1. Enthalpy, entropy and free energy Flashcards
What are the two ways in which energy is released from stores in molecules?
heat or work
What is internal energy?
It is the total energy of all particles (atoms and molecules) in a sample and is the letter U.
Equal to the sum of potential and kinetic energies of the particles of the system
What is the difference between potential and kinetic energy?
PE is energy that is stored or ready to be released
KE moving objects have energy of motion
How can energy be defined?
As the capacity to do work or supply heat
What is enthalpy? H
Function related to the heat absorbed or evolved by a chemical system
What is entropy? S
A measure of the number of ways energy is distributed throughout a chemical system
What is Gibbs energy? G
The energy that is available to do work
G= H - TS
What is the first law of thermodynamics?
delta U = q + w
Where q is the transfer of heat and w is the work done on a system or the system doing work
For the first law of thermodynamics, if heat enters/exits the system then what is the sign of q?
If heat enters the system then q is positive and if heat exits the system then q is negative
For the first law of thermodynamics, if work enters/exits the system then what is the sign of w?
If work is into the system then w s positive and if work is out of the system then w is negativ
What is the SI unit of energy?
1 joule = 1 kg m2 s-2
What happens in a closed system bs and isolated system for the first law of thermodynamics?
W=0 in a closed system and q=0 in an isolated system
What is the equation for work?
W = -P x delta V
What happens to W if v increases or decreases?
If V increases then the system is doing work (w is negative)
If V decreases then work is being done on system (w is negative)
What can enthalpy be defined as?
H = U + PV
What happens to enthalpy at constant pressure?
At constant pressure (where only PV work is allowed), change in enthalpy (delta H) equals the energy flow as heat
What happens to enthalpy at constant volume?
At constant volume no work is done (p delta V = 0)
What is the difference between heat and temperature?
Heat is not temperature
During a phase change such as evaporation, the substance undergoes a change in its enthalpy without experiencing a change in temperature
Heat is the actual energy measured in J
Heat is a measure of some of the energy in a substance
Explain the enthalpy of the physical changes of water
When liquid water is cooled, its temp falls steadily until it drops just below the freezing point at 0 degrees. The temp then remains constant at the freezing point while the water crystallises. Once the water is completely frozen its temperature continues to fall
What is the standard state of a substance?
Standard state of a substance is in its pure form at 1 atm pressure, where temp is usually 25 degrees
What is a standard state reaction?
A standard state reaction enthalpy, delta H, is measured when reactants in their standard states change to products in their standard states
What is delta H of formation if the element is in ts most stable form?
Zero
What is entropy?
Measure of the randomness or disorder of a system, the larger the entropy the greater the disorder
Which is more disordered out of a glass of ice chips or a glass of water?
The water is made of little molecules while the ice cubes are made of chunks of molecules, within which molecules are fixed in a lattice.
What is the formula to work out delta S
delta S = s(products) - s(reactants)
Units of delta s are J mol-1 K-1
If there are fewer gas molecules in products than reactants, then delta S is…?
Negative
If gas molecules appear in the products then delta S is…?
Positive
Why do gas molecules have higher entropy?
Gas molecules have more ways available to distribute their energy
What is the second law of thermodynamics?
The entropy of the universe increases in the course of a spontaneous change.
What is the formula to work out the entropy for a particular macro state?
S = k ln(W)
Where k is Boltzmann constant and W is the number of microstates
How does volume affect entropy?
For gases the entropy increases with increasing volume
How does temperature affect entropy?
The higher the temperature the higher the entropy
How does physical state affect entropy?
FRom solid to liquid to gas, entropy increases
How do number of particles affect entropy?
When all other things are equal, reactions that increase the number of particles in the system tend to have a positive entropy change
What is the third law of thermodynamics?
The entropy (S) of a perfect crystalline substance is zero at the absolute zero of temperature
What is the standard entropy change for a reaction given by?
Delta S= sum (s of products) - sum(s or reactants)
What is the equation for the spontaneity of the universe?
Delta(s univ) = delta(s system) + delta(s surr) > zero
Is a process spontaneous if delta Ssystem, delta Ssurr and delta Suniv are positve?
Yes it is a spontaneous process
If d(s sys), d(s surr) and d(s univ) are negative is the reaction spontaneous?
No, the process will occur in the opposite direction
If d(s sys) is + and d(s univ) is - then is the process spontaneous?
Yes, if d(s sys) is greater than d(surr)
If d(s sys) is - and d(s surr) is +, is the process spontaneous?
Yes, if d(s surr) has a larger magnitude than d(s sys)
Is d(s univ) is equal to zero, what does this tell you about the reaction?
It is at equilibrium
What happens if d(s) + and d(h) -?
Spontaneous at all temperatures
What happens if d(s) + and d(H) +?
Spontaneous at high temperatures (where exothermic is relatively unimportant)
What happens if d(s) - and d(H) -?
Spontaneous at low temperatures (where exothermicity is dominant)
What happens if d(s) - and d(h) is +
Process not spontaneous at any temperatures (reverse process is spontaneous at all temperatures)
How do you work out d(s Surr) for change of entropy in surroundings?
D(s surr) = - d(H)/T
What is another easier way to determine spontaneous reactions?
-Td(s univ) = -d(H sys) + Td(s sys) is less than 0
For the Gibbs free energy equation, what must does a negative d(G) indicate?
Forward reaction is spontaneous
What does a positve delta G indicate
Non spontaneous forward reaction or a spontaneous reverse reaction
What does delta G = 0 indicate?
Equilibrium
What is the equation for Gibbs free energy?
G = H - TS
What is the equation for delta G at constant temperature?
D(g) = d(H) - Td(S)
