Equilibrium - Week 13 - 16 Flashcards
What is the definition of an isolated system?
No energy or matter can go in or out
What is the definition of a closed system?
Energy can go in and out but not matter
What is the definition of an open system?
Energy and matter can come in and out
What is the definition of an adiabatic system?
No heat can go in or out
How do changes arise in closed systems?
Changes arise from loss or gain of heat and work done on or by the system.
Is heat a random or organised movement of particles?
= a random movement of particles
Is work done a random or organised movement of particles?
= an organised movement of particles
What are state functions?
Variables of the system with values independent of how they were produced
e.g. pressure, volume, temperature
Variables can be intensive or extensive
What is an extensive variable?
Value dependent of how much of the system there is (size of system)
What is an intensive variable?
Value is independent of how much of the system there is. (size of system)
What are some examples of extensive variables?
e.g. energy, volume, area, mass, heat capacity
What are some examples of intensive variables?
e.g. thermal conductivity, density, temperature, concentration, pressure, specific heat capacity, surface tension
What equation are changes in state functions designated?
ΔX = (Xf - Xi)
If q = heat absorbed by the system and w = work done on the system
ΔU = q+w where U is the internal energy
q and w are not state functions. U is
What is the equation for the 1st Law of thermodynamics (a statement of the Law of Conservation of Energy)
ΔU = q + w where U is the internal energy
What does the law of conservation of energy require - about the change in energy between two defined states ?
The change in energy between two defined states is independent of the route used between A and B.
What is the equation for enthalpy changes at a constant pressure? H ?
H = U + PV
For changes at constant pressure ΔH = q so ΔH values are usually measured rather than ΔU values.
What is a situation where adding or removing energy by heating or cooling does not produce a change in temperature?
= when changes of state occur for a substance
How do you measure heat entering a system?
q = the heat entering the system (therefore ΔH)
q is equal in magnitude but opposite sign to the heat entering the surroundings
Heat entering the apparatus (calorimeter) = Q
q = -Q
Why do different substances produce different temperature changes for the same value of Q?
Q produces a temperature change which depends on the magnitude and sign of Q, the mass of substance affected and nature of substance.
Therefore different substances may produce different temp changes for the same Q value.
What is the equation that describes the relationship change between Loss of heat (Q), and the temperature change produced by the substance?
Q=mcΔT
What do each of the symbols represent in the equation Q=mcΔT ?
m = mass affected
c = specific heat capacity of the substance
ΔT = temperature change
=> where m and c are positive quantities
For an exothermic change what (positive or negative) are Q and ΔH?
For an exothermic change:
Q = positive
ΔH = negative
What happens to ΔH (positive or negative) in an endothermic change?
Endothermic change = ΔH is positive
What are the units for enthalpy change of a reaction?
kJ mol^-1
What are the standard conditions and standard states for enthalpy changes?
Standard conditions:
- 298K
- 100 kPa (1 atm)
- 1 mol dm^-3
What does ΔH° mean?
ΔH° = standard enthalpy change
What is the standard enthalpy change of reaction, ΔrH°?
The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state.
What is the equation for Standard enthalpy change of formation, ΔH°f ?
ΔrH° = ΔfH° (prod) - ΔfH° (react)
ΔrH° = products - reactants
What is the ΔH°f of any element?
ΔH°f of any element = 0
What is the equation for enthalpy change of combustion ΔcH° ?
ΔrH° = ΔcH° (react) - ΔcH° (prod)
ΔrH°= reactants - products
What is the equation for bond enthalpy?
(of all gaseous molecules)
ΔrH° = BE (react) - BE (prod)
ΔrH° = reactants - products
What are the only energy changes possible in an isolated system?
Changes where energy becomes more spread out or disorganised
What is entropy (S) ?
Entropy measures how heat spreads out - the ‘disorder’
What is the Gibbs Free Energy equation?
ΔG = ΔH - ΔTS
If ΔG is negative what is TΔS?
TΔS will be positive
-> and the process is likely to occur
If ΔG is positive what is TΔS?
TΔS is negative
-> the process is unlikely to occur
What is Le Chatelier’s principle?
When a system at equilibrium is subject to a change in one of its constraints (pressure, concentration or temperature) the system responds in a way to minimise the effect
What is the effect of pressure on a reaction?
