C - Thermodynamics Flashcards
What is the First Law of Thermodynamics?
Energy cannot be created or destroyed.
This is known as the law of conservation of energy
What is specific heat capacity?
The gradient of a plot of energy against temperature at constant pressure
Cp = dH / dT
What is 1 calorie?
The energy change needed to increase the temperature of 1g of water by 1°C
1 calorie = 4.2 J
What is the equation for work done?
E = Fx
where F = force constant
and x = distance
What is the equation for gravitational potential energy?
PE = mgh
What is the equation for kinetic energy?
E = ½ mv²
What is the equation for the energy of an individual photon?
E = hν
What is a calorimeter?
An insulated device in which the temperature change associated with a specific chemical or physical change can be measured. It is normally calibrated electrically.
It has a stirrer, a thermometer, and a lid to keep constant pressure
eg. school calorimeter, ‘bomb’ calorimeter
The latter has the material in excess oxygen, and when given a spark it combusts very quickly
Heat evolved: qp = n x Cp x ΔT = ΔH
What is the equation for power dispersed?
P = dE / dt
P measured in Watts = J/s
What is Kleiber’s law?
Power output, P ∝ M3/4
What do the terms universe, surroundings and adiabatic mean?
Universe = both system and surroundings
Surroundings = everything around the system that has physical contact with it; may or may not exchange energy and matter
Adiabatic = Energy is transformed only in the form of work
What do the terms system, and this being open, closed or isolated, mean?
System = the chemical reaction or mechanism under observation
Open = system can exchange energy and matter with the surroundings
Closed = system can exchange energy but not matter with the surroundings
Isolated = no exchange can occur
Describe the Maxwell-Boltzmann distribution.
It is the distribution of energy of atoms in a gas. It is weighted towards lower speeds, and tends towards zero.
As temperature increases, the curve flattens and moves towards the right, given that with increasing temperature the probability of finding particles at higher energy increases.
The highest point on the curve, the stationary point, is the modal energy of the atoms.
How can the distribution within energy levels be described?
There is an exponential relationship.
As temperature increases, the higher energy states become more populated.
What is the equation for predicting the occupation of a particular energy level (Maxwell-Boltzmann distribution)?
(population in state En / population in state Eo ) = e-(En - Eo)/kT
if considering individual molecules, k = Boltzmann constant, 1.38 x 10^-23 J/K
if considering moles, k = R (gas constant) = 8.314 J
What is the Zeroth Law of Thermodynamics?
Energy flows between objects so as to equalise their temperatures.
Temperature is therefore to a liquid as pressure is to a gas.
Alternatively, if two systems A and B are individually in equilibrium with a third system, C, then A and B are also in equilibrium with each other.
What is the difference between intensive properties and extensive properties?
Intensive properties do not depend on the amount of material, whereas extensive properties do
eg. temperature = intensive, volume = extensive
What is temperature?
Temperature determines the distribution of thermal energy in a system.
Higher temperatures imply faster molecules or more populated higher energy levels.
What do the terms U, q and w mean?
U = internal energy ie the sum total of all possible energies in the system
q = heat transferred to the system
w = work done on the system
What is a state function? Give an example of one.
A state function is path independent; it does not matter how the change has come about, but the value of the change does matter.
U is a state function as they are denoted with capital letters.
What is heat?
The transfer of energy due to a difference in temperature between the system and the surroundings.
This results in a change in the random motion at the molecular level in the system.
What is work?
The transfer of energy to or from the system that results in a coordinated motion of molecules of a system.
What does it mean if q > 0 or q < 0 ?
q > 0 = energy is moving from the surroundings to the system = endothermic
q < 0 = energy moving from the systems to the surroundings = exothermic
What does it mean if w > 0 or w < 0 ?
w > 0 = surroundings do work on the system = system gains energy
w < 0 = system does work on the surroundings = system loses energy
What happens to ΔU, q and w if pressure says constant?
At constant pressure q = qp
w = -pΔV
qp = ΔU + pΔV = ΔH
What happens to ΔU, q and w if both pressure and volume stay constant?
ΔU ≈ ΔH
What is enthalpy?
Enthalpy is the thermal energy transferred in a process at constant pressure.
It is a state function and extensive.
What does it mean when ΔH > 0 or ΔH < 0 ?
ΔH > 0 = energy is moving from the surroundings to the system = endothermic
ΔH < 0 = energy is moving from the system to the surroundings = exothermic
What is Kirchoff’s Law? What does it allow us to do? What is its limitation?
ΔHT2 = ΔHT1 + CpT2 − T1
This allows us to correct enthalpies under standard conditions to physiological temperatures.
It can only be applied to small temperature changes,
What is isothermal calorimetry and what is it used for?
Two chambers are linked by feedback mechanisms.
A constant power is applied to the sample cell in order to keep its temperature the same as the standard cell.
