Question

Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by the liberation of heat, this process is called slaking of lime. Calcium hydroxide dissolves in water form its solution called lime water. Which among the following is (are) true about slaking of lime and solution formed?
(i) It is an endothermic reaction.
(ii) It is an exothermic reaction.
(iii) The pH of the resulting solution will be more than seven.
(iv)The pH of the resulting solution will be less than seven.
A. (i) and (ii)
B. (ii) and (iii)
C. (i) and (iv)
D. (iii) and (iv)

Answer 1

Hint: To answer this question we should know what is exothermic and endothermic and should have an idea of pH. The pH of the solution depends upon the acidity or basicity. The acidic solution has a pH of less than seven. The basic solution has a pH of more than seven.

Complete answer:
On the basis of the heat of the reaction, reactions are of two types:
Endothermic reaction: these reactions require the addition of heat.
Exothermic reaction: Heat is liberated during these reactions.
It is given that solid calcium oxide reacts vigorously with water to form calcium hydroxide and heat is liberated with this. This process is called slaking of lime.
The chemical formula of calcium oxide is ${\text{CaO}}$and calcium hydroxide is${\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}$.
The chemical reaction of slaking of lime is as follows:
${\text{CaO}}\,{\text{ + }}\,{{\text{H}}_{\text{2}}}{\text{O}}\, \to \,{\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}\,{\text{ + }}\,{\text{heat}}$
As heat is liberating during the reaction so, slaking of lime is an exothermic reaction.
Calcium hydroxide dissolves in water to give lime water.
${\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}\,{\text{ + }}\,{{\text{H}}_{\text{2}}}{\text{O}}\, \to \,{\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}{\text{(aq)}}$
The calcium hydroxide dissociated as follows:
$\,{\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}{\text{(aq}})\, \to {\text{C}}{{\text{a}}^{{\text{2 + }}}}{\text{(aq)}}\,{\text{ + }}\,{\text{2}}\,{\text{O}}{{\text{H}}^{\text{ - }}}{\text{(aq)}}$
The calcium hydroxide gives hydroxide group, so calcium hydroxide is basic. The base has a pH of more than seven, so the pH of calcium hydroxide solution will be more than seven. So, It is an exothermic reaction and the pH of the resulting solution will be more than seven are true about the slaking of lime and solution formed.

Therefore, the correct answer is option (B) (ii) and (iii).

Note: The acids are proton donors. The bases are proton acceptors or hydroxide donors. The pH is used to determine the acidity or basicity of the solution. The pH of the acidic solution ranges from one to six. The pH of the basic solution ranges from eight to fourteen. The solutions having pH equal to seven are known as neutral. The liberation of heat is shown by a negative sign and the addition of heat is shown by a positive sign.