\(CaCO_{ 3 }(s)\rightarrow CaO(s)+CO_{ 2 }(s)\)
\({ \Delta }_{ f }{ H }^{ o }={ \Delta }_{ f }{ H }^{ o }[CaO(s)]+{ \Delta }_{ f }{ H }^{ o }[CO_{ 2 }(g)]-{ \Delta }_{ f }{ H }^{ o }[CaCO_{ 3 }(s)]\)
\( { \Delta }_{ f }{ H }^{ o }=-635.1+(-393.5)-(-1206.9)=+178.3kJ{ mol }^{ -1 }\)
Because \(\Delta H\) value is positive, so the reaction is endothermic, Hence, according to Le-Chatelier's principle, reaction will proceed in forward direction on increasing temperature.

Since, pH = 5, hence [H⁺] = 10^-5 mol L^-1
On diluting the solution  100 times
 [H⁺] = \(\frac { { 10 }^{ -5 } }{ 100 } ={ 10 }^{ -7 }mol \ { L }^{ -1 }\)
Total H+ ion concentration = H+ ions from acid ₊H⁺ ion from water
[H⁺] = 10^-7+10^-7 = 2 x 10^-7 M
pH = -log[2 x 10^-7] = 6.699

\({ \overset { 0 }{ C } l }_{ 2 }(g)+2{ \overset { -2 }{ O } }\overset { +1 }{ { H }^{ - } } (aq)\longrightarrow { \overset { +1 }{ C } l }\overset { -2 }{ { O }^{ - } } (aq)+{ \overset { -1 }{ C } l }^{ - }(aq)+{ \overset { +1 }{ H } }_{ 2 }\overset { -2 }{ O } (l)\)
In this reaction, oxidation number of Cl increases from 0 (in Cl₂) to 1(in ClO^-) as well as decreases from 0 (in Cl₂₎ to -1 (in Cl^-).So, it acts as both reducing as well as oxidising agent. This is an example of disproportionation reaction. In this reaction , ClO^- species bleaches the substances due to its oxidising action.[In hypochlorite ion (ClO^-), Cl can decrease its oxidation number from +1 to 0 or -1].
Note 
Disproportionation reactions are a special type of redox reactions in which an element in one oxidation state is simultaneously oxidised and reduced. 

In a disproportionation reaction, the same species simultaneously get oxidised as well as reduced.Therefore, for such a redox reaction to occur,the reacting species must contain an element which has atleast three oxidation states.The element, in reacting species,is present in an intermediate state while lower and higher oxidation states are available for reduction and oxidation states are available for reduction and oxidation to occur(respectively).Fluorine is the strongest oxidising agent.It does not show positive oxidation state. That's why fluorine does not show disproportionation reaction.