What is the enthalpy change in a chemical reaction?

The enthalpy change in a chemical reaction is defined as the difference between the enthalpy of the reactants and the enthalpy of the products. Enthalpy, a state function denoted as H, represents the total heat content of a system at constant pressure. In a reaction, if the products have a higher enthalpy than the reactants, the reaction is endothermic, indicating that heat has been absorbed from the surroundings. Alternatively, if the products have a lower enthalpy than the reactants, the reaction is exothermic, showing that heat has been released into the surroundings.

This concept is crucial in thermodynamics as it allows chemists to understand the energy changes that accompany chemical reactions, enabling predictions about whether a reaction will occur spontaneously based on the enthalpy change. The other options refer to different concepts: speed does not relate to the energy change of a reaction, energy lost to surroundings may relate to exothermic reactions but does not explicitly define enthalpy change, and temperature variations during the reaction are not directly a measure of enthalpy change.