Question

What type of chemical reactions take place when:
A.A magnesium wire is burnt in the air?
B.Limestone is heated
C.Silver bromide is exposed to sunlight?
D.Electricity is passed through water?
E.Ammonia and hydrogen chloride are mixed?

Answer 1

Chemical reactions can be classified into various categories depending upon factors like: the number of reactants reacting, the type of reactants and products involved in the reaction, the reagent or conditions used and the type of changes observed.

Complete answer:
A.When a magnesium ribbon burns in air, then it undergoes a chemical change and forms magnesium oxide. The magnesium ribbon vigorously reacts with the oxygen present in the atmosphere and burns with a glow, giving out heat in the process.
$2Mg(s) + {O_2}(g) \to MgO(s) + heat$
Thus, this reaction can be classified as a combustion reaction.
B.Limestone is primarily made up of calcium carbonate $(CaC{O_3})$ which is a carbonate of an alkaline earth metal. The carbonates of second group metals have a tendency to disintegrate or break down by the action of heat on them. Therefore calcium carbonate breaks into calcium oxide and liberates carbon dioxide gas along with it. A single reactant gives two products upon heating.
$CaC{O_3}(s) + heat \to CaO(s) + C{O_2}(g)$
Thus, this reaction can be classified as a decomposition reaction.
C.Silver bromide contains bromide ion with oxidation number $( - 1)$ . When silver bromide is exposed to sunlight, the compound breaks apart to give solid silver metal and diatomic bromine in gaseous form which contains bromine in its zero oxidation state. Therefore it is observed that bromine loses an electron to gain a higher oxidation state.
$2AgBr(s)\xrightarrow{{sunlight}}2Ag(s) + B{r_2}(g)$
Thus, this reaction can be classified as an oxidation reaction.
D.Passing electricity through water leads to charge separation in it. The hydrogen ions and hydroxide ions get segregated and start migrating towards the electrodes of opposite polarity.
${H_2}O(l) \to {H^ + }(aq) + O{H^ - }(aq)$
Thus, this reaction can be classified as an electrolytic reaction.
E.Ammonia in itself is basic in nature and hydrogen chloride is a good source of protons i.e. a strong Arrhenius acid. An acid base reaction takes place between the two resulting in the formation of a salt.
$N{H_3} + HCl \rightleftharpoons N{H_4}Cl$
Thus, this reaction can be classified as a neutralization reaction.

Note:
A reaction can be classified into more than one category based on the changes involved. For example: the breaking apart of silver bromide in the presence of sunlight is both oxidation and decomposition. Also, combustion in itself is an oxidation reaction.