Chemical Reactions and Equations

Skeletal chemical equation

An equation that shows the correct formulae of the reactants and products but has not yet been balanced, so the number of atoms of an element differs between the two sides. It does not yet satisfy conservation of mass.

Balanced chemical equation

An equation in which the number of atoms of every element is the same on the reactant and product sides. Balancing is done by changing coefficients only — the formulae of the compounds themselves must never be altered.

How do you balance a skeletal chemical equation by the hit-and-trial method?

Draw a box round each formula so it cannot be altered, then list the atoms of each element on both sides. Start with the compound having the most atoms and equalise one element at a time using coefficients only. Recount every element at the end, and add state symbols if they are needed.

Law of conservation of mass

Mass can neither be created nor destroyed in a chemical reaction, so the total mass of the elements in the products equals the total mass in the reactants. This is why atom counts must match on both sides.

Combination reaction

A reaction in which two or more substances, elements or compounds, combine to form a single product. Quicklime reacting with water to give slaked lime, and coal burning to give carbon dioxide, are examples.

Decomposition reaction

A reaction in which a single reactant breaks down into two or more simpler products. It is the opposite of a combination reaction, and always needs energy supplied from outside to break the reactant apart.

Displacement reaction

A reaction in which a more reactive element takes the place of a less reactive element in its compound. Zinc and lead both displace copper from copper salts because each is more reactive than copper.

Double displacement reaction

A reaction in which two compounds exchange ions with each other. Unlike a displacement reaction, where one element replaces another, here it is ions that swap partners between the two reactants.

Precipitation reaction

Any reaction that produces a precipitate — a substance formed in solution that is insoluble in water and separates out as a solid. Mixing sodium sulphate and barium chloride solutions gives a white precipitate.

Oxidation

The gain of oxygen, or the loss of hydrogen, by a substance during a reaction. When copper is heated in air it gains oxygen to form black copper(II) oxide, so the copper has been oxidised.

Reduction

The loss of oxygen, or the gain of hydrogen, by a substance during a reaction. When hydrogen is passed over heated copper(II) oxide, the oxide loses oxygen and is reduced back to brown copper.

Exothermic reaction

A reaction in which heat is released along with the products, warming the surroundings. Burning natural gas, respiration, and the decomposition of vegetable matter into compost are all exothermic.

Endothermic reaction

A reaction in which energy is absorbed from the surroundings. Decomposition reactions are endothermic, since they need heat, light or electricity supplied to break the reactant down.

Corrosion

The gradual attack on a metal by substances around it such as moisture and acids. Rusting of iron is the familiar case; the black coating on silver and the green coating on copper are also corrosion.

What observations tell you that a chemical reaction has taken place?

Any of four changes: a change in state, a change in colour, the evolution of a gas, or a change in temperature. Observing one of these is the practical test that the identity of the substance has altered.

Why must a chemical equation always be balanced?

Because mass is conserved in a chemical reaction. Atoms are neither created nor destroyed, only rearranged, so each element must appear in equal numbers on both sides. An unbalanced equation would imply mass had changed.

Water is added to quicklime in a beaker and the beaker becomes hot to touch. What kind of reaction is this, and why does it warm up?

Calcium oxide and water combine to form a single product, slaked lime, so it is a combination reaction. It is also exothermic: a large amount of heat is released along with the product, which is what warms the beaker.

In what three forms can the energy needed for a decomposition reaction be supplied, and what is an example of each?

Heat, light or electricity. Limestone decomposes to quicklime and carbon dioxide on heating; silver chloride turns grey in sunlight as it breaks into silver and chlorine; water splits into hydrogen and oxygen when a current is passed through it.

Why is a substance being oxidised in a reaction always accompanied by another being reduced?

Because the oxygen or hydrogen lost by one reactant is exactly what the other gains. When hydrogen is passed over copper(II) oxide, the oxide loses oxygen while the hydrogen gains it. Such paired reactions are called redox reactions.

Why are packets of oily foods such as chips flushed with nitrogen?

Fats and oils turn rancid — their smell and taste change — when they are oxidised. Nitrogen is unreactive, so displacing the air keeps oxygen away from the food and slows that oxidation. Antioxidants and airtight containers work the same way.

What happens when water is added to quicklime, and what is the equation?

It reacts vigorously to give slaked lime, releasing a large amount of heat: CaO(s) + H2O(l) --> Ca(OH)2(aq) + heat. The slaked lime solution is used for whitewashing walls.

What is observed when lead nitrate is heated, and what is the equation?

Brown fumes of nitrogen dioxide are given off: 2Pb(NO3)2(s) --heat--> 2PbO(s) + 4NO2(g) + O2(g). The brown colour of the fumes is the giveaway that the gas is nitrogen dioxide.

What is observed when ferrous sulphate crystals are heated, and what is the equation?

The green crystals lose their water of crystallisation and change colour, and the characteristic smell of burning sulphur is given off: 2FeSO4(s) --heat--> Fe2O3(s) + SO2(g) + SO3(g).

An iron nail is left in copper sulphate solution. What changes are seen, and what is the equation?

The nail turns brownish as copper deposits on it and the blue colour of the solution fades: Fe(s) + CuSO4(aq) --> FeSO4(aq) + Cu(s). Iron is more reactive, so it displaces copper from the salt.

What happens when sodium sulphate and barium chloride solutions are mixed, and what is the equation?

A white precipitate of barium sulphate, insoluble in water, forms at once: Na2SO4(aq) + BaCl2(aq) --> BaSO4(s) + 2NaCl(aq). The sodium chloride formed stays dissolved in the solution.

Machine-readable version: /api/decks/cbse-class-10-science-chemical-reactions-and-equations