Question

Values of heats of formation for $SiO_2$ and $MgO$ are $-48.4$ and $-34.7 kJ$ respectively. The heat of the reaction
$2Mg + SiO_2 \to 2MgO + Si$ is

• A. $21.16\, kJ$
• B. $- 21.00\, kJ$
• C. $- 13.62\, kJ$
• D. $13.6\, kJ$

Answer 1

Answer: (B)

$\Delta H=\left[2\times\left(-48.4\right)+0\right]-\left[\left(-34.7+0\right)\right]$
$=-69.4+48.4 kJ$
$=-21.00kJ$