October 6, 2011
CH-111
Chapter 5, Section 2
Thermochemistry
The first law of thermondynamics states that energy is conserved. My favorite way of relating this idea is to say that energy is neither created nor destroyed. If energy exists, it may change forms and be transferred back and forth from system to surroundings, but it never ceases to exist. Any energy lost by a system must be gained by surroundings, and vice versa.
In order to analyze energy changes in a chemical system, we must consider all the sources of kinetic and potential energy in the system we are studying.
Internal Energy: internal energy of a system (E) is the sum of all the kinetic and potential energies of the components of the system.
In thermodynamics, we are generally concerned only with the change in E, which can be determined by subtracting Einitial from Efinal. This is usually denoted by a delta symbol, (delta being the generally accepted symbol to denote change) and E: ΔE.
Generally, we cannot determine numerical values for Einitial and Efinal for any system of practical interest. We need only the value of ΔE to apply the law, which can often be determined without numerical values for Einitial and Efinal.
Thermodynamic quantities, such as ΔE, have three parts:
1. a number
2. a unit
3. a sign that gives direction
1 and 2 together give the magnitude of the change.
- A positive value of ΔE results when Efinal > Einitial, indicating that the system has gained energy from its surroundings.
- A negative value of ΔE results when Efinal<Einitial, indicating that the system has lost energy to its surroundings.
We take the point of view of the system rather than than the surroundings in discussing energy changes. It is important to remember that any increase in the temperature of the system results in a decrease of the temperture in its surroundings and vice versa. In a chemical reaction, the initial state of the system refers to the reactants and the final state refers to the products.
2H2(g) + O2(g) ---------> 2H2O(l) In this reaction, 2H2(g) + O2(g) is the initial state and 2H2O is the final
When hydrogen and oxygen form water at a given temperature, the system loses energy to the surroundings. Because energy is lost from the system, the internal energy of the products (final state) is less than that of the reactants (initial state), making ΔE for the process negative.
The internal energy of a system changes in magnitude as heat is added to or removed from the system or as work is done on or by the system. When a system undergoes any chemical or physical change, the accompanying change in internal energy, ΔE, is the sum of the heat added to or liberated from the system, q, and the work done on or by the system, w:
ΔE=q+w
When heat is added to a system or work is done on a system, its internal energy increases.
When heat is transferred to the system from surroundings, q is +
When work is done on the system by the surroundings, w is +
When heat is lost by system to surroundings, q is -
When work is done on the surroundings by the system, w is -
The following are the sign conventions for q, w, and ΔE:
For q: + means gain of heat, - means loss of heat
For w: + means gain of heat, - means loss of heat
For ΔE: + means neet gain of energy by system, - means net loss of energy by system
A process occurring in which a system absorbs heat is called endothermic. During endothermic processes, systems gain heat.
A process occurring in which a system loses heat is called exothermic. During exothermic processes, systems lose heat.
Examples of Endothermic and Exothermic Processes:
Endothermic: (heat flowing in), ice melting
Exothermic: (heat flowing out), gasoline combustion
Although we have no way of knowing the exact value of the internal energy of E, there is a fixed value for a given set of conditions. Conditions influencing E include temperature and pressue. Also, the internal energy of a system is proportional to the total quantity of matter in the system because energy is an extensive property.
(Recap: extensive property: a property that depends on the amount of material considered, ie: mass or volume. Extensive properties are dependent upon the amount of substance! Therefore, in this case, energy being extensive means that the amount of energy contained in a substance varies with the size of the sample of substance we are working with.)
State function: a property of a system that is determined by specifying the system's condition or state. The value of a state function depends only on the present state of the system, not on the path the system took to reach that state.
E is a state function, and therefore ΔE depends only on Einitial and Efinal and not how the change occurs.
Book example:
"Suppose you drive from Chicago, which is 596ft above sea level, to Denver, which is 5280ft above sea level. No matter which route you take, the altitude change is 4684ft. The distance you travel, however, depends on your route. Altitude is analogous to state function because the change in altitude is independent of path taken."
Here's some fun Google images of exothermic reactions!
Although ΔE and the equation ΔE=q+w are both state functions, the values for q and w are not. That means they are in fact dependent on the process in which they occurred. The specific amounts of heat (q) and work (w) produced depend on the way in which the change occurs. Thus, if changing the path by which a system goes from initial state to final state increases the value of q, the change will simulatneously decrease the value of w by the exact same amount. Our final result, ΔE, is the same for the two paths.