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Edexcel Separate Sciences · Chemistry · Paper 1

SC13 · Transition metals, alloys and corrosionTopic 5 — Separate chemistry 1

Metal properties, rust prevention, electroplating and useful alloys

Revise the key ideas

Transition metals and choosing metals

  • Transition metals occupy the central block of the periodic table. Typical examples include iron, copper and nickel; their properties differ from the very reactive, low-density Group 1 metals.
  • Many transition metals have high densities, high melting points and good strength. They conduct electricity and thermal energy; these are typical trends, not rules without exceptions.
  • Transition-metal compounds are often coloured, and the metals or their compounds can act as catalysts. Iron catalyses the Haber process; a catalyst speeds a reaction without being used up overall.
  • Copper is useful for wiring because it conducts electricity well and can be drawn into wires. Its good thermal conductivity also suits heat-transfer applications.
  • Aluminium has low density and forms a protective oxide layer. Its alloys are useful where low mass and strength matter, such as aircraft parts; low density alone does not establish suitability.
  • Gold is unreactive, attractive and malleable, but pure gold is relatively soft. Jewellery often uses gold alloys for greater hardness; cost and appearance also influence the choice.
  • Choose a metal using data on density, conductivity, corrosion resistance, strength, price and the environment in which it will operate. No material is best for every use.

Corrosion and rust prevention

  • Corrosion is gradual destruction of a metal by chemical reaction with its surroundings, involving oxidation of the metal. Rusting is the corrosion of iron, not a name for every metal’s corrosion.
  • Iron rusts when both water and oxygen are available. Salt can speed rusting; a fair investigation compares identical iron samples while changing one condition.
  • To show the need for both reactants, compare iron in moist air, dry air with a drying agent, and boiled water protected by an oil layer. Boiling removes dissolved oxygen; oil limits oxygen returning.
  • Paint, oil, grease and plastic coatings act as barriers that exclude water and/or oxygen. Damage to the barrier can expose iron and allow rusting.
  • Galvanising coats iron with zinc. The zinc keeps water and oxygen away from the iron. It is also more reactive than iron, so it oxidises instead of the iron (sacrificial protection), even if a small scratch exposes the iron.
    Sacrificial protectionZinc or magnesium attached to iron The attached metal is more reactive → Attached metal oxidises first It loses electrons in preference to iron → Iron is protected while it remains Replace the sacrificial metal when used upZinc or magnesium attached to ironThe attached metal is more reactiveAttached metal oxidises firstIt loses electrons in preference to ironIron is protected while it remainsReplace the sacrificial metal when used up
    Sacrificial protection can still work at exposed areas while electrical contact remains.
  • Sacrificial protection connects iron to a more reactive metal such as zinc or magnesium. That metal loses electrons in preference to iron and eventually needs replacement.
  • A less reactive coating such as tin protects intact iron as a barrier, but a scratch can permit faster iron corrosion. A coating and a sacrificial metal are not always interchangeable.

Electroplating and alloys

  • Electroplating deposits a thin metal layer using electrolysis. The object to be plated is the negative cathode; positive metal ions gain electrons there and form metal atoms on its surface.
    Copper electroplatingPositive copper anode Cu atoms lose electrons to form Cu²⁺ → Copper ions in electrolyte Carry charge through the solution → Negative object: cathode Cu²⁺ gains electrons, coating the objectPositive copper anodeCu atoms lose electrons to form Cu²⁺Copper ions in electrolyteCarry charge through the solutionNegative object: cathodeCu²⁺ gains electrons, coating the object
    The object is connected to the negative terminal of the power supply.
  • For copper plating, use a copper-containing electrolyte and usually a copper anode. Copper dissolves at the positive anode, replacing ions removed at the cathode: Cu²⁺ + 2e⁻ → Cu and Cu → Cu²⁺ + 2e⁻.
  • Clean the object before plating and use controlled current and time for a suitable coating. Plating can improve appearance or corrosion resistance without making the entire object from an expensive metal.
  • An alloy is a mixture containing a metal and one or more other elements. Different-sized atoms distort the regular layers of a pure metal, making it harder for layers to slide; alloys are often harder and stronger.
    Pure metal and alloy layersRegular pure-metal layers slide more readily; different-sized alloy atoms disrupt the arrangement.Pure metal: regular layersAlloy: distorted layers
    Different-sized atoms make sliding harder; this model explains a common strengthening effect.
  • Adding carbon and other elements makes steels with different properties. Stainless steels contain chromium, which helps form a protective surface layer; not every steel is stainless or equally corrosion-resistant.
  • Brass is mainly copper and zinc; it is useful for items requiring a workable, corrosion-resistant material. Magnalium contains aluminium and magnesium, combining low density with improved mechanical properties.
  • Gold alloys can improve hardness; copper alloys can trade some conductivity for strength. Evaluate property data rather than assuming every alloy improves every property.
  • The proportion of an element in an alloy (%) = mass of that element ÷ total alloy mass × 100. Compare samples with the same total mass when investigating the effect of composition.

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