Chemistry Questions and Answers

Answer: A

Fluorine's reactivity follows from its very high electronegativity.

At 4.0 on the Pauling scale it is the most electronegative element, so it pulls electrons from almost anything and forms strong bonds with the resulting energy release. The weakness of the F–F bond - only about \(155\ \mathrm{kJ\,mol^{-1}}\), far less than Cl–Cl - reinforces this, since little energy is needed to break the molecule apart. Fluorine has no d-orbitals available.

Answer: B

Such a substance is a compound.

Its elements are chemically combined in a fixed ratio by mass - the law of constant proportions - and it has properties quite unlike those of its constituents. A mixture can be in any proportion and keeps the properties of its components.

Answer: B

Temporary hardness is due to calcium bicarbonate (and magnesium bicarbonate).

It is called temporary because boiling decomposes the bicarbonate: \[\mathrm{Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3\downarrow + H_2O + CO_2}\] The insoluble carbonate settles out as the fur in a kettle. Sulphates and chlorides cause permanent hardness, which boiling cannot remove.

Answer: C

The mass number is 35.

Seven electrons in the M shell means the configuration is \(2, 8, 7\), so the atom has \[Z = 2+8+7 = 17\ \text{electrons and protons}\]

which identifies it as chlorine. Then \[A = Z + N = 17 + 18 = 35\]

Answer: C

Misch metal is used in cigarette lighters.

It is an alloy of cerium, lanthanum and other rare earths with iron, and is the 'flint' of a lighter: it is pyrophoric, so scraping it against the striker throws off sparks. It is also added to steel as a deoxidiser.

Answer: D

The neutron was discovered late because it carries no charge.

The detectors of the day - cloud chambers, electroscopes, ionisation counters - all responded to ionisation caused by charged particles. A neutral particle leaves no track. Chadwick identified it only in 1932, by measuring the recoil of protons knocked out of paraffin.

Answer: D

Ferric chloride works because it coagulates the blood and so seals the vessels.

Blood is a negatively charged colloidal sol. The trivalent \(\mathrm{Fe^{3+}}\) ion neutralises that charge very efficiently - by the Hardy–Schulze rule, coagulating power rises sharply with the charge on the oppositely charged ion - so the sol clots at once.

Answer: B

The atomic number gives the number of electrons in a neutral atom, because it equals the number of protons.

The proton count defines the element and is what the atomic number really counts; the electron count matches it only while the atom is uncharged. The number of nucleons is the mass number instead.

Answer: C

Carbon tetrachloride was used as a fire extinguisher, under the name pyrene.

Its vapour is much denser than air, so it blankets the fire and cuts off oxygen, and it does not conduct electricity, which made it useful on electrical fires. It is no longer used, because hot \(\mathrm{CCl_4}\) can form phosgene.

Answer: C

Such species are isoelectronic.

\(\mathrm{N^{3-}}\), \(\mathrm{O^{2-}}\), \(\mathrm{F^-}\), \(\mathrm{Ne}\), \(\mathrm{Na^+}\) and \(\mathrm{Mg^{2+}}\) all have 10 electrons. Isotopes share the proton number, isobars the mass number, and isotones the neutron number.