}$: blue-green.

How are sulfate, chloride and carbonate ions identified in solution?

Sulfate: add dilute nitric acid then barium nitrate/chloride — white precipitate. Chloride: add dilute nitric acid then silver nitrate — white precipitate. Carbonate: add dilute acid — effervescence of $\ce{CO2}$ that turns limewater milky.

List the main gases of clean, dry air and their approximate percentages.

Nitrogen about $78\%$, oxygen about $21\%$, argon (noble gases) about $1\%$, and carbon dioxide about $0.04\%$ (plus variable water vapour).

Name two atmospheric pollutants from car engines and state how each forms and its harmful effect.

Carbon monoxide ($\ce{CO}$) from incomplete combustion — a toxic gas reducing oxygen transport in blood. Oxides of nitrogen ($\ce{NO_x}$) from nitrogen and oxygen reacting at high engine temperatures — cause acid rain and respiratory problems.

Explain how a catalytic converter reduces pollution, giving an equation.

A catalytic converter (platinum/rhodium catalyst) converts harmful gases into less harmful ones, e.g. $\ce{2CO + 2NO -> 2CO2 + N2}$, removing carbon monoxide and nitrogen oxides.

How does carbon dioxide contribute to climate change (the greenhouse effect)?

$\ce{CO2}$ (a greenhouse gas, like methane) absorbs and re-emits infrared (heat) radiated from the Earth's surface, trapping heat in the atmosphere and raising the average global temperature (enhanced greenhouse effect).

Define a homologous series and state three of its characteristics.

A homologous series is a family of organic compounds with the same general formula and similar chemical properties. Members differ by $\ce{CH2}$, show a gradual trend in physical properties down the series, and share the same functional group.

What is structural isomerism? Give the molecular formula common to the butane isomers.

Structural isomers are compounds with the same molecular formula but different structural (atom arrangement) formulae. For example, butane and methylpropane both have the formula $\ce{C4H10}$.

State the general formulae of alkanes and alkenes.

Alkanes: $\ce{C_nH_{2n+2}}$ (saturated). Alkenes: $\ce{C_nH_{2n}}$ (unsaturated, containing a $\ce{C=C}$ double bond).

How do you test for the presence of a carbon-carbon double bond (an alkene)?

Add bromine water (orange/brown). An alkene decolourises bromine water (turns it colourless) via an addition reaction; a saturated alkane does not.

What is cracking, why is it carried out, and give an example equation?

Cracking is breaking large, less useful long-chain alkanes into smaller, more useful alkanes and alkenes using heat and a catalyst. Example: $\ce{C10H22 -> C8H18 + C2H4}$. It produces more petrol-range fuels and alkenes for plastics.

Compare a substitution reaction of alkanes with an addition reaction of alkenes.

Alkanes (saturated) undergo substitution, e.g. with chlorine in UV light: $\ce{CH4 + Cl2 -> CH3Cl + HCl}$. Alkenes (unsaturated) undergo addition across the double bond, e.g. $\ce{C2H4 + Br2 -> C2H4Br2}$.

Name the functional group of alcohols and give two ways to manufacture ethanol.

Alcohols contain the hydroxyl group $\ce{-OH}$. Ethanol is made by (1) fermentation of glucose: $\ce{C6H12O6 -> 2C2H5OH + 2CO2}$ (yeast, $\sim 30\,^{\circ}\text{C}$), and (2) catalytic addition of steam to ethene: $\ce{C2H4 + H2O -> C2H5OH}$.

What is produced when a carboxylic acid reacts with an alcohol, and name the catalyst?

An ester (plus water) is produced in esterification, catalysed by concentrated sulfuric acid. Example: ethanoic acid + ethanol $\to$ ethyl ethanoate + water.

Distinguish between addition polymerisation and condensation polymerisation.

Addition: many small unsaturated monomers (alkenes) join to form one long polymer with no other product, e.g. ethene $\to$ poly(ethene). Condensation: monomers with two functional groups join, losing a small molecule (e.g. water) each time, forming polymers like polyesters and nylon.

Describe the repeat unit relationship for poly(ethene) made from ethene.

Ethene monomer $\ce{CH2=CH2}$ undergoes addition polymerisation; the double bond opens and units link to give poly(ethene) with repeat unit $\ce{-(CH2-CH2)-}_n$.

Why are many synthetic addition polymers a disposal problem, and give a drawback of two disposal methods?

They are non-biodegradable (microorganisms cannot break them down). Landfill wastes space and they do not decompose; incineration (burning) can release toxic gases such as $\ce{HCl}$ and carbon dioxide (a greenhouse gas).