Chemical Reactions and Equations

The defining concept of a double displacement reaction is the exchange of ions between two compounds, usually ionic compounds in aqueous solution. Its general pattern is \(AB + CD \rightarrow AD + CB\): the positive or negative ions change partners and two new compounds may form. Burning a metal is combustion, while melting and liquefaction are physical changes rather than ionic chemical reactions.

This is a double displacement reaction: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions combine to form insoluble barium sulphate, a white precipitate. Sodium chloride remains dissolved in water, so it does not form the precipitate.

A precipitation reaction commonly occurs when two aqueous solutions are mixed and one of the possible products is insoluble in water. That insoluble product separates from the solution as a solid and is called a precipitate. It may make the mixture cloudy or settle at the bottom. A gas, a liquid, or a substance that remains completely dissolved does not meet this definition.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Oxygen is removed from copper(II) oxide to form water, leaving copper behind. Removal of oxygen is reduction; therefore, \(\mathrm{CuO}\) is reduced. In contrast, hydrogen gains oxygen and is oxidised.

On exposure to air, magnesium ribbon develops a thin layer of magnesium oxide on its surface. This layer prevents direct contact between the metal and oxygen and hinders burning. Rubbing with sandpaper removes the layer and exposes fresh magnesium for combustion. It does not lower the ribbon’s melting point.

The governing concept is that oxidation and reduction are complementary processes. If one substance is oxidised, another substance in the same reaction is reduced; electrons, oxygen, hydrogen, or oxidation state may be used to describe the change. Such a reaction is called a redox reaction. Combustion and precipitation are particular reaction situations, while condensation is generally a change of state, so none of those names gives the required definition.

At the introductory level, oxidation can be identified as the addition of oxygen to a substance. For example, magnesium combines with oxygen to form magnesium oxide, so magnesium is oxidised. In broader definitions, oxidation may also mean loss of electrons or removal of hydrogen, but those are not the simple oxygen-based criterion asked here. Removal of oxygen is reduction, and freezing is a physical change.

In reduction, oxygen is removed from a substance. For example, when oxygen is removed from copper(II) oxide to form copper, copper(II) oxide is reduced. In contrast, oxidation involves the addition of oxygen. Release of heat or evolution of a gas may occur in some reactions, but neither is a definite sign of reduction.

A decomposition reaction follows the pattern \(AB \rightarrow A + B\): one compound breaks into two or more simpler substances. For example, on heating, calcium carbonate decomposes as \(CaCO_3 \rightarrow CaO + CO_2\). This is the reverse structural pattern of combination, which forms one product from multiple reactants. Displacement involves replacement by an element, and double displacement involves exchange between two compounds.

Rusting of iron is an oxidation process that requires both oxygen and water. In the presence of water, oxygen reacts with iron to form hydrated iron(III) oxide, commonly called rust. Carbon dioxide may affect the rate of corrosion, but it is not essential for rusting.

The reaction is \(CaO + H_2O \rightarrow Ca(OH)_2\). Two reactants, quicklime and water, combine to form only one product, calcium hydroxide; therefore it is a combination reaction. The number of products is the useful test here. It is not decomposition because nothing breaks apart, not displacement because no element replaces another, and not double displacement because two ionic compounds do not exchange ions.

A chemical change produces one or more new substances with different properties. During rusting, iron reacts with oxygen and moisture to form iron oxides, commonly represented as hydrated iron(III) oxide, so the original iron is chemically changed. Melting ice changes only the physical state, dissolving sugar usually allows recovery by evaporation, and tearing paper changes size and shape without forming a new substance.

Rancidity is the spoilage of oils and fats caused mainly by their reaction with oxygen. Oxidation produces unpleasant-smelling and unpleasant-tasting substances, which is why stale fried food or old oily foods may become unacceptable. Preventive methods include airtight storage, refrigeration, and adding suitable antioxidants. Precipitation forms an insoluble solid, neutralisation involves acid and base, and distillation separates liquids by boiling points.

Rancidity occurs when fats and oils in food react with oxygen to form substances with an unpleasant smell and taste. Antioxidants slow this oxidation, so rancidity occurs later. In contrast, increasing the rate of oxidation would make rancidity happen faster.

Chips contain fats and oils that can oxidise on contact with oxygen in air. This causes rancidity and gives the chips an unpleasant taste and smell. Nitrogen is relatively unreactive; it reduces the amount of oxygen in the packet and therefore helps prevent oxidation. It is not filled to increase moisture.

Iron is more reactive than copper, so it displaces copper from copper sulphate: \\(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\\). The iron sulphate formed makes the solution pale green, and displaced copper appears as a reddish-brown deposit on the nail. Unlike option B, the solution does not become colourless after the reaction.

Combustion is a chemical reaction in which a fuel reacts rapidly with oxygen and releases heat, often with light. Therefore oxygen is the gas generally required to support burning. Nitrogen is abundant in air but does not normally support combustion, while helium is inert and chlorine is not the usual gas identified as the supporter of ordinary fuel burning. Thus option B is the unambiguous answer.

In combustion, a fuel reacts with oxygen to form new products such as carbon dioxide and water. Energy is required to break bonds in the reactants, but more energy is released when new, more stable bonds form in the products. Therefore, heat is released overall, making combustion exothermic. Option B describes the opposite energy balance, which would correspond to an endothermic reaction.

If the temperature of the reaction mixture rises, the reaction is generally releasing heat to the surroundings. Such a reaction is called exothermic. In contrast, an endothermic reaction absorbs heat and may cause a temperature decrease unless heat is continuously supplied from outside.

A fall in temperature during a reaction generally indicates that the reacting system is absorbing heat from its surroundings. Such a reaction is called endothermic. Therefore, option A is correct. Release of heat would usually raise the temperature, while light emission and rusting are possible reaction features but do not specifically explain a temperature decrease.