For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up ΔH°_f for each substance in Appendix C. (a) NO₂(g) (b) SO₃(g) (c) NaBr(s) (d) Pb(NO₃)₂(s).

Verified step by step guidance

1. Step 1: Identify the elements that make up the compound. In this case, the compound SO3(g) is made up of sulfur (S) and oxygen (O2).
2. Step 2: Write the unbalanced equation for the formation of the compound from its elements in their standard states. The standard state of sulfur is solid (S) and for oxygen is diatomic gas (O2). So, the unbalanced equation is: S(s) + O2(g) -> SO3(g).
3. Step 3: Balance the equation. The balanced equation is: S(s) + 3/2 O2(g) -> SO3(g). This equation represents the formation of one mole of SO3 from its elements in their standard states.
4. Step 4: Look up the standard enthalpy of formation (ΔH°f) for the compound in Appendix C or a similar reference. The ΔH°f value is specific to the compound and is measured in kJ/mol.
5. Step 5: Write the balanced thermochemical equation by including the ΔH°f value in the equation. The general form of the equation is: S(s) + 3/2 O2(g) -> SO3(g) ΔH°f = [value from Appendix C] kJ/mol.

Key Concepts

Thermochemical Equations

Thermochemical equations represent the heat changes associated with chemical reactions. They show the relationship between the enthalpy change (ΔH) and the stoichiometry of the reaction. In the context of formation reactions, these equations depict how one mole of a compound is formed from its elements in their standard states, allowing for the calculation of enthalpy changes.

Standard State

The standard state of a substance refers to its physical state (solid, liquid, gas) at a specified temperature and pressure, typically 1 bar (or 1 atm) and 25°C. For elements, the standard state is the most stable form at these conditions. Understanding standard states is crucial for accurately writing formation equations and determining standard enthalpy of formation (ΔH°f).

Enthalpy of Formation (ΔH°f)

The enthalpy of formation (ΔH°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a key thermodynamic quantity used to assess the stability of compounds and predict reaction behavior. Values of ΔH°f can be found in thermodynamic tables, and they are essential for calculating the overall enthalpy change in chemical reactions.