This relationship can help to determine whether a reaction is endothermic or exothermic. At constant pressure, an endothermic reaction is when heat is absorbed. On the other hand, an exothermic reaction at constant pressure is when heat is released. When the temperature increases, the amount of molecular interactions also increases. When the number of interactions increase, then the internal energy of the system rises.
We can use the equation for heat capacity and Equation 2 to derive this relationship. This is the enthalpy of a substance at standard state. The standard state is defined as the pure substance held constant at 1 bar of pressure. Phase transitions , such as ice to liquid water, require or absorb a particular amount of standard enthalpy:. The enthalpy of condensation is the reverse of the enthalpy of vaporization and the enthalpy of freezing is the reverse of the enthalpy of fusion.
The enthalpy change of a reverse phase transition is the negative of the enthalpy change of the forward phase transition. Also the enthalpy change of a complete process is the sum of the enthalpy changes for each of the phase transitions incorporated in the process. Enthalpy Change Accompanying a Change in State When a liquid vaporizes the liquid must absorb heat from its surroundings to replace the energy taken by the vaporizing molecules in order for the temperature to remain constant.
Effect of Temperature on Enthalpy When the temperature increases, the amount of molecular interactions also increases. Outside Links Canagaratna, Sebastian G. Internal energy specifies the total kinetic and potential energy of molecules in the system. Internal energy of a system can be changed either by doing work on the system, or heating it. However, the change in internal energy is not equal to the energy that transfers as heat when the system is capable of changing its volume.
Enthalpy is a thermodynamic property and we can denote it by H. The mathematical relationship for this term is as follows:. Here, H is enthalpy and U is the internal energy, P is the pressure and V is the volume of the system. This equation shows that the energy supplied as heat at a constant pressure is equal to the change in enthalpy.
The term pV accounts for the energy required by the system to change volume against the constant pressure. Therefore, enthalpy is basically the heat of a reaction at constant pressure.
If this value is negative, then the reaction is exothermic. If the value is positive, then the reaction is said to be endothermic. The change in enthalpy between any pair of reactants and products is independent of the path between them. Moreover, enthalpy change depends on the phase of the reactants. For example, when oxygen and hydrogen gases react to produce water vapor, the enthalpy change is But, when the same reactants react to produce liquid water, the enthalpy change is The capacity of a system to do work is the energy of that system.
We can do work on the system or the system can do work, which leads to increase or decrease the energy of the system accordingly. Energy of a system can be changed, not only by the work itself, by other means too. When the energy of a system changes as a result of temperature difference between the system and its surroundings, we refer to that energy transferred as heat q ; that is, energy has been transferred as heat.
Heat transfer takes place from high temperature to low temperature, which is according to a temperature gradient.
0コメント