Explain the terms molecularity and order of a reaction. Give one example from each of first and second order reactions. A 0.1 M HCN solution contained 0.2 moles of KCN per litre of the solution. What was the hydronium ion concentration of the solution.
`(K_(a)` for `HCN = 7.2 xx 10^(-10) )`

Explanation

Hydrogenation of ethene is 1st order reaction and dissociation of HI is 2nd order reaction.
For weak acid, `HCN" " HCN hArr H^(+) + CN^(-)`
For salt of strong base and weak acid (KCN)
Thus, `[CN^(-) ] = [ KCN] = 0.2" mol L"^(-1) ["HCI".] =0.1" mol/L"`
`therefore" "K_a = 7.2 xx 10^(-10) K_a = ([ H^(+ )] [ CN^(-) ] )/( [HCI])`
`7.2 xx 10^(-10) = [(H^(+) ] [0.2])/[0.1] = 2 xx [H^(+) ]`
`therefore [ H_(3) O^(+) ]` Or `[H^(+) ] = (7.2 xx 10^(-10) )/( 2) = 3.6 xx 10^(-10) " mol/L"`