Video Transcript
Using the standard enthalpies of
formation in the given table, what is the standard enthalpy of reaction for the
decomposition of ammonium chloride? NH4Cl solid reacts to form NH3 gas
plus HCl gas.
Because it can be difficult to
measure the enthalpy of certain types of reactions, chemists calculate enthalpy
changes by using enthalpy data from other reactions. In this question, we need to
calculate the enthalpy change for the decomposition of ammonium chloride. We can calculate this enthalpy
change indirectly by using the standard enthalpies of formation and creating a Hess
cycle.
The standard enthalpy of formation
is the enthalpy change when one mole of substance is formed from its constituent
elements in their standard states and under standard conditions. To create our Hess cycle, we can
start by writing the balanced equation that was provided in the problem and labeling
it reaction one. Below the cycle, we should write
the constituent elements of the reactants and products, which are nitrogen,
hydrogen, and chlorine. We need to use a subscript of two
for all of these elements, because they exist as diatomic molecules in their
standard state.
Now, let’s draw an arrow from the
elements to the reactant ammonium chloride. This reaction represents the
formation of one mole of ammonium chloride from its constituent elements. To balance this equation, we need a
coefficient of one-half in front of N2, a coefficient of two in front of H2, and a
coefficient of one-half in front of Cl2. Let’s label this reaction as
reaction two.
Now, let’s draw an arrow from the
elements to the products and label this reaction as reaction three. This equation is already balanced
and represents the formation of one mole of ammonia gas and one mole of hydrogen
chloride gas from their constituent elements. Using the Hess cycle we created, we
can state that the enthalpy change of reaction one is equal to the enthalpy change
of reaction two plus the enthalpy change of reaction three.
However, it’s important to note
that when moving through the cycle, we complete reaction two in the opposite
direction of the arrow. Because of this, we will need to
change the sign of the enthalpy change of reaction two in our equation.
Now, to calculate the enthalpy
change of reaction one, we will need to substitute the correct values from the given
table. We should substitute negative 314
kilojoules per mole, which is the standard enthalpy of formation of ammonium
chloride, as the enthalpy change of reaction two. We should add together the standard
enthalpies of formation for ammonia gas and hydrogen chloride gas to calculate the
enthalpy change of reaction three. After simplification, we get Δ𝐻
one equals 314 kilojoules per mole minus 138 kilojoules per mole. After subtraction, the result is
positive 176 kilojoules per mole.
Therefore, the standard enthalpy of
reaction for the decomposition of ammonium chloride is positive 176 kilojoules per
mole.