Chemistry Questions and Answers

Answer: D

All the above describe glycolysis.

It is the first stage in the oxidation of glucose, and it breaks one molecule of glucose down to two of pyruvate: \[\mathrm{C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2P_i \rightarrow 2\,CH_3COCOO^- + 2NADH + 2ATP + 2H^+ + 2H_2O}\] It takes place in the cytoplasm and needs no oxygen, which is why it is common to both aerobic and anaerobic respiration.

Answer: D

Nitrogen has the highest first ionisation potential of the four.

Ionisation energy rises across a period, so oxygen should exceed nitrogen - but nitrogen's \(2p^3\) sub-shell is exactly half filled and therefore unusually stable, while oxygen's \(2p^4\) has one paired orbital whose electron–electron repulsion makes removal easier. So the order is N > O > C > B.

Answer: A

Nitrogen makes air a moderate supporter of combustion.

Being chemically inert under ordinary conditions, it dilutes the oxygen. In pure oxygen things burn fiercely and fires would be uncontrollable; with 78 per cent nitrogen present, combustion proceeds at a manageable rate.

Answer: C

Fluorine is the most electronegative element, at 4.0 on the Pauling scale.

Electronegativity rises across a period as nuclear charge increases, and falls down a group as the atom grows. Fluorine sits at the top right of the reactive elements, so it holds bonding electrons more strongly than anything else; oxygen follows at 3.5.

Answer: B

When a bond forms, the atoms share or transfer electrons.

Transfer gives an ionic bond and sharing a covalent one; either way the atoms reach a more stable electron configuration and the system falls to lower energy. Protons and nuclei are not involved - moving those would be a nuclear process, not a chemical one.

Answer: D

Ammonia is by far the most soluble of these gases in water.

About 700 litres of ammonia dissolve in one litre of water at room temperature, because \(\mathrm{NH_3}\) hydrogen bonds to water and partly reacts with it. Carbon dioxide dissolves to less than one litre per litre, nitrogen barely at all.

Answer: D

The mixture is separated by fractional distillation.

Ethanol boils at 78 °C and water at 100 °C, a difference too small for simple distillation, so a fractionating column is needed. Even then only 95.6 per cent ethanol can be obtained, because the mixture forms an azeotrope at that composition.

Answer: D

Acid rain is caused by oxides of nitrogen and sulphur dioxide.

\(\mathrm{SO_2}\) from burning coal and \(\mathrm{NO_x}\) from vehicle engines are oxidised in the air and dissolve in rain: \[\mathrm{2SO_2 + O_2 + 2H_2O \rightarrow 2H_2SO_4},\qquad \mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}\] Rain below about pH 5.6 counts as acid rain.

Answer: B

Laughing gas is nitrous oxide, \(\mathrm{N_2O}\).

Humphry Davy named it for the euphoria it induces, and it is still used as a mild anaesthetic and analgesic mixed with oxygen. Nitric oxide, NO, and nitrogen dioxide, \(\mathrm{NO_2}\), are quite different and are toxic.

Answer: B

Hydrogen is used to make vanaspati.

Passed through the warm oil over a nickel catalyst, it adds across the carbon–carbon double bonds, converting the unsaturated liquid oil into a saturated semi-solid fat. The process is called hydrogenation, or hardening of oils.