Properties of Metals Worked ExamplesIGCSE Chemistry 0620

(a) P and S: they conduct electricity when solid and they are malleable (they flatten instead of shattering). Their melting points are also fairly high.

(b) R is brittle — it shatters when hammered — so it is not malleable. (R behaves like graphite, a form of carbon: a non-metal that conducts.)

(c) Any two: good thermal conductors; ductile (can be drawn into wires); high boiling points; high density; shiny when freshly cut.

(a) Any two: fizzing / bubbles; the zinc gets smaller / disappears (if the acid is in excess); the test-tube gets warm.

(b) Metal + acid → salt + hydrogen. Zinc forms $\text{Zn}^{2+}$, so the salt is zinc sulfate.

(c) Hold a lighted splint at the mouth of the tube: the gas burns with a squeaky pop — hydrogen.

(a) A: a positive ion (metal cation). B: a delocalised electron.

(b) Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a 'sea' of delocalised electrons.

(c) The delocalised electrons are free to move through the lattice, so they can carry charge (flow) when a potential difference is applied.

(d) The positive ions are in layers. When a force is applied, the layers can slide over each other; the delocalised electrons still hold the ions together, so the metal changes shape instead of breaking.

(a) The magnesium glows / burns with a bright white light and a white solid forms.

(b) With steam, magnesium forms the oxide (not the hydroxide) and hydrogen:

(c) Hydrogen.

(d) Steam is at a much higher temperature, so the particles have more energy and more collisions have enough energy to react.

(e) As the tube cools, the gas inside contracts and liquid could be sucked back into the hot tube, which could crack it.

(a) Any two: fizzing / bubbles; the calcium sinks and then moves / gets smaller; the water turns cloudy (milky) because calcium hydroxide is only slightly soluble; the water level in the cylinder falls.

(b) Calcium forms $\text{Ca}^{2+}$, so two hydroxide ions are needed:

(c) Moles of calcium ($A_r = 40$), then the $1 : 1$ ratio of Ca to $\text{H}_2$:

(d) Some hydrogen escapes before the cylinder is in place, or some gas bubbles miss the cylinder. (A layer of oxide on the calcium would also reduce the amount of metal that reacts.)

The surface of aluminium is covered by a thin, unreactive layer of aluminium oxide, which stops the acid reaching the metal at first. Once the acid removes the layer, the aluminium reacts vigorously. Answer: B.

A contradicts the information given (aluminium is above zinc). C and D are true statements but do not explain the observation.

(a)

(b) To let air (oxygen) in so that the magnesium can keep burning; lifting it only slightly stops the white solid escaping.

(c) $n(\text{Mg}) = 0.36 \div 24$; the ratio Mg : MgO is $1 : 1$; $M_r(\text{MgO}) = 24 + 16 = 40$.

(d)

(e) Some white magnesium oxide smoke escaped when the lid was lifted, or not all the magnesium reacted (heating was stopped too soon).

(a) Readings (to the nearest $0.5\ ^{\circ}\text{C}$): magnesium $38.5$, zinc $27.0$, iron $24.5$, copper $21.0\ ^{\circ}\text{C}$. Subtract $21.0$:

(b) A larger temperature rise means a more vigorous reaction: magnesium > zinc > iron > copper.

(c) No temperature rise: copper does not react with dilute hydrochloric acid because it is below hydrogen in the reactivity series.

(d) Iron forms iron(II) chloride with dilute hydrochloric acid:

(a) Rub each strip with sandpaper (emery paper) until it is shiny. This removes the oxide layer, so the acid reacts with the metal itself from the start.

(b) Put a measured volume (e.g. $25 \text{ cm}^3$) of the acid in a conical flask connected to a gas syringe.

Add one strip, replace the bung at once and start a stopwatch. Record the volume of hydrogen every 10 s (or after a fixed time, e.g. 60 s).

Repeat with the other metal using the same volume and concentration of acid, the same size (mass and surface area) of strip and the same temperature.

The metal that gives the greater volume in the same time (steeper graph) reacts faster.

(a)

(b) Magnesium oxide reacts with water to form magnesium hydroxide, an alkaline solution: litmus turns blue — magnesium oxide is basic.

Sulfur dioxide dissolves to form an acidic solution: litmus turns red — sulfur dioxide is acidic.

(c) Metals form basic oxides; non-metals form acidic oxides.

(a) The plunger is at the fourth small division after 40: $48 \text{ cm}^3$.

(b) A 2+ metal forms $\text{MCl}_2$:

(c) Moles of hydrogen, which equal moles of M ($1 : 1$):

Relative atomic mass = mass ÷ moles:

From the Periodic Table, $A_r = 65$ is zinc.

(d) Copper is below hydrogen in the reactivity series, so it does not react with dilute hydrochloric acid to give hydrogen.

(a)(i) With cold water, the hydroxide forms:

(ii) With steam, the oxide forms:

(b)(i) Moles of magnesium; ratio Mg : $\text{H}_2$ = $1 : 1$.

(ii) $M_r(\text{MgO}) = 40$:

(c) $M_r(\text{Mg(OH)}_2) = 24 + 2(16 + 1) = 58$, and the ratio is still $1 : 1$:

The same number of moles of product forms, but magnesium hydroxide contains extra hydrogen and oxygen (it has a larger $M_r$ than magnesium oxide), so its mass is greater. The volume of hydrogen is the same, $600 \text{ cm}^3$, in both.