First slide

Hess law of constant summation

Question

$\large H_{aq}^ + \; + \;OH_{aq}^ - \; \to \;{H_2}{O_{(l)}},\Delta H = \; - {q_1}KJ\$
$\large HC{N_{aq}} + \;KO{H_{aq}}\; \to \;KC{N_{aq}}\; + \;{H_2}{O_{(l)}};\;\Delta H\; = \;-\;{{\text{q}}_2}{\text{ KJ}}\$
$\large HC{l_{aq}} + N{H_4}O{H_{aq}} \to N{H_4}C{l_{aq}} + {H_2}{O_{(l)}};\;\Delta H\; = \;{\text{ - }}{{\text{q}}_3}{\text{ KJ}}\$
Then ΔH for HCN_aq+NH₄OH_aq → Products (in KJ)

Moderate

Solution

$\Delta {H_n }\; = \;\Delta {H_i}\; + \;\Delta {H_{\left( {{H^ + } + O{H^ - } \to {H_2}O} \right)}}\$

${(\Delta {H_i})_{HCN}}\; = \;\Delta {H_n }\; - \;\Delta {H_{\left( {{H^ + } + O{H^ - } \to {H_2}O} \right)}}$

= -q₂-(-q₁) = (-q₁ - q₂)KJ

${(\Delta {H_i})_{N{H_4}OH}}\; = \; - {q_3}\; - ( - {q_1}) = ({q_1} - {q_3})KJ$

$\Delta {H_n }\; \;for\;HCN\; + \;N{H_4}OH\; \to \;products\;is\$

$\Delta {H_n }\; = \;{(\Delta {H_i})_{HCN}}\; + \;{(\Delta {H_i})_{N{H_4}OH}}\; + \;\Delta {H_{({H^ + } + O{H^ - } \to {H_2}O)}}\$

= (q₁-q₂) +(q₁-q₃) +(-q₁)
ΔH_n = q₁ - q₂- q₃
= [-q₂-q₃]+q_{1 KJ}

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