chemistry

Question 1

metal oxides

Answer 1

the oxide ion in metal oxides can accept protons and neutralise acids. for example, calcium oxide will neutralise sulfuric acid to form calcium sulfate.

Question 2

metal hydroxides

Answer 2

metal oxides will form hydroxides when they react with water. the hydroxide ions will accept protons and neutralise acids. for example, magnesium hydroxide is used as an antacid to relieve the symptoms of indigestion and heartburn by neutralising hydrochloric acid that has entered the oesophagus.

Question 3

amphoteric oxides

Answer 3

amphoteric oxides can behave as either acids or bases. elements that form such oxides tend to be in the middle of a period, e.g. aluminium. aluminium oxide won’t dissolve in water but will react with acids to form a salt and water in a similar way to other oxides. it will also react with bases to form aluminates. for example, it’ll react with sodium hydroxide to form sodium tetrahydroxoaluminate, which is used in the bayer process to convert the mineral bauxite to aluminium oxide.

Question 4

extraction of aluminium

Answer 4

bauxite is mined and then processed to form alumina, which is alumnium oxide. molten alumina is then electrolysed using the Hall-Héroult process.
cryolite is added to the alumina to lower the melting point and save energy. the lining of the steel tank is made from carbon, which acts as the negative electrode. aluminium ions are reduced to form molten aluminium: Al³⁺ + 3e⁻ ￫ Al.
the molten aluminium can be drained off and cast into ingots.
the positive electrodes are made from carbon. here, oxide ions are oxidised to form oxygen:
2O²⁻ ￫ O₂ + 4e⁻

Question 5

electrolysis of brine

Answer 5

brine is a solution of sodium chloride and water. the process of electrolysis involves using an electric current to bring about a chemical change and make new chemicals. the electrolysis of brine is a large-scale process used to manufacture chlorine from salt. two other useful chemicals are obtained during the process, sodium hydroxide and hydrogen.

it is important that the chlorine and sodium hydroxide produced in the process are separated they react when they come into contact with each other.

products of electrolysis of brine

chlorine - disinfectant and purifier, manufacture of hydrochloric acid and making plastics
sodium hydroxide - processing food products, removing pollutants from water and manufacture of paper
hydrogen - manufacture of hydrochloric acid and potential as a pollution-free fuel

Question 6

molecular formulae

Answer 6

molecular formulae tell you the number and type of each atom present in a molecule. for example,
• butane, C₄H₁₀, contains 4 carbon atoms and 10 hydrogen atoms.
• phenol, C₆H₆O, contains 6 carbon atoms, 6 hydrogen atoms and an oxygen atom.

Question 7

displayed formulae

Answer 7

displayed formulae show the bonds present in a molecule, so it’s clear to how each joins to others.

Question 8

structural formulae

Answer 8

a graphic representation of the molecular structure, showing how the atoms are possibly arranged in the real three-dimensional space

Question 9

skeletal formulae

Answer 9

simplified formulae with any hydrogen atoms removed and the carbon chain reduced to a single line

Question 10

alkanes

Answer 10

organic compounds that consist of single bonded carbon and hydrogen atoms

Question 11

bonding of alkanes

Answer 11

the overlap of orbitals results in the formation of sigma bonds between carbons and hydrogen, and between adjacent carbons. these single bonds are free to rotate.

the carbon orbitals involved in the bonding are called sp³ hybrid orbitals. they form when the 2s orbital and three 2p orbitals in carbon’s outer shell rearrange themselves into four identical orbitals, each containing one unpaired electron. this process is called hybridisation.

Question 12

types of alkanes

Answer 12

alkanes can be classified, according to their structure. alkanes with a single chain are called straight chain alkanes, eg. hexane, CH₃CH₂CH₂CH₂CH₂CH₃.

alkanes with one or more carbon atoms attached to a carbon is the main chain are called branched alkanes, e.g. 2-methylpentane, CH₃CH(CH₃)CH₂CH₂CH₃.

alkanes with carbon atoms joined together in a ring are called cyclic alkanes, e.g. cyclohexane, C₆H₁₂.

Question 13

boiling points of alkanes

Answer 13

the longer the carbon chain of a straight chain alkane, the higher the boiling point.

longer carbon chains have stronger London forces as they have more electrons. the electron density of the larger electron clouds fluctuates more readily, so instantaneous dipoles are stronger in magnitude. hence, more energy is required to break the London forces.

Question 14

structural isomers

Answer 14

molecules with the same molecular formula but a different structural formula. alkanes can have a variety of isomers, due to a range of possible branches on a carbon chain. for example, there are three isomers with the molecular formula C₅H₁₂.

Question 15

alkenes

Answer 15

organic compounds that consist of a carbon-carbon double bond

Question 16

bonding in alkenes

Answer 16

the overlap of orbitals in alkenes results in the formation of sigma bonds between carbons and hydrogens, and forms one of the bonds between adjacent carbons.

the carbon orbitals involved in the bonding are called sp² hybrid orbitals. they form when the 2s and two of the 2p orbitals in carbon’s outer shell rearrange themselves into three identical orbitals, each containing one unpaired electron. the remaining p orbitals in each carbon overlap sideways to form a pi (π) bond.

Question 17

stereoisomerism in alkenes

Answer 17

alkenes can form structural isomers. however, restricted rotation about the carbon-carbon double bond can lead to steroisomers. stereoisomers are compounds with the same structural formula, but their atoms have a different arrangement in space.

Question 18

combustion of alkane fuels

Answer 18

combustion is the reaction with oxygen.
vehicles burn hydrocarbon fuels, such as petrol or diesel.
petrol contains hydrocarbons with 4-12 carbons.
diesel contains hydrocarbons with 8-21 carbons.
complete combustion produces CO₂ and H₂O.
incomplete combustion produces CO and C.

Question 19

reactions of alkanes with halogens

Answer 19

this reaction is called a free radical substitution. the mechanism of a reaction shows what occurs at each stage of the process. the three stages in the mechanism are:
• initiation - homolytic bond fission of a halogen to form halogen free radicals.
• propagation - steps where one free radical reacts and forms a different free radical.
• termination - combination of any two free radicals to form a stable product.

Question 20

addition reaction mechanism

Answer 20

the functional group, C=C, allows alkenes to undergo addition reactions. for example, ethene reacts with bromine to form 1, 2-dibromethane:
CH₂=CH₂ + Br₂ ￫ CH₂BrCH₂Br
it is easier to see what happens using structures with all their covalent bonds.
the reaction is a addition reaction because one molecule combines with another molecule, forming one larger molecule and no other products.
alkanes cannot take part in addition reactions.