The standard enthalpy of reaction denotes the heat lost or gained during a chemical reaction when one molar mass of a substance reacts under standard conditions. The standard enthalpy of reaction is shown as "?H" or "delta H" in scientific notation.
Standard enthalpies of reaction can be positive or negative numbers. When the value is negative, that means the reaction is exothermic, and substances release heat into the surrounding environment. A positive value means the reaction is endothermic, and chemicals take in heat. The units for standard enthalpy of reaction are kilojoules per mole, and standard conditions are 25 degrees Celsius and 1 atmosphere of pressure.