Question

What is the heat of formation of $C{S}_2$, if the heats of combustion of $C,S$ and $C{S}_2$ are x, y and z kJ mol${}^{-1}$ ?

• A. $2y-x+z$
• B. $x-2y+z$
• C. $x+2y-z$
• D. $x+y+z$

Answer 1

The correct option is C $x+2y-z$

Required equation is
$C\left(s\right)+2S\left(s\right)\to C{S}_2\left(l\right){\Delta }_{f}H=?$
We have to find ${\Delta }_{f}H$ for this;


Given
a. $C\left(s\right)+O_2\left(g\right)\to C{O}_2\left(g\right){\Delta }_{f}H=x$

b.$S\left(s\right)+O_2\left(g\right)\to S{O}_2\left(g\right){\Delta }_{f}H=y$

c.$C{S}_2\left(l\right)+3O_2\left(g\right)\to C{O}_2\left(g\right)+2S{O}_2\left(g\right){\Delta }_{f}H=z$

Multiply the equation (b) by 2.
d. $2S\left(s\right)+2O_2\left(g\right)\to 2S{O}_2\left(g\right){\Delta }_{f}H=2y$
Adding equation (a) and (d) and subtracting (c),

We get the equation,
$C\left(s\right)+2S\left(s\right)\to C{S}_2\left(l\right){\Delta }_{f}H=?$

This is the required equation.
Thus ${\Delta }_{f}H=x+2y-z$
Standard heat of formation of $C{S}_2\left(l\right)=x+2y-z$