Quiz 3: Chapter 4 Flashcards
What does Clausius say?
The entropy of an isolated system always increases in the course of a spontaneous process
What is the Clausius equation?
What is ΔGsys? What type is it? What type of equation is it? What is it best thought of as?
- ∆𝐺_sys is the change in Gibbs free energy.
- Gibbs free energy is not a “normal energy” because it is not conserved (if a spontaneous process occurs in the universe, the Gibbs free energy of the universe becomes more negative).
- The Gibbs equation is an “all-entropy” equation.
- ∆𝐺_sys is best thought of as the “dispersible amount of energy” or the “maximum non-pV work”.
How do you calculate the free energy change during the expansion/compression of a gas?
What is Q?
the reaction quotient
What is K?
the equilibrium quotient
Describe Q and K at equilibrium
At equilibrium ….
the partial pressures of the quotient Q correspond to the equilibrium partial pressures.
Therefore, Q = K and the Gibbs free energy is zero per definition ∆𝐺=0.
What is the standard gibbs free energy equation? What does this value represent?
Explain what the ratio of Q and K means for gibbs free energy.
What is one way Gibbs free energy can be interpreted?
Give an example of a partial molar quantity.
chemical potential
What is the best way to rationalize a partial molar property?
“the partial molar volume”
What is partial molar volume?
What does the partial molar volume depend on?
depends on how molecules pack in the liquid
Partial molar volume is ___________
- a derivative
- The partial molar volume of a substance A is the slope of the plot of the total volume as the amount of A is changed.
- The partial molar volume is not necessarily positive (addition of MgSO4 into water leads to a slight reduction of the total volume).
How do volumes of mixtures change? How can this change be calculated?
- the total volume of any mixture is the partial molar volumes at the composition of the mixture multiplied by the amounts.
What is chemical potential? What is the equation? What is the graph?
- The chemical potential of a substance A in a mixture is defined as the partial molar Gibbs energy
- The chemical potential is the contribution of that substance to the total Gibbs free energy
What is the fundamental equation of chemical thermodynamics? What happens to this equation under constant temperature and pressure?
- First two terms are dropped under constant T and P
What is 𝜉?
extent of reaction
What are the two interpretations of ∆𝑟𝐺?
- As a derivative and a difference
The free energy change for the reversible and isothermal expansion/compression of an ideal gas is:
Entirely entropic
What is the chemical potential of gas A in a mixture of ideal gases?
How are chemical potential and molar gibbs free energy related?
What is the origin of the minimum on this graph?
The Gibbs free energy of mixing (dilution) give rise to a minimum in the plot of ∆𝑟𝐺 vs the extent of the reaction, which is the equilibrium position defined by K
What does the Van’t Hoff equation describe?
describes the T-dependence of the equilibrium constant
Explain the relationship between enthalpy and the equilibrium constant.
What is Le Chatelier’s principle?
When a chemical system at equilibrium is disturbed, it reattains equilibrium by undergoing a net reaction that reduces the effects of the disturbance (change in concentration, temp, or pressure).
When does K change?
K will only change when temp is changed
For an endothermic reaction ________
- Heat is a reactant
- According to Le Châtelier’s principle, the equilibrium will shift away from the added heat.
- Equilibrium constant increases if heat is added as a reactant
What is the integrated Van’t Hoff equation?
What is activity?
effective concentration or effective pressure and accounts for non-ideal behavior
How do you define the reaction quotient in terms of activities?
What happens to chemical potential under standard conditions?
- Under standard concentrations the activity always equals 1 and ln(1)=0 so the concentration term disappears
What is standard state for ideal gas?
The standard state for an ideal gas is the gas with a partial pressure equal to 1 bar.
How does pressure relate to activity? (ideal gas)
What is the equation for activity for real gases? What variable is required and what is it a function of?
Describe the activity of a pure liquid or solid.
The Gibbs free energy of a solid or liquid does not change significantly with pressure. So for a pure solid or liquid at any pressure close to 1 bar:
A = 1
How do you find activity for a component of a solution?
mole fraction (Xa) = (moles of species A)/(total moles in solution)
How does the activity of a real solution compare to the activity of an ideal solution?
The activity of a real solution is lower than an ideal solution as concentration increases
What do biochemical processes necessitate in relation to activity?
- biochemical processes occur in very complex environments
- biochemical processes occur at a pH of 7 and not a pH of 0 (1 M H+)
-therefore Ah+ = 1 at a pH of 7
What happens to H+ in the biochemical standard state?
activity of H+ = 1 and can be omitted from K
What is activity?
Total concentration,
- In biological systems we often do not know the concentrations of related species
- Therefore, we define the activity of a molecule as the total concentration of all species of that molecule at pH 7:
What is ionic strength?
How does ionic strength relate to activity?
As the ionic strength of a solution increases, the activity coefficient (γ) of an ion decreases according to the Bebye-Huckel theory
What is Debye-Huckel?
- only applies to very dilute solutions (<10mM)
How are activity coefficients treated in complex solutions?
What problem arises with activity coefficients of ionic solutions?
- For small uncharged molecules and low concentrations, we often assume an activity coefficient of unity.
- For ions, even at low concentrations, this approximation is typically not valid.
- However, ions always come in pairs (overall, a solution is electrically neutral).
- Therefore, we cannot measure the activity coefficients of cations and anions separately.
- We can only measure a mean activity coefficient.
How do you calculate the chemical potential of an NaCl solution accounting for activity?
Describe the activity coefficients of ions. What causes them to change?
- Activity coefficients of ions strongly deviate from unity, even at moderate concentrations of ~0.1 M.
- The activity coefficients are strongly dependent on the valency of the ion (the larger the valency, the higher the rate of decay of 𝛾_±).
What does a positive or negative change in enthalpy imply about bond formation?
If more energy is produced in bond formation than that needed for bond breaking, the reaction is exothermic and the enthalpy is negative
What is Vm?
molar volume
Vm equations
∆Gm = Vm∆p
Vm = molar mass / density
What does a change in enthalpy imply about bond formation?
- negative change - bond formation
- positive change - bond breaking
What is the entropy of a process at constant pressure, temperature and surroundings?
