Chemistry · Chemistry of the elements · Grade 9

Metals and alloys

Why does an iron nail turn red in copper sulfate, and why is bronze harder than copper?

The situation

The school chemistry room has put together a “Metals” exhibition: metal cubes of equal size, a cast-iron cauldron, a bronze bell, a duralumin wing rib and a gold ring. The lab technician put an iron nail into a copper sulfate solution — within a few minutes the nail took on a reddish tint. Why do metals shine and conduct current, how does an alloy differ from a pure metal, and which metal displaces another from the solution of its salt?

Topic

Metals and alloys: properties and the reactivity series. Exhibition of metals: an iron nail dipped in copper sulfate turns red; bronze, brass, duralumin and 585-purity gold

Key ideas

Metallic bonding
Alloys
Reactivity series of metals

Formulas

ρ = \frac{m}{V}

Density: the cube edge is 2 cm, the volume $V$ = 8 cm³; the mass is measured on a balance.

Fe + CuSO_{4} \overset{}{\to} FeSO_{4} + Cu

A more active metal displaces a less active metal from the solution of its salt.

Cu + 2 AgNO_{3} \overset{}{\to} Cu (NO_{3})_{2} + 2 Ag

Copper displaces silver: the wire becomes covered with silvery crystals and the solution turns blue.

Δ m = n (Cu) (M (Cu) - M (Fe))

Increase in the mass of the nail: for every mole of iron that dissolves, one mole of copper is deposited (63.55 − 55.85 = 7.70 g/mol).

ω (Au) = \frac{fineness}{1000}

585 fineness: 58.5 % of the mass of the alloy is pure gold.

Theory

Metallic bonding and physical properties

In a metal crystal the atoms make their outer electrons “common”: at the lattice sites there are positive cations, and between them a freely moving electron gas — this is the metallic bond. The shared properties come from it: electrical and thermal conductivity (when a voltage is applied, the free electrons drift in one direction — see it in the model), metallic lustre (the electrons reflect light) and malleability (layers of cations slide without the bond breaking — the metal can be hammered and drawn into wire). Electrical conductivity: Ag > Cu > Au > Al. Metals with a density below 5 g/cm³ are light metals (Li, Na, Mg, Al, Ti), and those above it are heavy (Fe, Cu, Pb, Au, W). The lightest is lithium (0.534 g/cm³), the densest is osmium (22.59 g/cm³). Mercury solidifies at −38.8 °C — the only metal that is liquid under ordinary conditions; tungsten melts at 3422 °C. Iron, nickel and cobalt are attracted by a magnet. More about the movement of electrons in the “Electric current in metals” experiment.

Alloys

An alloy is a substance obtained by melting two or more metals together (or a metal with a non-metal such as carbon or silicon) and cooling it. Atoms of different sizes hinder the sliding of the cation layers — so an alloy is usually harder and stronger than a pure metal, has lower electrical conductivity, and often melts at a lower temperature. Bronze — Cu + Sn, brass — Cu + Zn, duralumin — Al + Cu + Mg + Mn, steel — Fe + C (< 2.14 %), cast iron — Fe + C (> 2.14 %), stainless steel — Fe + 18 % Cr + 10 % Ni. An alloy of tin and lead with 61.9 % Sn melts at a single temperature — 183 °C (the eutectic): this is far below tin (231.9 °C) and lead (327.5 °C). The fineness of gold articles is the number of grams of pure gold in 1000 g of alloy.

