Chemistry 0620 | Paper 4 Mock 01 | Theory (Extended)

This question tests combustion of ethanol, the energy calculation from a data table, and the danger of incomplete combustion.

(a) Complete combustion in plenty of oxygen gives carbon dioxide and water:

\[\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\]

correct formulae [1]; balanced [1].

(b) The two products are carbon dioxide and water [1]. A confirming test: bubble the gas through limewater, which turns milky (carbon dioxide), or the liquid turns anhydrous copper(II) sulfate from white to blue (water) [1].

(c) From the table ethanol releases 30 kJ per gram, so for 9.2 g:

\[E = 9.2 \times 30\ [1] = 276\ \text{kJ}\ [1]\]

(d) Incomplete combustion happens because there is insufficient (limited) oxygen for complete combustion [1]; instead of carbon dioxide, carbon monoxide (and carbon / soot) is formed [1]; the poisonous gas is carbon monoxide, which is toxic because it binds to haemoglobin and reduces oxygen transport in the blood [1].

(e) Per gram, ethanol is the better fuel [1]; because it releases more energy per gram than methanol (30 kJ/g compared with 23 kJ/g) [1].

This question tests the reactivity series through the reaction of metals with a dilute acid: the more reactive the metal, the faster it displaces hydrogen.

(a) The completed table, in order of decreasing reactivity, is:

zinc [1]; iron [1]; copper [1]; and the fourth mark is for showing the correct order of rate magnesium > zinc > iron > copper [1]. The pale green colour with iron is the Fe²⁺ ion, a useful identifying detail.

(b) Copper does not react because it is below hydrogen in the reactivity series [1]; being less reactive than hydrogen, it cannot displace hydrogen from the acid [1]. Only metals above hydrogen liberate hydrogen from dilute acids.

(c) The gas is hydrogen [1]; the test is to hold a lighted splint at the mouth of the tube, which gives a squeaky pop [1].

(d) Zinc is above hydrogen, so it displaces it:

\[\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\]

correct formulae [1]; balanced [1].

(e) Hydrogen ions gain electrons (reduction) to form hydrogen gas, so two electrons are needed to balance the two positive charges:

\[2\text{H}^+ + 2e^- \rightarrow \text{H}_2\]

2e^- on the left [1]; balanced [1].

(f) Any one change that gives the particles more frequent or more energetic collisions increases the rate: raise the temperature, use more concentrated acid, or use zinc powder instead of pieces to increase the surface area [1].

What this tests: the definition of isotopes and using nuclide notation to find neutrons and electrons.

(a) Isotopes [2] Isotopes are atoms of the same element [1] that have the same number of protons but different numbers of neutrons (same proton number, different nucleon numbers) [1].

(b) Neutrons [1] In nuclide notation the top number (37) is the nucleon number and the bottom number (17) is the proton number, so neutrons \(= 37 - 17 = 20\) [1].

(c) Electrons [1] A neutral atom has equal numbers of protons and electrons, so there are 17 electrons [1].

(d) Other isotope [1] With nucleon number 35, neutrons \(= 35 - 17 = 18\) [1].

(e) Different physical property [1] The two isotopes have different masses (accept different density or different rate of diffusion) [1]. Their chemical properties are identical because they have the same electron arrangement.

This question links a metal's reactivity to how it is extracted, and reviews redox and recycling.

(a) Filling the extraction-method column by comparing each metal with carbon:

Metals above carbon (potassium, aluminium) need electrolysis; those below carbon but still reactive (zinc, iron) are reduced with carbon; gold is unreactive enough to be found uncombined.

(b) The most reactive metals are more reactive than carbon, so they cannot be extracted by carbon reduction and can only be obtained by electrolysis [1]. Electrolysis needs an electric current, which was not available until a supply of electricity was developed in the early nineteenth century [1].

(c) The more reactive a metal is, the more strongly it holds onto the oxygen in its oxide [1], so more energy is needed to remove that oxygen and the metal is harder to reduce/extract [1].

(d) Low-grade copper ores are now used because the rich, high-grade ores are running out [1], while demand and the price of copper remain high, making it economic to process ores that contain only a little copper [1].

(e) The general half-equation for reducing a metal ion \( M^{3+} \) to the metal is:

This scores [1]; the ion gains three electrons.

(f) In terms of electron transfer, reduction is the gain of electrons [1].

(g) Any two reasons recycling matters, one mark each up to [2]: it conserves limited ore/metal reserves; it saves energy compared with extracting from ore; it reduces waste sent to landfill; it reduces mining and the damage it causes; it lowers cost.

What this tests: the apparatus and correct terms for separating an excess solid from a salt solution by filtration.

(a) [1] A is the (filter) funnel.

(b) [1] B is the filter paper.

(c) [2] The solid left behind on the paper is the residue [1]; the liquid that passes through and is collected as C is the filtrate [1].

(d) Why excess zinc [2] Adding zinc in excess makes sure all the acid is used up, so none is left in the product [1]; the leftover zinc is insoluble, so it can simply be filtered off, leaving a pure salt solution [1].

(e) [1] The salt in solution C is zinc sulfate.

(a) Both magnesium-24 and magnesium-26 have 12 protons and 12 electrons [1]; they differ only in neutrons, magnesium-24 having 12 neutrons and magnesium-26 having 14 neutrons [1]. Isotopes are atoms of the same element with different numbers of neutrons.

(b) Relative atomic mass is the weighted mean of the three isotope masses [method 1]:

(c) Magnesium has 12 electrons, so its full configuration is 2,8,2 [1].

(d)(i) The ion drawn shows only 10 electrons (2,8), but the nucleus still holds 12 protons, so the charge is 2+ and the symbol is \(\text{Mg}^{2+}\) [2+, 1; symbol, 1].

(d)(ii) The atom (2,8,2) loses its 2 outer electrons [1]; it then has 12 protons but only 10 electrons, giving a 2+ charge [1]. The ion is smaller than the atom because the outermost occupied shell has been removed, and the 12 protons now pull the remaining 10 electrons in more strongly [1].

(e) Group II tells you there are 2 electrons in the outer shell [1]; Period 3 tells you there are 3 occupied electron shells [1].

(f) Magnesium burns in oxygen to form magnesium oxide [formulae 1; balanced 1]:

Magnesium is a metal, so its oxide is a basic oxide [1] (metal oxides are basic and react with acids to form a salt and water).