Heat and Enthalpy Changes

Heat and Enthalpy Changes - H, equals the heat, q p , added...

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Heat and Enthalpy Changes When a chemical reaction occurs in an open container most of the energy gained or lost is in the form of heat . Almost no work is done (i.e. nothing is being moved). Heat flows between the system and surroundings until the two are at the same temperature. When a chemical reaction occurs in which the system absorbs heat, the process is endothermic (it feels cold) When a chemical reaction occurs in which the system produces heat it is exothermic (it feels hot) Enthalpy Under conditions of constant pressure (e.g. most biological processes under constant atmospheric pressure) the heat absorbed or released is termed enthalpy (or "heat content"). We do not measure enthalpy directly , rather we are concerned about the heat added or lost by the system, which is the change in enthalpy (or H). In formal terms: The change in enthalpy,
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Unformatted text preview: H, equals the heat, q p , added to or lost by the system when the process occurs under constant pressure : H=q p H represents the difference between the enthalpy of the system at the beginning of the reaction compared to what it is at the end of the reaction: H = H final- H initial We are considering the enthalpic state of the system . Thus: if the system has higher enthalpy at the end of the reaction, then it absorbed heat from the surroundings ( endothermic reaction) if the system has a lower enthalpy at the end of the reaction, then it gave off heat during the reaction ( exothermic reaction) Therefore: For endothermic reactions H final > H initial and H is positive (+ H) For exothermic reactions H final < H initial and H is negative (-H)...
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