Chemical properties: metals are reducing agents

Metal atoms give up electrons and become cations: $M - n e^{-} \to M^{n +}$ — metals are reducing agents. With non-metals: $2 Mg + O_{2} \overset{}{\to} 2 MgO$ (a dazzling flame), $2 Cu + O_{2} \overset{}{\to} 2 CuO$ (blackens when heated), $3 Fe + 2 O_{2} \overset{}{\to} Fe_{3} O_{4}$ , $Fe + S \overset{}{\to} FeS$ , $2 Fe + 3 Cl_{2} \overset{}{\to} 2 FeCl_{3}$ . With water: alkali metals react violently ( $2 Na + 2 H_{2} O \overset{}{\to} 2 NaOH + H_{2} ↑$ ), magnesium very slowly in cold water and faster in hot water ( $Mg + 2 H_{2} O \overset{}{\to} Mg (OH)_{2} + H_{2} ↑$ ), iron only when red-hot with steam: $3 Fe + 4 H_{2} O \overset{}{\to} Fe_{3} O_{4} + 4 H_{2}$ . With acids: metals to the left of hydrogen displace hydrogen: $Zn + 2 HCl \overset{}{\to} ZnCl_{2} + H_{2} ↑$ ; copper, silver and gold do not (more in the “Acids and metals” experiment).

The activity series and reactions with salts

The activity series of metals (N. N. Beketov): Li, K, Ba, Ca, Na, Mg, Al, Zn, Fe, Ni, Sn, Pb, (H), Cu, Hg, Ag, Pt, Au. In the series a metal on the left displaces a metal on the right from the solution of its salt: iron becomes coated with copper in copper sulfate, copper becomes coated with silver in silver nitrate. The series corresponds to the standard electrode potentials $E °$ : Mg −2.37 V, Zn −0.76 V, Fe −0.44 V, Cu +0.34 V, Ag +0.80 V — the more negative the potential, the stronger the reducing agent. Very active metals such as sodium, potassium and calcium react with water first in a salt solution, so they are not used for displacement. Lab work 6: four metals are put into four salt solutions and a table is made — a reaction occurs only when the metal is to the left of the metal in the salt.

Lab work and practical

Lab work 4 — examining metal samples: colour, lustre, hardness (tested by scratching), attraction to a magnet, density: $ρ = m / V$ (cubes of equal volume are weighed). Lab work 5 — samples of alloys: composition, colour and use are compared with pure metals. Practical 3 (experimental problem): an iron nail is weighed ( $m_{1}$ ), held in a CuSO₄ solution, taken out and dried, and weighed again ( $m_{2}$ ). The nail gets heavier: for every mole of Fe that dissolves (55.85 g), one mole of Cu (63.55 g) is deposited. So $n (Cu) = Δ m / 7.70$ mol, $m (Cu) = n \cdot 63.55$ g, and the same number of moles of CuSO₄ is used up.

Safety and the model

Safety: silver nitrate stains the skin black and copper sulfate is harmful — wear gloves and goggles; pour hydrochloric acid into water, never the other way round; wash your hands after handling lead samples; mercury vapour is toxic — it is kept only in a sealed ampoule; a hot plate and molten metal cause burns. Model: the volume of the cubes is exactly 8 cm³, and the densities are reference values at 20 °C; the reaction rates in the test tubes are schematic (time sped up 20 times), and which reaction occurs follows the activity series; the Sn–Pb phase diagram is simplified to straight lines, the power and heat capacity of the hot plate are typical values (time 4 times faster); the deposition of copper on the nail is calculated with first-order kinetics ( $τ$ = 600 min — only a few per cent of the Cu²⁺ is used up in an hour), time 60 times faster.

Did you know?

Silver is the best electrical conductor among metals (resistivity 1.59·10⁻⁸ Ω·m at 20 °C). Copper conducts only about 5 % worse, but is much cheaper — that is why wires are made of copper.
Pure gold is very soft (2.5 on the Mohs scale) — jewellery would wear away quickly. That is why rings are made of an alloy: in a 585 fineness article, 58.5 % of the mass is gold and the rest is silver and copper.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Nitrogen, ammonia and nitric acid — Grade 8
Phosphorus and mineral fertilisers — Grade 8
Carbon and its compounds — Grade 9
Silicon and silicates — Grade 9
Making cast iron and steel — Grade 9
Alkali metals and soda — Grade 9
Aluminium and its compounds — Grade 9
Copper, silver, gold and zinc — Grade 9
Chromium and manganese: coloured ions — Grade 9
Iron and its compounds — Grade 9