Chemistry: TestAS practice testlets

The three-dimensional shape of a molecule influences its physical and chemical properties, from its boiling point to the way it fits into an enzyme. A simple model predicts these shapes: the electron-pair repulsion model, also called the VSEPR model. It assumes that the electron pairs around a central atom, both bonding pairs and non-bonding (lone) pairs, repel each other and arrange themselves as far apart as possible.

With four electron pairs around the central atom, the pairs point towards the corners of a tetrahedron. In methane (CH₄), all four pairs are bonding pairs, so the molecule is tetrahedral, with bond angles of 109.5°. In ammonia (NH₃), one of the four pairs is a lone pair; the atoms form a trigonal pyramid, and because lone pairs repel slightly more strongly than bonding pairs, the angle shrinks to about 107°. In water (H₂O), two lone pairs push the two hydrogen atoms closer together, giving a bent (angular) molecule with an angle of about 104.5°. Double bonds count as one region of electron density: carbon dioxide (O=C=O), with two such regions and no lone pairs on carbon, is linear.

Shape determines polarity. A bond between atoms of different electronegativity is polar: the more electronegative atom carries a partial negative charge. Whether the whole molecule is polar, a dipole, depends on whether the bond dipoles cancel out. In carbon dioxide, the two C=O bonds are polar, but they point in exactly opposite directions, so their effects cancel, and the molecule is non-polar. In water, the bent shape means the bond dipoles do not cancel, and the molecule is a strong dipole. Methane is non-polar because its bonds are only weakly polar and arranged symmetrically.

Polarity has visible consequences. Polar substances dissolve well in polar solvents such as water, while non-polar substances dissolve in non-polar solvents, following the rule that like dissolves like. This is why oil and water do not mix, and why washing-up liquid, whose molecules have a polar and a non-polar end, can remove grease. A thin stream of water from a tap is deflected by a charged plastic rod, while a stream of a non-polar liquid is not.

In a galvanic cell, a spontaneous redox reaction produces electrical energy. In electrolysis, the process is reversed: an external voltage source forces a non-spontaneous redox reaction to take place. The electrode connected to the negative pole is the cathode, where reduction occurs; the electrode connected to the positive pole is the anode, where oxidation occurs.

Electrolysis has many industrial uses. Aluminium is produced by the electrolysis of molten aluminium oxide; chlorine and sodium hydroxide are produced by the electrolysis of sodium chloride solution; hydrogen for the energy transition can be produced by the electrolysis of water. In electroplating, a metal object is made the cathode in a solution of metal ions, which are reduced and deposited on its surface as a thin, even layer, for example of copper, nickel, chromium or silver.

Michael Faraday discovered in the 1830s that the amount of substance deposited at an electrode is proportional to the electric charge that has flowed. The charge Q equals the current multiplied by time. One mole of electrons carries a charge of about 96,500 coulombs, a quantity called the Faraday constant F. To deposit one mole of a metal whose ions carry the charge z, z moles of electrons are needed: one mole of silver from Ag⁺ needs one mole of electrons, one mole of copper from Cu²⁺ needs two.

Q = I·t n(electrons) = Q ÷ F n(metal) = n(electrons) ÷ z m = n·M F ≈ 96,500 C/mol

Electrolysis consumes a great deal of electrical energy, especially in the production of aluminium, which is why aluminium smelters are often located near cheap sources of electricity and why recycling aluminium, which needs only about five percent of the energy of primary production, is so worthwhile. The questions refer to the electroplating of a metal part with copper from a copper(II) sulphate solution; the molar mass of copper is 63.5 g/mol. In practice, the deposited mass is often slightly lower than calculated, because part of the current is used for side reactions such as the formation of hydrogen at the cathode. The ratio of the actual to the calculated mass is called the current efficiency. To obtain an even layer, the part must be thoroughly cleaned and degreased before plating, and the current density must be kept within a suitable range, since too high a current produces rough, poorly adhering deposits.