Supposethat we are interested in studying some physical or chemical change. The substance or mixture of substances in which this change occurs i called the system. The surroundings are everything in the vicinity of this system. For example, if we are interested in the reaction of barium hydroxide octahydrate with ammonium thiocyanate, this reaction mixture is our system, and the reaction vessel and everything outside it are the surroundings. Heat is defined as energy that flows into or out of a system, because of a difference in temperature between the system and its surroundings. As long as a system and its surroundings are in thermal contact (that is, not thermally insulated freom each other), energy-heat-flows between them to establish temperature equality or thermal equilibrium. Heat flows from a system at high temperature to one at low temperature; but once thermal equilibrium is established, heat flow stops.
Suppose, for example, that a system and its surroundings are both initially at 25 C. They are in thermal equilibirum and no heats flows. Imagine that we initiate a chemical reaction in the system so taht the temperature rises. Heat begins to flow from the system to the surroundings. It continues to flow until the reaction stops and the temperature of the system equals the temperature of the surroundings. If, on the other hand, the temperature of the system falls because of the reaction, heat flows from the surroundings to the system – the system absorbs heat. It will continue to absorb heat until the reaction stops and the temperature of system equals the temperature ef the surroundings
The total amount of heat that is evolved or absorbed by a system at a particular temperature (25 C in the previous example) because of chemical reaction is called the heat of reaction. A chemical reaction or physical change is exothermic when heat is evolved, and endotermic when heat is absorbed. Experimentally, we note that in the exothermic case the reaction flask warms; in the endothermic case, the reaction flask cools.
We will use the symbol q to denote heat. When heat is absorbed by a system, energy added to it; by convention, the number assigned to q is positive. Thus, for an endothermic reaction, the heat, q, is a positive quantity. On the other hand, when heat is evolved durin a reaction, energy is substract from the system. The number assigned to q for an exothermic reaction is negative. The sign conventions for q are summarized below.
Enthalpy
When one mole methane burns in oxsygen to give carbon dioxide and liquid water, the amount of heat that is released depends on how the experiment is carried out. If the reaction occurs in the atmosphere, where the pressure is constant (equal to that of atmosphere), the quantity of heat evolved at 1 atm and 25 C is 890 kj (q= -890kj). On the other hand, if this same reaction is done in a closed container, the heat evolved at 25 C is 883 kj (q = -883 kj). In this cesa, although the volume of the system is constant (equal to the volume of the container), the pressure changes during the reaction.
Most reactions are carried out in beakers or flasks that are open to the atmosphere. In these cases, the volume changes but the pressure is constant, since it must equal that of the surrounding atmosphere. Yhe heat of reaction at constant pressure,qp, is related to a property of the reactants and products called enthalpy (or heat content). Enthalpy (denoted H) is a property of a substance, like mass and volume. Every substance has a definite quantity of enthalpy, just as it definite mass and definite volume. The enthalpy of a substance depends on the amount of substance. Two moles of substance have twice as much enthalpy as one mole of substance. The quantity of enthalpy in a mol of substance depends only on those variables, such as temperature and pressure, that determine the state of the substance. Therefore, enthalpy is said to be a state function, or a state property.
Using the symbol D (meaning ‘’change in’’), we write the change in enthalpy as DH . The change in enthalpy for a reaction (called the enthalpy of reaction) is obtained by subtracting the enthalpy of the reactants from the enthalpy of the product:
DH = H (products) – H (reactants)
The heat of reaction at constant pressure equals the enthalpy of a reaction: qp = DH
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