Final answer:
The enthalpy change for a reaction represented by ΔH is a measure of the heat content change. It can be calculated from bond energies or looked up if known, considering the states of reactants and products. The sign of ΔH indicates if a reaction is exothermic or endothermic.
Step-by-step explanation:
The enthalpy change for a chemical reaction, often represented by the symbol ΔH, is a measure of the total heat content change during the reaction. For instance, if the given reaction has an enthalpy of h, the enthalpy change is simply ΔH = h. This change can be calculated by using the bond energies of reactants and products, or if it's a known reaction under standard conditions, it can be found in tables of standard enthalpy changes. It's important to consider the physical states of reactants and products, as these can affect the magnitude of enthalpy change. Furthermore, according to Hess's Law, if we know the enthalpies of formation for reactants and products, we can calculate the overall enthalpy change.
An example of enthalpy change in a balanced chemical equation is:
2 H₂(g) + O₂(g) → 2 H₂O(l), ΔH° = -570 kJ. The negative enthalpy change indicates that the reaction is exothermic, meaning it releases heat into the surroundings. Conversely, a positive enthalpy change would imply an endothermic reaction, which absorbs heat from the surroundings.
To determine whether a reaction is exothermic or endothermic, you look at the sign of ΔH; a negative sign indicates an exothermic reaction while a positive sign suggests an endothermic reaction. For example, the standard enthalpy change for the reaction H₂(g) + Br2(g) → 2HBr (g) can be calculated using bond energies or found in reference materials.