INTERNAL ENERGY Internal energy depends on many things Chemical identity Sample size Temperature and pressure Physical states However internal energy does not depend on the system s history Only current temperature pressure etc matter not what these values were an hour or day ago State function A function or property whose value depends only on the present state or condition of the system not on the path used to arrive at that state Pressure volume and temperature are all state functions WORK Work is not a state function Physics define work w as force F needed to move an object times the distance moved d w F x d Your leg muscles provide the force needed to walk up stairs and overcome gravity Many chemical reactions involve work most commonly expansion work Also sometimes called pressure volume work or short PV work Expansion work refers to work that results in a volume change in the system This is especially important for reactions that involve gases either on the reactant or product side w P x change in V A system can exchange energy with its surroundings in two ways Transer of heat q Doing work w for example expansion work ENERGY AND ENTHALPY Enthalpy is a state function whose value depends only on the current state of the system not on the path taken to arrive at that state We cannot determine absolute values for H It is important to pay attention to states when dealing with enthalpy changes The enthalpy change will be different for the production of gaseous liquid or solid forms of the same product Change in H depends on many factors Molar quantities physical states temperature pressure Thermodynamic Standard State a standard for all factors so they can be measured in a comparable way
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