Acid or salt is injected into the sample cell to unfold the protein. A ligand could be added instead to measure binding enthalpy.
The added substance causes a change in temperature; power decreases if sample heats up, power increases if sample cools down.
Power input is then measured over time
What is the Second Law of Thermodynamics?
The entropy of any isolated system not in thermal equilibrium almost always increases
What is entropy the ratio of?
The ratio of heat transferred to temperature:
ΔS = q/T
What does entropy establish?
Entropy establishes the direction of spontaneous change, but it does not tell its speed
What is the equation for maximum efficiency of work?
wmax = qtransferred ( 1 - [Tcold / Thot] )
What does entropy measure?
Entropy is the measure of the “quality of energy”, and how narrowly it is distributed. In other words, it is the measure of disorder.
What is the equation for entropy in terms of microstates?
S = kB ln W
where kB = constant of proportionality (Boltzmann constant)
W = microstates
What is the order of entropy for solids, liquids and gases?
Gases → liquids → solids
Most entropy → least entropy
What is the equation for the mole fraction, x?
xA = moles of A / total moles of everything in sample
What is the equation for molality?
molality = moles of solute (moles) / mass of solvent (kg)
What does isothermal mean?
Isothermal is a system that doesn’t rely on temperature differences to do work
What is the equation for ΔG under constant pressure?
ΔG = ΔH - T ΔS
only under constant temperature and pressure
What happens when ΔG > 0 , ΔG < 0 and ΔG = 0 ?
ΔG > 0 → endergonic
ΔG < 0 → exergonic
ΔG = 0 → equilbrium
What is the chemical potential, µ?
µ measures the driving force for a process.
It is a measure of how G changes per molecule.
What is the equation for ΔG°?
ΔG° = - RT ln Keq
What is Gibbs free energy?
The energy from a change in the system that will be available to do non-pV work
Will the reaction take place if both ΔH and ΔS are positive?
The reaction will not be spontaneous at low temperatures.
Above a critical temperature, it will switch over to spontaneous behaviour.
Will the reaction take place if ΔH is negative and ΔS is positive?
The reaction will be spontaneous at all temperatures.
Will the reaction take place if ΔH is positive and ΔS is negative?
The reverse reaction will be spontaneous at all temperatures.
Will the reaction take place if both ΔH and ΔS are negative?
The reaction will be spontaneous at low temperature.
Above a critical temperature, the reverse reaction will be favoured.
What are osmosis and diffusion driven by?
Chemical potential
What are mixing and unmixing driven by?
Entropy
What causes protein folding and membrane formation?
They are driven by entropy.
Clustering into micelles or folded proteins reduces the contact of non-polar molecules with the solvent.
When proteins form membranes they release water, thereby increasing the entropy.
What is a spontaneous process?
A process that occurs in a system without any input of energy from the surroundings.
It is favoured by a decrease in H and an increase in S.
What is the relationship between kB (Boltzmann constant) and R (gas constant)?
In the equation for the Maxwell-Boltzmann distribution, using kB gives the answer in J/molecule, where R gives the answer in J/mol.
This is because kB = R / Na
where Na = Avogadro’s number
What is the enthalpy of combustion?
The enthalpy change when one mole of a compound burns in oxygen to produce H2O and CO2 under standard conditions.
What is the enthalpy of formation?
The enthalpy change when one mole of a compound is formed from its constituent elements, with all substances in their standard states at standard conditions.
How can the enthalpy of combustion be determined?
Calorimetry
What is the Third Law of Thermodynamics?
For any perfect crystal at 0K, the entropy is zero.
What is specific heat?
The energy needed to increase the temperature of 1g of a substance by 1°C
C = q / ΔT
Does an increase in gravitational potential energy cause an increase in U?
Yes because it is positive work (+w)
What is the formula for expansion work?
w = -p x ΔV
What is the thermodynamic standard state?
The most stable state of a substance at 1 atm, 25°C and 1M of all substances in solution
What is the standard enthalpy of reaction?
The enthalpy change for a reaction measured under standard conditions
What is the molar heat capacity of water?
4.186
What is Raoult’s Law?
The partial vapour pressure of a component in a mixture is equal to the vapour pressure of the pure component at that temperature multiplied by its mole fraction in the mixture.
The freezing and boiling points of an ideal solution are respectively depressed and elevated relative to that of the pure solvent by an amount proportional to the mole fraction of solute.
BASF’s Insulator Micronal PCM is a polystyrene foam filled with specialised wax droplets.
How does it work to heat buildings?
During the heat of the day (≈23°C), the wax melts (endothermic) and so stores potential chemical energy, which is then released at cooler times as thermal energy via the wax solidifying (exothermic).
This keeps the temperature of the building constant; the wax absorbs energy when heat is excessive and releases energy when the building cools below comfortable temperatures.
