In order to grasp thermochemistry, you first have to know what energy is. In short, it's the capacity to do work or to transfer heat. Work is defined as force * distance.
W = Fd
Energy can be found in type forms: kinetic and potential. Potential energy is usually associated with attractions and repulsions. For example, the pull of gravity on a car 30 feet in the air is potential energy. On the other hand, kinetic energy is present when the car is on its way down.
No matter what, energy is always conserved. This is known as the Principle of the Conservation of Energy.
PEi + KEi = PEf + KEf
This equation shows that when the potential energy and the kinetic energy combined of an object before some action takes place is equal to the total potential and kinetic energy after the action.
Three main types of energy exist:
1) Heat - Thermal Energy; Thermal Energy <> Temp.; Thermal Energy
= change of Temp.
2) Radiant Energy - Radiation
3) Electrical Energy - Generated by Transfer of Electrons
Since the heading of this page is Thermochemistry, we'll mostly be dealing thermal energy, a.k.a. heat.
To most people in the food business, energy is measured as Calories. This is different from calories, because one Calorie = one thousand calories. So those of you out there who only use all lowercase letters or all CAPITAL letters better figure something out. Anyways, one thousand calories is equal to a kilocalorie (in other words, a thousand calories), so one Calorie is equal to a kilocalorie.
So how did the calorie come into play anyways, huh? Well, coincidentally enough, one calorie is the amount of energy required to raise 1.00 grams of pure H20 from 14.5 Co to 15.5 Co (1.0 Co).
Well, while we Americans were using the calorie, the rest of the world came up with its own unit of energy without even asking us!! As if it weren't bad enough that we weren't told about this, the conversion turned out to be very ugly, too. This other unit of energy is known as the joule. One calorie equals 4.184 joules. And unlike calories and Calories, one Joule is equal to one joule. One joule is equal to 1 newton (not the fig variety (ok, I know it's a lame joke, but i was bored, alright?)) * 1 meter.
Heat capacity is the amount of heat required to change the temperature of 1.0 g of an object by 1.0 Co. The heat capacity of a specific substance is known as specific heat capacity, a.k.a. specific heat (who would've thought of that?). Specific heat is symbolized by the letter c. Don't confused this with c as in the speed of light.
The amount of heat generated from an a change in temperature of an object is shown below:
Q = mc /\T
where Q = heat, m = mass, c = specific heat, and /\T = change in temp (delta T)
If /\T is positive, then the overall reaction is endothermic.
If /\T is negative, then the overall reaction is exothermic.
Molar heat capacity is (specific heat) * (molar mass of object).
There are two types of heat (no, not hot and cold!). Sensible heat is associated with changes in temperature. This is the type of heat that most of us can recognize; e.g. heating up water. Latent heat is associated with changes in phase (gas, solid, and liquid). When something changes phase, the temperature change is 0. When a solid changes phase into a liquid, that is known as Latent Heat of Fusion.
In a chemical process, the material that is being studied is called the system and the rest of the universe is called the surroundings. One thing to remember is that the system does work on the surroundings. The change in internal energy / system is symbolized by /\E. The equation to show the work done on the system is shown below:
/\E = q - w*
*This is known as the First Law of Thermodynamics.
If /\E < q, then the reaction is endothermic.
If /\E > q, then the reaction is exothermic.
Enthalpy is heat that is transferred in/out of a system at a constant pressure. The symbol for enthalpy is /\H.
If /\H is positive, then the reaction is endothermic.
If /\H is negative, then the reaction is exothermic.
Hess's Law states that if a chemical reaction is the sum of two or more reactions, then the overall enthalpy must be the sum of all the enthalpies of the reactions.
Standard conditions are used in order to allow experiments that are taken at different locations to come out with the same results. Standard pressure is 1 atmosphere or 1.0135 x 105 pascals. Standard temperature is 25o C. Standard state is the physical state at which an element or a compound exists at standard conditions.