Chemistry Questions and Answers

Answer: C

Both statements are correct.

The sign convention of thermodynamics is written from the system's point of view: work done on the system is positive, and work done by the system on the surroundings is negative. With this convention the first law reads \[\Delta U = q + w\]

Answer: A

Ozone is produced in photochemical smog.

Sunlight splits nitrogen dioxide from vehicle exhaust, \[\mathrm{NO_2 + h\nu \rightarrow NO + O},\qquad \mathrm{O + O_2 \rightarrow O_3}\] and the ozone then reacts with unburnt hydrocarbons to give PAN and aldehydes. Hydrocarbons and nitrogen oxides are the ingredients rather than the products.

Answer: C

Chlorine is the most commonly used bleaching agent.

In water it forms hypochlorous acid, \[\mathrm{Cl_2 + H_2O \rightarrow HCl + HOCl}\] which releases nascent oxygen and oxidises the coloured compound to a colourless one. Bleaching by chlorine is therefore permanent, unlike bleaching by sulphur dioxide, which works by reduction and reverses in air.

Answer: C

Silver iodide is used to produce artificial rain.

Its crystal lattice closely resembles that of ice, so supercooled droplets in a cloud freeze on it and grow heavy enough to fall. It is dispersed from aircraft or fired from the ground into a cloud that already holds enough moisture.

Answer: D

Aspirin is acetylsalicylic acid.

It is made by acetylating salicylic acid with acetic anhydride, and works by irreversibly inhibiting the cyclo-oxygenase enzymes, so cutting the production of prostaglandins that cause pain, fever and inflammation. Methyl salicylate is oil of wintergreen.

Answer: B

This is the thermal capacity, or heat capacity, of the body.

\[C=\frac{Q}{\Delta T}\] measured in \(\mathrm{J\,K^{-1}}\). It depends on the mass of the particular body. Specific heat is the heat needed per unit mass for the same rise, and is a property of the substance rather than of the object.

Answer: C

Cadmium does not liberate hydrogen from water.

Sodium, lithium and potassium are all alkali metals, high in the reactivity series, and react with even cold water. Cadmium lies below hydrogen in the series and is unaffected by water at ordinary temperatures.

Answer: D

Carbon monoxide is the most toxic of these.

It binds to haemoglobin about 240 times more strongly than oxygen does, forming carboxyhaemoglobin, so the blood can no longer carry oxygen. A few hundred parts per million can be fatal, and because the gas is colourless and odourless there is no warning.

Answer: C

One-eighth of the isotope remains.

\[\text{number of half-lives} = \frac{6\ \mathrm{h}}{2\ \mathrm{h}} = 3\]

\[\frac{N}{N_0} = \left(\frac{1}{2}\right)^{3} = \frac{1}{8}\]

The quantity halves at 2 h, quarters at 4 h and falls to an eighth at 6 h.

Answer: C

The pH of pure water is 7 at 25 °C.

Water self-ionises to the extent that \[[\mathrm{H^+}] = [\mathrm{OH^-}] = 10^{-7}\ \mathrm{mol\,L^{-1}}\] so \(\mathrm{pH} = -\log(10^{-7}) = 7\). The value drifts below 7 at higher temperatures, but the water stays neutral because the two concentrations remain equal.