The standard enthalpy of reaction (also known as the heat of reaction) is the change in enthalpy that occurs when a chemical reaction takes place under standard conditions. Standard conditions refer to a specific set of conditions, including a temperature of 25°C (298 K) and a pressure of 1 atmosphere (101.3 kPa).
In the case of the reaction CaCO3(s) → CaO(s) + CO2(g), the standard enthalpy of reaction would be the change in enthalpy that occurs when 1 mole of calcium carbonate decomposes to form 1 mole of calcium oxide and 1 mole of carbon dioxide, under standard conditions. The value Hr = +177.8kJ represents the enthalpy of reaction under these conditions.
In general, the standard enthalpy of reaction is a measure of the amount of heat absorbed or released during a chemical reaction under standard conditions. It is expressed in units of energy per mole of reactants or products. Positive values indicate that heat is absorbed during the reaction (endothermic), while negative values indicate that heat is released during the reaction (exothermic).