Unit 6 Flashcards
Thermodynamics
study of
energy and its conversion from
one form to another
* Thermodynamics is useful because it
has so many practical applications,
including predicting the direction of
chemical reactions.
* Among other things, this knowledge
helps us learn how to capture and
store energy efficiently
- a branch
of thermodynamics, which highlights
how heat is involved in chemical and
physical transformations
There are two basic kinds of energy:
potential energy (Ep) and kinetic energy (Ek).
➢ The internal energy, E, of a system is the sum of all energy
(potential and kinetic) of everything in that system.
EK is the energy of motion = 1/2 mv2EP is stored energy
chemical bond energy
potential energy stored in molecular bonds
- Many of our cellular processes are
driven by energy released upon hydrolysing a
phosphate bond in an adenosine triphosphate
(ATP) molecule to create more stable
products: ADP and inorganic phosphate
energy of molecular motion
- translational, rotational and vibrational
- sometimes also called “thermal energy”
- temperature is a common measure of the energy of mol. motion
Energy and Change in Energy
Energy - the capacity to do work or supply heat
Change in energy = work plus heat
First law of thermodynamics
Energy cannot be created or destroyed (although as we will discuss,
energy can be converted from one form to another)
- Note that is the total energy of the universe that is conserved
System
The system is chosen to include whatever you are focusing on
(and things that cannot usefully be separated from that)
Surroundings
The surroundings is everything not in the system.
System + Surroundings = Universe
- If energy leaves the system, it must enter the surroundings (and visa
versa). Thus, we can determine the energy change of a system of
interest by measuring the energy change in the surroundings.
Change in energy
Any change in the energy of the system must be accompanied by
an equal magnitude change in the energy of the surroundings,
but the signs of these changes must be opposite.
∆𝐸𝑢𝑛𝑖𝑣𝑒𝑟𝑠𝑒 = ∆𝐸𝑠𝑦𝑠𝑡𝑒𝑚+∆𝐸𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔𝑠= 0
e.g., if a reaction occurs in a beaker of water and the temperature of
the water increases, we can deduce that the reaction produced heat.
Moreover, if we can determine how much heat flowed into the water,
we know how much heat the reaction produced
Barrier
For practical reasons, we often use a barrier to limit the surroundings
in thermochemical experiments. Its role is to stop (or at least
minimize) transfer of energy (heat and work).
Thus, we can determine the energy change in system of interest
by measuring energy change in limited, well-defined surroundings
Sign Conventions
In thermodynamics, all signs reflect the system’s perspective
State function:
A function or property whose value depends
only on the present state (condition) of the system.
Stated another way… the path does not matter
Energy
the capacity to do work or supply heat
heat
also called “thermal energy”) will flow from
higher-temperature objects to lower-temperature objects
Work
Mechanical work is the product of force (F) and
distance (d).
w = F × d
- The larger the required force, or the longer the distance an
object is moved, the more work is done on it
expansion work
Gas forming reaction in an insulated container that is attached to
a piston-cylinder assembly that pushes against something
- The system pushes the piston out,
doing work on the surroundings.
The system releases (loses) energy.
Pressure Volume
Expansion work is also often known as pressure volume (PV) work,
because the amount of work done depends on both P and V
* At constant pressure (P = Psurr),
𝑤 = −𝑃∆𝑉 = −𝑃 (𝑉𝑓𝑖𝑛𝑎𝑙 − 𝑉𝑖𝑛𝑖𝑡𝑖𝑎𝑙)
➢ More work is done when the volume change (DV) is larger
and/or when pushing against a higher external pressure
Expansion works = units
You will have many opportunities to calculate PV work in the
homework, problem solving sessions and tutorials.
* For this, it will be useful to realize that the units of work, heat
and energy must be the same. In SI, they are in Joules (or in kJ).
* Here is a handy conversion between Joules and our most
commonly used units of pressure and volume:
101.3 𝐽 = 1 atm.× 𝐿
Expansion work
𝑤 = −𝑃∆𝑉
-The negative sign is required to fit with the convention that signs
reflect the system’s perspective.
Consider a system that expands against a pressure
-P, which is the constant pressure of the surroundings, is always > 0.
* The system does work. It loses internal energy by doing this work.
Internal Energy (E)
is transferred as heat and work (w)
Enthalpy
is a useful scale for keeping track of the energetics of these
constant-pressure transformations, in the kitchen or on the benchtop
H=E+PV
Change in Enthalpy
is equal to the amount of heat absorbed or released in a transformation
(a real or imagined reaction) at constant pressure (see next slide). This means
enthalpy change (DH) is often straightforward to determine via experiment
Exothermic
Evolved heat flows out
of the system into the
surroundings. Heat is a
product of the reaction
Endothermic
Heat flows into the system
from the surroundings. Heat
is a reactant
