October 6, 2011
CH-111
Chapter 5, Section 1
Thermochemistry
The study of energy and its transformations is known as thermodynamics. The portion of thermodynamics concerned with chemical reactions and energy changes that involve heat is thermochemisty.
Energy is defined as the capacity to do work or transfer heat. Work is the energy used to cause an object to move against a force. Heat is the energy used to cause the temperature of an object to increase.
Energy: the capacity to do work or transfer heat.
Work: the energy used to cause an object to move against a force.
Heat: the energy used to cause the temperature of an object to increase.
Kinetic energy is the energy of motion. The magnitude of kinetic energy is expressed as Ek. This magnitude is dependent on an object's mass and speed, m and v, respectively. The equation to calculate magnitude of an object's kinetic energy is Ek=1/2mv^2.
By this equation, we see that the kinetic energy of an object increases as its speed increases. Energy and speed are relative. All other kinds of energy are potential energy. An object has potential energy by virtue of its position relative to other objects.
Potential energy is essentially the stored energy that arises from attractions and repulsions an object experiences in relation to other objects.
Potential energy may be converted into kinetic energy. As potential energy decreases, kinetic energy must increase (as potential is converted into kinetic). This is a cornerstone in the foundation of thermodynamics.
One of the most important forms of potential energy in chemistry is electrostatic potential energy, or Eel (how I will refer to it here for lack of available scientific symbols). Electrostatic potential energy, Eel, arises from the interactions between charged particles. The energy is proportional to the electrical charges on the two interacting objects Q1 and Q2 and inversely proportional to the distance, d, that they are separated by. Thus, we derive the equation:
Eel = kQ1Q2/d ---> constant of Q1Q2 divided by distance
*k simply being a constant of proportionality, 8.99X10^9J-m/C^2
(C = Coulomb, the SI unit for electrical charge)
(J = joule, a unit of energy)
At a molecular level, the charges Q1 and Q2 are typically the magnitude of a charge of an electron:
electron charge = 1.60X10^-19C
Electrostatic potential energy goes to zero as d becomes infinite. Electrostatic potential energy is defined as infinite separation of the charge particles.
In other words, if particles are too far away from each other to attract of repell eachother (distance increasing), there is no potential energy created. The further apart two charged particles, the less electrostatic potential energy. The following graph further explains charged particles and electrostatic potential energy:
(- - denotes distance)
Eel > 0 Like charges (repulse): Q1 <- - -> Q2 > Q1 <- - - - - - - - - - - - - - -> Q2
Eel = 0 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -> infinity
Eel < 0 Opposite charges (attract): Q1 <- - -> Q2 > Q1< - - - - - - - - - - - - - - -> Q2
When Q1 and Q2 have the same sign, the two charged particles will repel eachother and the repulsive force will push them apart. In this case, Eel is positive and the potential energy decreases (becomes more negative) as the particles move farther and farther apart. When the two particles (Q1 & Q2) have opposite signs, the particles attract and move toward eachother. In this scenario, the Eel is negative and the potential energy increases (becomes less negative) as the particles move apart.
Q1- <------decreasing potential energy------> Q2-
Q1+ <----------increasing potential energy--------->Q2-
Many substances release chemical energy when they react, energy due to the potential energy stored in the arrangements of their atoms. The energy a substance possesses because of its temperature (its thermal energy) is associated with the kinetic energy of the molecules in the substance.
The SI Unit for energy is the joule, abbreviated J.
1J =1kg-m^2/s^2
EX: mass of 2kg moving at a speed of 1m/s possesses a kinetic energy of 1J:
Ek = 1/2mv^2 = 1/2(2kg)(1m/s)^2 (mass is 2kg, speed is 1m/s)
1/2(2kg)(1m/s)^2 = 1kg-m^2/s^2 (1/2 multiplied by 2kg = 1kg, (1m/s)^2 = 1m^2/s^2)
1kg-m^2/s^2 = 1J
A joule is not a particularly large amount of energy, therefore energy is typically described in kJ, kilojoules. The traditional expression of energy in calories is not a standard SI Unit, but is still widely used in chemistry and other sciences (other sciences not named because NO science is as important as chemistry ;).
Calorie: the amount of energy required to raise the temperature of 1g of water from 14.5 celsius to 15.5 celsius (1 degree). This is the traditional definiton of calorie, however, we now define the calorie in terms of the joule:
1 calorie = 4.184 J (EXACTLY)
On a fun side note: the calorie we all know and love (well, some of us love them, some of us do battle with them on a daily basis), perhaps better said, the calorie we are all familiar with in terms of nutrition is the Calorie. Note the capital C. A Calorie is different than a calorie. 1 Cal = 1000 cal = 1 kcal, and therefore:
So if 1 Calorie is equal to 1,000 calories and 4.184 joules is equal to 1 calorie, your 2,000 Calorie-a-day diet consists of 8,368 joules. If I had that many joules I'd be rich!! (Silent Audience) .......But since the joule represents such a small amount of actual electricity, let's convert it to kJ, you know, for fun:
8,368J(1kJ/1000J)= 8.368kJ
Wasn't that fun??
....
Okay, without further digression.
When analyzing energy changes, we need to focus on a limited and well-defined part of the universe to keep track of the energy changes that occur. The portion we single out for study is called the system, and everything else is called the surroundings.
In the chemical reactions we study, the reactants and products consitute our system. The container and everything beyond is considered surrounding. Systems may be open, closed, or isolated.
An open system is one in which matter and energy can be exchanged with the surroundings.
A closed system is a system that can exchange energy but not matter with its surroundings.
In thermochemistry, the preferred system is one that is closed. A cylinder fitted with a piston is the surrounding for which a chemical reaction can take place and produce energy, which is transferred from the system to the surrounding in the form of work and heat. Matter, however, is not transferred. Thus a cylinder fitted with a piston is a closed system.
An isolated system is one in which neither matter nor energy can escape.
A force is any push or pull exerted on an object. We define work (w), as the energy transferred when a force moves an object. The magnitude of this work is equal to the product of the force (F) and the distance (d):
w=Fd
Heat is the energy transferred from a hotter object to a colder one. A combustion reaction, for example, releases the chemical energy stored inside the molecules that are being reacted. When fuel is burned (as in a combustion reaction) for energy, what is really happening is that chemical energy is being liberated from the molecules that the fuel consists of. This liberated energy causes an increase in system temperature, which is transferred in the form of heat to the cooler surroundings.