Thermochemistry is a branch of thermodynamics that involves the study of energy transformation in chemical reactions.
Mouse over energy, heat, and work to see their definitions.
Calorimeter measures
energy of reactions
Consider the following chemical reaction, which we can assume is a
A closed system may exchange energy (but not matter) with its surroundings.
Click on the flask to mix two solutions that react to form a red solid.
Was energy transferred from the system to the surroundings or the surroundings to the system?
In this case, the system lost energy and that energy was gained by the surroundings as heat. We can see this because of the temperature increase of the surroundings.
Click on the flask to view a molecular level view of the energy transfer. In this animation, the container wall is simplified to a line.
Energy produced by the reaction is transferred to the molecules in the solution as kinetic energy. This raises the temperature of the solution.
The molecules in the solution collide with the walls of the container and transfer some of their energy. As this process occurs, the temperature of the solution drops as the molecules lose energy to the container.
The molecules of air surrounding the container collide with the container and gain energy as kinetic energy. These molecules dissipate their increased kinetic energy farther out into the surroundings and eventually everything settles to a new, slightly higher temperature.
Energy
In order to study energy exchange in a chemical process, we must define where the energy change is occurring. Click on the words system and surroundings in the image below for definitions of each of these.
In general in chemistry, the chemical reaction is the system.
System: The part of the universe whose change will be observed.
Surroundings: Often considered to be the rest of the universe outside of the system.
A system can interact with its surroundings in different ways.
If both matter and energy can be exchanged between system and surroundings, this is an OPEN SYSTEM.
If only energy—and not matter—can flow between system and surroundings, this is a CLOSED SYSTEM.
An ISOLATED SYSTEM cannot exchange matter OR energy with its surroundings.
The sum of the potential and kinetic energy of all the particles in a system is called the internal energy (E). When an exchange of energy between a system and its surroundings occurs, there is a change in the internal energy of each.
Energy lost to surroundings
When the initial state of the system is
higher in energy than the final state of the system, there was energy lost to the surroundings and the change in the internal
energy of the system is negative (< 0).
Energy gained from surroundings
When the initial state of the system is lower
in energy than the final state of the system,
there was energy gained from the
surroundings and the change in the internal
energy of the system is positive (> 0).
A change in the internal energy of the system, either positive or negative, is always opposite in sign to the change in the internal energy of the surroundings.
Heat is the thermal energy transferred as a result of a difference in temperature between a system and its surroundings.
Work is the energy associated with moving an object through a given distance.
Mouse over heat, q and work, w to see the definitions.
If the system absorbs energy through heat, the sign of q is defined to be positive, and we call that process ENDOTHERMIC.
If the system loses energy through heat, the sign of q is defined to be negative, and we call that process EXOTHERMIC.
Note that endo- and exothermic refer only to HEAT not WORK.
When work is done ON a system, it increases the energy content of the system, and the sign of work is defined to be positive.
When work is done BY a system, it decreases the energy content of the system, and the sign of work is defined to be negative.
Notice that the sign used for heat and work is always with respect to the system. If the system is gaining energy, the sign will be positive. If the system is losing energy, the sign will be negative.
Check your understanding. If a system loses heat AND does work on its surroundings, what are the signs of heat and work?
HEAT WORK
Since the system is losing energy from heat and losing it by doing work, we know the overall ΔE must be negative without having to know the exact values of q and w.
Consider the transition between water and ice. The work involved in this process is negligible, and the system exchanges energy as heat only. Hence, w ≈ 0 and ΔE ≈ q.
Ice melts because heat energy is being
absorbed by the system (glass of water)
from the surroundings. The change in
the internal energy of this system is
positive (>0). Since heat was gained by
the system, q is positive (>0) and the
process is said to be endothermic.
Water freezes because heat energy is lost
by the system to the surroundings and the
change in the internal energy of the system
is negative (<0). Since heat (q) energy was lost by the system, q is also negative (<0)
and the process is said to be exothermic.
Let's look at a system where work is the only means of energy transfer. Here, the reaction is being conducted under conditions where no heat is lost. Hence, q ≈ 0 and ΔE ≈ w.
If the system is the expanding gas, how does the energy of the system change?
Consider the following situation in which the system is underlined. FOR THE SYSTEM, identify the primary means of energy transfer and its direction.
Question 1:
Select one button from each group.
Consider the following situation in which the system is underlined. FOR THE SYSTEM, identify the primary means of energy transfer and its direction.
Question 2:
Select one button from each group.
Consider the following situation in which the system is underlined. FOR THE SYSTEM, identify the primary means of energy transfer and its direction.
Question 3:
Select one button from each group.
Question 4:
Question 5:
If a system loses heat and has work done on it by the surroundings, what is the sign of ΔE?
c. Cannot determine the sign without knowing the magnitudes of q and w.
Question 6:
Congratulations! You completed the Internal Energy ChemTour!
In this tour, you learned:
• The first law of thermodynamics: energy is conserved. ΔEsys + ΔEsurr = 0.
• Energy may be transferred between a system and its surroundings as heat, work, or both.
• The sign of the energy transferred is negative if it leaves the system and positive if it enters
the system.
• How to calculate the net change in internal energy of a system when energy transfer
occurs, using another statement of the first law of thermodynamics ΔE = q + w.
Since energy transfer is due to an object being moved against a force (force of gravity), the transfer process is WORK. The man uses some of his energy to lift the box. Since the man is defined as the system in this problem, work is negative. Energy is transferred from system to surroundings.
Since energy transfer is due to an object being moved against a force (force of gravity), the transfer process is WORK. The box increases in potential energy with the act of being lifted. The man (surroundings) transferred some of his energy to the box. Since the box is defined as the system in this problem, work is positive. Energy is transferred from surroundings to system.
The appropriate equation is ΔE = q + w.
Substituting these values into the equation: ΔE = q + w.
Systems can change their internal energy via heat transfer or work according to the following equation:
ΔE = q + w
If the system loses heat, the sign of q is negative. If the system has work done on it by the surroundings, the sign of w is positive.
With one component, w, INCREASING the energy of the system and the other component, q, DECREASING the energy of the system (q), we must know both signs on q and w and their magnitudes in order to determine whether the internal energy of the system will undergo a net INCREASE or DECREASE.
−
+
Step 1: Define the system and the surroundings.
System = gasoline reaction and products
Surroundings = pistons, rest of car
Step 2: Insert the proper values for each variable into the internal energy equation to determine the change in internal energy. Make sure to use the proper signs and units for heat (q) and work (
w).
Since q and w do not have the same units, we must convert units to make them consistent.