Use the data in Table 9.3 to calculate an approximate ∆H° in kilojoules for the synthesis of hydrazine from ammonia: 2 NH3(g) + Cl2(g) → N2H4(g) + 2 HCl(g)

Enthalpy change (∆H°) is the heat content change of a system at constant pressure during a chemical reaction. It indicates whether a reaction is exothermic (releases heat, ∆H° < 0) or endothermic (absorbs heat, ∆H° > 0). Calculating ∆H° for a reaction often involves using standard enthalpy values of formation for the reactants and products.

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This principle allows for the calculation of ∆H° for a reaction by using known enthalpy changes of related reactions, facilitating the determination of enthalpy changes for complex reactions.

The standard enthalpy of formation (∆H°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. These values are tabulated for many substances and are essential for calculating the overall enthalpy change of a reaction using the formula: ∆H° = Σ(∆H°f products) - Σ(∆H°f reactants).