Hess's law states that the enthalpy change of a reaction:
Is independent of the route taken
Depends on the reaction pathway
Is always positive
Equals the activation energy
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WhyHess's law states that the total enthalpy change is independent of the route taken, depending only on the initial and final states (conservation of energy).
2Multiple choice · Medium
Which expression gives the standard enthalpy change of a reaction from enthalpies of formation?
What is the standard enthalpy of formation of an element in its standard state?
Zero
$+100\,\text{kJ mol}^{-1}$
$-286\,\text{kJ mol}^{-1}$
Always positive
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WhyBy definition the standard enthalpy of formation of an element in its standard state (e.g. $\text{O}_{2}(g)$) is zero.
4Multiple choice · Medium
What are the standard conditions used when quoting standard enthalpy changes?
$100\,\text{kPa}$ and $298\,\text{K}$
$101\,\text{kPa}$ and $0\,\text{K}$
$1\,\text{kPa}$ and $373\,\text{K}$
$100\,\text{kPa}$ and $1000\,\text{K}$
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WhyIB standard conditions are a pressure of $100\,\text{kPa}$ and a stated temperature, usually $298\,\text{K}$ ($25\,^{\circ}\text{C}$).
5Multiple choice · Hard
For a reaction, $\sum \Delta H_{f}^{\circ}(\text{products}) = -600\,\text{kJ}$ and $\sum \Delta H_{f}^{\circ}(\text{reactants}) = -400\,\text{kJ}$. What is $\Delta H^{\circ}$?
Hess's law is a direct consequence of which physical principle?
Conservation of energy
Conservation of mass
Le Chatelier's principle
The ideal gas law
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WhyBecause energy cannot be created or destroyed (conservation of energy / first law of thermodynamics), the enthalpy change must be the same regardless of route.
7Fill in the blank · Easy
The standard enthalpy of formation of oxygen gas, $\text{O}_{2}(g)$, is $\text{kJ mol}^{-1}$.
Answer:
0 / zero
WhyAn element in its standard state has $\Delta H_{f}^{\circ} = 0$.
8Fill in the blank · Medium
For a reaction $\Delta H_{1} = -100\,\text{kJ}$ and $\Delta H_{2} = -150\,\text{kJ}$ along a two-step route to the same products. By Hess's law the overall $\Delta H$ is kJ.
Answer:
-250 / -250 kJ
WhyBy Hess's law the overall enthalpy change is the sum of the steps: $-100 + (-150) = -250\,\text{kJ}$.
9Multiple choice · Medium
Hess law is a direct consequence of which principle?
Conservation of energy
Conservation of mass only
Le Chatelier principle
The ideal gas law
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WhyHess law follows from the conservation of energy: enthalpy is a state function.