Enthalpy is a measure of the total energy of a chemical system. The enthalpy change of a process is the heat energy that is transferred between the system and its surroundings at constant pressure.The difference in the enthalpy of the reactants and products is called the enthalpy change ΔH.ΔH = Hproducts - HreactantsExothermic processes transfer heat energy from the system to the surroundings. The temperature increases. ΔH is negative.
Endothermic processes transfer heat energy from the surroundings to the system. The temperature decreases. ΔH is positive.Enthalpy changes are measured under standard conditions of 100kPa and 298KThe standard state of an element is the physical state it exists in at 298K.
A reaction is over when the products have cooled back to 298K. The heat given out to the surroundings as the product cools is the enthalpy change of the reaction.With endothermic reactions that take in heat from the surroundings and cool down the surroundings, the reaction is over when the products have warmed back up to 298K.If a gas is given out energy is required to push away the molecules of gas in the atmosphere so pressure affects the enthalpy change.
Endothermic processes transfer heat energy from the surroundings to the system. The temperature decreases. ΔH is positive.Enthalpy changes are measured under standard conditions of 100kPa and 298KThe standard state of an element is the physical state it exists in at 298K.
A reaction is over when the products have cooled back to 298K. The heat given out to the surroundings as the product cools is the enthalpy change of the reaction.With endothermic reactions that take in heat from the surroundings and cool down the surroundings, the reaction is over when the products have warmed back up to 298K.If a gas is given out energy is required to push away the molecules of gas in the atmosphere so pressure affects the enthalpy change.