4.4.1.1,4.4.1.2,4.4.1.3 - Metal reactions, reactivity and extraction
Learn how metal oxides form in oxidation reactions (4.4.1.1), how reactions with water, dilute acids and other metal compounds reveal a reactivity order (4.4.1.2), and how that order helps choose a method of extracting a metal (4.4.1.3). The connection is that a metal's readiness to react determines both how it behaves in a laboratory and how difficult it is to obtain from its compounds.
Metal oxides and oxidation
A metal reacting with oxygen forms a new compound called a metal oxide.
Magnesium burns with a bright white flame and forms a white solid, magnesium oxide. The solid is a different substance from the shiny grey metal. Its formula shows that oxygen has become chemically combined with magnesium:
The equation is balanced: two magnesium atoms and two oxygen atoms occur on each side. When copper is heated in air, its surface becomes coated with black copper oxide:
These reactions are oxidation because the metals gain oxygen. Here “gain” means combining with oxygen in a chemical reaction, rather than simply collecting oxygen gas nearby. If a measured sample of metal becomes completely converted to its oxide, its mass increases because oxygen atoms from the air are now part of the solid. Conservation of mass still holds when that oxygen is included.
Oxidation and reduction by oxygen transfer
Oxidation is gain of oxygen. Reduction is loss of oxygen.
A metal oxide can be reduced back to its metal if another substance removes the oxygen. The definitions concern the change in a named substance: a reaction may oxidise one substance while reducing another. An observed colour change is evidence of a chemical change, but deciding which substance is oxidised requires following the oxygen in the equation.
Water, acids and metal reactivity
Metals differ in how readily their atoms form positive ions when they react.
A positive ion has fewer electrons than the corresponding atom. A more reactive metal has a greater tendency to form its positive ions. Reactions with water or dilute acid let us compare this tendency through observations such as gas bubbles, disappearance of the metal and warming. The reactivity series arranges metals from most to least reactive.
The required order is potassium, sodium, lithium, calcium, magnesium, zinc, iron, then copper. Carbon and hydrogen are useful reference substances in the series even though they are non-metals. Carbon is below magnesium and above zinc; hydrogen is below iron and above copper. Aluminium, needed later for extraction, lies between magnesium and carbon.
| Metal | Water at room temperature | Dilute hydrochloric or sulfuric acid at room temperature |
|---|---|---|
| Potassium | Very vigorous; produces potassium hydroxide and hydrogen; hydrogen may ignite | Extremely vigorous; forms a salt and hydrogen; hazardous and not a suitable classroom comparison |
| Sodium | Vigorous; produces sodium hydroxide and hydrogen; heat often melts the metal into a moving ball | Very vigorous; forms a salt and hydrogen; hazardous |
| Lithium | Fizzes and moves on the surface; produces lithium hydroxide and hydrogen | Very vigorous; forms a salt and hydrogen; hazardous |
| Calcium | Fizzes; forms hydrogen and calcium hydroxide; the water can become cloudy | Vigorous reaction producing a salt and hydrogen |
| Magnesium | Very slow reaction; little visible change in a short cold-water experiment | Rapid fizzing; forms a salt and hydrogen |
| Zinc | No observable reaction in the usual cold-water experiment | Fizzes less rapidly than comparable magnesium |
| Iron | No observable reaction in the usual cold-water experiment | Slow fizzing, slower than comparable zinc |
| Copper | No reaction | No reaction with these dilute acids |
For example, calcium reacts with water to make a hydroxide and hydrogen:
Metals above hydrogen can displace hydrogen from dilute hydrochloric or sulfuric acid. Copper is below hydrogen, so it does not. These comparisons concern room temperature; reactions with steam are outside this lesson's required scope.
To deduce an order from evidence, compare similar-sized clean pieces under the same temperature and acid concentration. Use the initial rate of bubbling or a measured gas volume per unit time. Total gas collected after completion also depends on how much metal was used, so more gas alone does not prove greater reactivity. Wear eye protection, use small quantities and keep ignition sources away from hydrogen; the very reactive metals require teacher-controlled procedures.
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