# Question Video: Identifying Which Compound Has the Highest Thermal Stability from Standard Enthalpy of Formation Values Chemistry

Which of the following has the highest thermal stability? [A] CuO (s), which has Δ𝐻°_(𝑓) = −157.3 kJ/mol [B] CuCl₂ (s), which has Δ𝐻°_(𝑓) = −220.1 kJ/mol [C] CuCl (s), which has Δ𝐻°_(𝑓) = −137.2 kJ/mol [D] CuSO₄ (s), which has Δ𝐻°_(𝑓) = −771.4 kJ/mol

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### 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.