The block in the middle

Most metals are transition metals — they sit in the central block of the periodic table, between Group 2 and Group 3. Their typical properties are:

• High melting points and high densities (iron melts at ~1538 °C).
• They form coloured compounds (e.g. blue Cu²⁺, green Fe²⁺, orange Fe³⁺).
• They (and their compounds) are useful as catalysts (e.g. iron in the Haber process).
• They have variable oxidation states, forming more than one ion (e.g. Fe²⁺ and Fe³⁺).

Compared with Group 1

A favourite exam comparison: transition metals against the Group 1 alkali metals.

Watch out: a common error is to call transition metals "more reactive" because they're famous. They are actually less reactive than Group 1 — sodium and potassium fizz and burn on water; iron and copper do not.

Spot the transition-metal property

Rusting needs air AND water

Corrosion is the oxidation of a metal at its surface. For iron this is rusting, which forms hydrated iron(III) oxide. Rusting needs BOTH oxygen and water:

Barriers and sacrifice

Edexcel lists two ideas: keep oxygen/water out, or use a more reactive metal to corrode instead.

• Barrier methods — paint, oil/grease, or plastic coating exclude oxygen and water.
• Galvanising — coating iron with zinc. Zinc is a barrier and, because it is more reactive, it gives sacrificial protection.
• Sacrificial protection — attach blocks of a more reactive metal (zinc or magnesium). It oxidises in preference to the iron, so the iron is protected even if scratched.

Watch out: the sacrificial metal is the more reactive one, not the less reactive one. A less reactive coating (like tin on a tin can) protects only as a barrier — scratch it and the iron rusts faster.

Protecting a steel pipe

Why alloys are harder

An alloy is a mixture of a metal with other elements. Steels are alloys of iron with carbon (and sometimes other metals). Alloys are usually harder than the pure metal.

Iron is alloyed to make alloy steels (e.g. adding chromium and nickel makes stainless steel, which resists corrosion). Other metals are matched to uses by their properties: aluminium (low density), copper (conducts), gold (unreactive); alloys include brass (copper + zinc) and magnalium (aluminium + magnesium).

Concentration of a solution

Concentration says how much solute is dissolved per dm³ (1 dm³ = 1000 cm³ = 1 litre). You meet two units:

To convert g/dm³ → mol/dm³, divide by the molar mass (M_r in g/mol). To go back the other way, multiply.

Your turn — concentration

The titration

A titration finds an unknown concentration accurately, e.g. a strong acid against a strong alkali. You measure a fixed volume of one solution with a pipette into a conical flask, add an indicator, then run in the other from a burette until the colour just changes (the end-point).

Watch out: the mean titre uses only the concordant results — discard the rough trial and any anomalies. Never just average every reading.

Titration calculations

Use the balanced equation. For a strong acid–strong alkali like HCl + NaOH → NaCl + H₂O the ratio is 1 : 1.

Your turn — titration

Percentage yield

The percentage yield compares what you actually made with the theoretical maximum from the equation:

The actual yield is usually less than the theoretical because of:

• Incomplete reactions (some reactant is left over).
• Practical losses when transferring or filtering.
• Competing (side) reactions making unwanted products.

Your turn — percentage yield

Atom economy

The atom economy measures how much of the reactant mass ends up as the useful product (the rest is wasted by-products):

% yield vs atom economy: they are different. Percentage yield is about how much you actually got in the lab; atom economy is about how much of the atoms in your equation are useful — even a 100%-yield reaction can have a poor atom economy.

Your turn — atom economy

The molar volume of a gas

At room temperature and pressure (rtp), one mole of any gas occupies the same volume — the molar volume:

By Avogadro's law, equal volumes of gases (at the same temperature and pressure) contain equal numbers of molecules, so the mole ratio in the equation is also the volume ratio.

Your turn — gas volume

Empirical formula from masses

The empirical formula is the simplest whole-number ratio of atoms. From masses (or % composition):

• Divide each element's mass ÷ its A_r to get moles.
• Divide each by the smallest to get the simplest ratio.

The molecular formula is a whole-number multiple of the empirical formula (find it by comparing the empirical M_r with the real M_r).

Your turn — empirical formula

The Haber process

In a sealed container a reversible reaction reaches dynamic equilibrium: forward and backward reactions still occur, but at equal rates, so concentrations stay constant. The Haber process makes ammonia for fertilisers:

Typical industrial conditions: about 450 °C, 200 atmospheres, with an iron catalyst.

Choosing the conditions

Changing the conditions shifts the equilibrium position (Le Chatelier's idea). For the Haber process the chosen conditions are a compromise between yield, rate and cost:

• Pressure ~200 atm — high pressure favours the side with fewer gas molecules (the NH₃ side, 4 → 2), giving more ammonia; but very high pressure is expensive and unsafe.
• Temperature ~450 °C — the forward reaction is exothermic, so a lower temperature gives a higher yield, but too low is far too slow. 450 °C is a compromise giving an acceptable yield at an acceptable rate.
• Iron catalyst — speeds up equilibrium being reached; it does not change the position/yield.

Watch out: a catalyst speeds up how fast equilibrium is reached but does not change the equilibrium yield. The 450 °C is a compromise, not the temperature that gives the most ammonia.

Reading the equilibrium

Chemical cells and fuel cells

A simple chemical cell is two different metal electrodes in an electrolyte. The more reactive metal loses electrons more readily, so a voltage is produced — until one reactant is used up (then it goes flat).

A hydrogen–oxygen fuel cell is fed hydrogen and oxygen continuously and produces a voltage with water as the only product:

Fuel cells vs rechargeable cells

Compared with rechargeable cells: fuel cells don't run down or need recharging and have no toxic metals, but hydrogen is hard to store/transport and is often produced from fossil fuels.

The fuel-cell product

Topic 5 at a glance

You've covered the whole of Edexcel Topic 5 — Separate chemistry 1. Press Finish to see your score.