Question

When manipulating thermochemical equations, if I divide the coefficients of the chemical reaction of one step by 3, what do I do to the enthalpy change value?

• A. Multiply it by 3
• B. Divide it by 3
• C. Reverse the sign
• D. I don't change the enthalpy value
• E. Divide it by 3 and reverse the sign

Answer 1

The correct option is B Divide it by 3

When manipulating thermochemical equations, if I divide the coefficients of the chemical reaction of one step by 3, I divide the enthalpy change value by 3. For example, consider the following thermochemcial equation
$C\left(graphite\right)+O_2\left(g\right)\to C{O}_2\left(g\right)$ $\Delta H^o=-393.5kJ$
I divide the coefficients of the chemical reaction by 3
$\frac{1}{3}C\left(graphite\right)+\frac{1}{3}{O}_2\left(g\right)\to \frac{1}{3}C{O}_2\left(g\right)$
I divide the enthalpy change value by 3
$\Delta H^o=\frac{-393.5kJ}{3}=-131.2kJ$