Video Transcript
Which of the following has the
highest thermal stability? (A) CuO solid, which has a standard
enthalpy of formation of minus 157.3 kilojoules per mole. (B) CuCl2 solid, which has standard
enthalpy of formation of minus 220.1 kilojoules per mole. (C) CuCl solid, which has standard
enthalpy of formation of minus 137.2 kilojoules per mole. Or (D) CuSO4 solid, which has
standard enthalpy of formation of minus 771.4 kilojoules per mole.
In this question, we have been
given four ionic compounds along with their standard enthalpies of formation,
shortened to Δ𝐻 plimsoll symbol, or a degree symbol which is seen here, 𝑓. The standard enthalpy of formation
is the enthalpy change when one mole of a substance is formed from its constituent
elements in their standard states under standard conditions.
The question lists the enthalpies
of formation for four different substances and asks us to find the molecule with the
highest thermal stability. Thermal stability is the tendency
to not break down when exposed to heat. A more negative enthalpy of
formation will correlate with more stable products. Therefore, the more negative the
enthalpy of formation is for a substance, the greater its thermal stability.
To answer the question, we will
therefore need to determine which substance has the most negative enthalpy of
formation. From the options given, CuSO4 has
the most negative enthalpy, with a value of minus 771.4 kilojoules per mole. This means that the species with
the highest thermal stability is (D), CuSO4 solid, which has a standard enthalpy of
formation of minus 771.4 kilojoules per mole.
