• A single replacement reaction is one in which a free element takes the place of another element in a compound.
• The more reactive element pushes out the less reactive one.
• General pattern A + BC → AC + B
• The element that is replaced is released in its free form.
• Before we study the examples we need a tool: the chemical reactivity series.

1.4.1 Replacement of hydrogen of an acid by a metal

• Activity.

• Place a piece of sanded magnesium ribbon in a test tube and add about 5 mL of dilute hydrochloric acid.
• At once bubbles of gas appear.
• Close the mouth of the tube for a few seconds, then bring a burning splint near: the gas burns with a "pop".
• That is the test for hydrogen.
• Mg + 2HCl → MgCl₂ + H₂↑

• The magnesium has replaced the hydrogen of the acid hydrogen was released as a gas and the salt (magnesium chloride) was formed.
• In general:
• Reactive metal + dilute acid → salt + hydrogen gas

• Do all metals react with the same speed?
• Place equal masses of zinc, iron and copper in equal volumes of the same dilute hydrochloric acid and compare the rate of bubbling:

• Magnesium is faster than zinc, zinc is faster than iron, and copper does not react.
• The rate of reaction with acids (and with oxygen and water) is a measure of the metal’s chemical reactivity.

• The chemical reactivity series is the arrangement of the metal elements in descending order of their chemical reactivity.
• Hydrogen, which is not a metal, is placed in the series as a reference point.
• Most reactive (top) to least reactive (bottom):
• Potassium (K) – Sodium (Na) – Lithium (Li) – Barium (Ba) – Calcium (Ca) – Magnesium (Mg) – Aluminium (Al) – Zinc (Zn) – Iron (Fe) – Tin (Sn) – Lead (Pb) – Hydrogen (H) – Copper (Cu) – Mercury (Hg) – Silver (Ag) – Platinum (Pt) – Gold (Au)
• Important rules from the series:

• Metals above hydrogen replace hydrogen from dilute acids (and the most reactive ones also from water).
• Metals below hydrogen (Cu, Hg, Ag, Pt, Au) do not react with dilute acids or with water, because they cannot replace hydrogen.
• A metal higher in the series replaces a lower metal from the solution of its salt.
• Platinum and gold are inactive elements. This is why gold and platinum stay shiny and are used in jewellery.

• Enrichment why does aluminium seem slow? Aluminium is above hydrogen, yet it does not react with dilute hydrochloric acid immediately.
• The reason is the thin, tough layer of aluminium oxide (Al₂O₃) on its surface, which acts as an insulating layer and prevents the acid from reaching the metal.
• When this layer is slowly eaten away by the acid, bubbles of hydrogen start to appear.
• So the delay does not mean aluminium is unreactive.

• Example. If small pieces of copper, zinc, silver and iron are put separately in dilute hydrochloric acid, hydrogen is released only from zinc and from iron, because both are above hydrogen.
• Copper and silver are below hydrogen, so nothing happens.

1.4.3 Replacement of hydrogen of water by a metal

• Highly reactive metals at the top of the series react with water and replace hydrogen, which is released as a gas that burns with a pop.
• Activity (done only by the teacher).

• Half-fill a glass trough with water.
• Using forceps, take a very small piece of sodium (the size of a grain of rice) from the kerosene, dry it with filter paper, and place it on the water.
• Observations the sodium floats because its density is less than that of water; it darts about quickly across the surface because it reacts instantly and violently; a yellowish-orange flame may form; the solution becomes alkaline (it turns red litmus blue).

Na + 2H₂O → 2NaOH + H₂↑

1. Sodium replaces hydrogen from water
2. forming sodium hydroxide (an alkali) and hydrogen gas.

• Safety. A great deal of heat is released, which can ignite the hydrogen or cause a small explosion.
• Water must never be used to put out a sodium fire; dry sand is kept ready.
• Sodium is stored under kerosene so that it does not react with air and moisture.
• Do all metals react with water equally? No. Compare sodium and magnesium: sodium reacts instantly with cold water, while magnesium ribbon does not react with cold water and needs thermal energy (hot water) before bubbles form:

Mg + 2H₂O(hot) → Mg(OH)₂ + H₂↑

• This proves magnesium is less reactive than sodium. Metals lower in the series such as copper do not react with water at all.

1.4.4 Replacement of one metal by another in its salt solution

• A more reactive metal can displace a less reactive metal from the solution of its salt.
• Activity. Pour blue copper(II) sulphate solution into a beaker. Clean a zinc rod with sandpaper and immerse it for about ten minutes.
• Observations the blue colour of the solution gradually fades, and a reddish-brown deposit of copper forms on the rod.
• Zn + CuSO₄ → ZnSO₄ + Cu
• (blue) (colourless) (red-brown solid)

• Zinc is higher than copper in the series, so zinc replaced copper.
• Had the experiment been done the other way round (a copper rod in zinc sulphate), nothing would happen, because copper cannot displace the more reactive zinc.
• Rule a metal that comes earlier in the series replaces any metal that follows it from its salt solution.

• Life application cleaning silverware. Silverware slowly develops a black layer of silver sulphide (tarnish).
• Place it in boiling water with sodium bicarbonate and a piece of aluminium foil.
• Aluminium is more reactive than silver, so it takes the place of silver in the tarnish, and the silver is restored to its shiny state.

• Cross-cutting concept patterns. Reactions are not random.
• They follow predictable patterns (combination, decomposition, replacement), and the reactivity series is a pattern that lets us predict what will happen before we do the experiment.