Chemical Reactions and Equations: questions with answers

Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. The bubbles are hydrogen produced by the reaction, not the liquid boiling. A gas that burns with a pop sound is hydrogen.

Rubbing with sandpaper only removes the coating on the surface, so the ribbon looks shiny but no new substance is formed. The colour and the length of the ribbon before burning describe the starting state, not a change. When the ribbon burns, a white powder is left behind. This powder is a new substance, so a chemical reaction has taken place.

On the left, 2Al gives 2 aluminium atoms and 3CuCl2 gives 3 × 2 = 6 chlorine atoms. On the right, 2AlCl3 gives 2 aluminium atoms and 2 × 3 = 6 chlorine atoms. So each side has Al 2 and Cl 6.

The coefficients 6, 8, 2, 8 share a common factor of 2. Dividing every coefficient by 2 gives 3, 4, 1, 4 and preserves every atom balance. Changing Fe3O4 changes the substance.

Balancing changes how many units of each substance take part, not what the substances are. A coefficient stands before a formula and gives the number of units, so coefficients may be changed. A subscript is part of the formula: changing the 4 in Fe3O4 gives a different substance. State symbols show physical condition and do not change any atom count. So only the coefficients written in front of the formulae may be changed.

Zinc and sulphuric acid are the starting substances, so they are the reactants and are written on the left of the arrow. Zinc sulphate and hydrogen are formed in the reaction, so they are the products and are written on the right. The word equation is Zinc + sulphuric acid → zinc sulphate + hydrogen.

In ZnCO3 → ZnO + CO2 a single reactant breaks down into two products. That pattern is decomposition. Zn + CuSO4 → ZnSO4 + Cu is displacement. Na2SO4 + BaCl2 → BaSO4 + 2NaCl is double displacement. 2Mg + O2 → 2MgO is combination.

A decomposition reaction starts with a single reactant that breaks down into two or more products. Here one compound, AgCl, gives silver and chlorine. The count of products alone is not enough, because a double displacement also gives two products but starts with two compounds.

The reaction absorbs energy from its surroundings all the while it proceeds. Reactions in which energy is absorbed are called endothermic reactions. An exothermic reaction is the opposite: heat is given out along with the products, so it does not need energy to be supplied to keep going.

Quicklime is calcium oxide. It combines with water to give slaked lime, calcium hydroxide. The mixture becomes hot on its own, so heat is given out along with the product. The reaction is exothermic. No outside heating is needed and no heat is taken in. Calcium carbonate is not formed here.

The complete equation, observation, and reaction pattern determine the result. 2H2O → 2H2 + O2 produces hydrogen and oxygen in a 2:1 volume ratio.

The complete equation, observation, and reaction pattern determine the result. The first is single displacement by a free element; the second is ion exchange between compounds.

Both reactants are in solution, so their ions exchange partners: Na2SO4(aq) + BaCl2(aq) gives BaSO4(s) + 2NaCl(aq). Barium sulphate is insoluble, so it separates as a white precipitate and is written (s). Sodium chloride is soluble, so it stays dissolved and is written (aq). Two products form, not one, and no free element is released. The answer is BaSO4 solid, NaCl aqueous.

Copper oxide loses its oxygen to hydrogen, and the brown solid left behind is copper. Loss of oxygen is reduction, so the copper oxide is reduced. The hydrogen gains that oxygen and forms water, so hydrogen is oxidised. Both changes happen together in the same reaction.

Iron rusts when its surface stays in contact with air and moisture. A coat of paint covers the surface and keeps air and moisture away from the iron, so rusting is prevented. Paint acts as a barrier only; it does not change rust back into iron.