Enthalpy changes are measured on a set of standard conditions as the enthalpy changes can vary with temperature, pressure or concentration. These are called standard enthalpy changes ΔHθStandard pressure is 100kPa (1 atmosphere of pressure is 101kPa)Standard temperature is 298KStandard concentration is 1 moldm-3The standard state of a substance is the physical state of a substance (solid, liquid or gas) under standard conditions.Enthalpy changes are measured under standard conditions of 100kPa and 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.
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.