Q.a. Reversible reaction
b. Rate of reaction

An irreversible reaction proceeds only from reactants to products, in a single direction, until one of the reactants is completely used up -- shown with a single arrow, as in $C(s)+O_2(g)\xrightarrow{\Delta}CO_2(g)$ or $2KClO_3(s)\xrightarrow{\Delta}2KCl(s)+3O_2(g)$. A reversible reaction, by contrast, proceeds in both the forward (reactants to products) and backward/reverse (products to reactants) directions simultaneously, shown with a double arrow ($\rightleftharpoons$), e.g. $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$. Whether a change behaves reversibly can depend on the system it is carried out in: decomposition of $CaCO_3$ looks irreversible in an open vessel (escaping CO2 prevents recombination) but is genuinely reversible in a closed vessel. When a reversible reaction is run in a closed system, the forward rate (high at first) falls as reactants are used up, while the reverse rate (initially near zero) rises as products build up, until the two rates become exactly equal -- this is dynamic equilibri …