The equilibrium will move to the side of the equation with fewer moles.
What is the effect of temperature on a reaction?
Increasing the temperature will shift the equilibrium in the endothermic direction.
Decreasing the temperature will shift the equilibrium in the exothermic direction.
What is the effect of a catalyst on a reaction?
Catalyst has no effect on position of equilibrium just speeds up the rate that equilibrium is attained.
How do some reactions occur despite having a positive ΔrG value?
Because the reactions are coupled with other reactions that have negative ΔrG values
- so the overall value of ΔrG is negative
How do gas - gas solutions mix?
Spontaneously
- Entropy driven
Very small ΔH
Large positive ΔS value
How do gas - liquid solutions mix?
More difficult to predict outcome
Temperature has important role - determining size + possible influence of entropy.
Pressure has an effect - gas solubility will decrease with increasing pressure
What is Henry’s Law, and what is the equation?
For gases that dont react with the solvent
Cgas = kH P gas
at constant T and low values of c and p
KH = the henry’s law constant - is different for
different gasses
How do liquid - liquid solutions mix?
- Even more difficult to predict
A-A and B-B interactions have to be overcome (endothermic process) and replaced by A-B interactions (exothermic process).
How do solid - liquid solutions interact?
Similarly to liquid - liquid.
A+B- interactions must be overcome as well as Water - water interactions.
WA+ and WB- interactions must be made
What is the definition of solubility?
A measure of the amount of a substance needed to produce a saturated solution of unit volume at a given temperature.
For sparingly soluble ionic substances its linked to the equilibrium constant Ksp that characterises the solubility product.
What happens if Qsp > Ksp ?
a precipitate will form
What is the equation and units for molarity?
Molarity = amount of solute / volume of solution
Units = mol dm^-3, mol L^-1 , M
What is the equation for Molality?
Molality = amount of solute / mass of solvent
Symbol = b
Units = mol kg^-1
What is the equation for mole fraction?
Mole fraction = amount of solute or component / total amounts in the mixture
Symbol xi for component i
What is the equation for partial pressure?
Partial pressure (for gas component i)
= Pxi (total pressure x mole fraction)
What is Dalton’s law of partial pressures equation?
P = Σ pi
What is colligative properties?
Properties that depend only on the number of solute particles in solution
What are colligative properties for non-volatile solutes?
- Vapour pressure changes
- Boiling point elevation
- Freezing point depression
- Osmotic pressure
What is Raoult’s Law equation?
P solution = P 0 solvent x X solvent ( p0 is the vapour pressure for the pure solvent)
(X solute + X solvent = 1)
What is boiling point elevation and what is the molal boiling point elevation constant?
ΔTb = Kb x b
where Kb is the molal boiling point elevation constant
What is the freezing point depression equation and constant?
ΔTf = Kf x b
where Kf is the molal freezing point depression constant
Kb and Kf are properties of the solvent only
What is the equation for osmotic pressure?
for osmotic pressure:
pi = cRT or pi = (n/V)RT or pi V = nRT
Osmotic pressure pi is measured in Pa if c is in mol m^-3 and T in K.
What is molality?
Number of moles per kg
Number of moles / 1000g
What is the Bronsted - Lowry definition of an acid?
An acid is any species that acts as a proton (H+) donor.
What is the Bronsted - Lowry definition of an base?
Any species that acts as a proton acceptor
What are the conjugate base pairs?
The conjugate base of HA is A-
The conjugate base of B is HB+
- one cannot function without the other
What are monoprotic (or monobasic) acids?
Acids that dissociate to provide one proton
- diprotic, triprotic etc
How is Ka for dissociation made greater?
Any effect which spreads or delocalises the negative charge in a conjugate base makes the loss of the associated proton easier and makes Ka for the dissociation greater.
- Electronegative atoms pull electron density away, decreasing likelihood of the dissociated protons reattaching.
What is the equation for pH?
pH = -log10 [H+]
What is the equation for pKw?
pKw = -log10 Kw
What is a buffer solution?
a solution that will maintain its pH when acidic or basic species are added to it or generated within it.
Either:
- a mix of weak acid and conjugate base in
solution
- a mixture of weak base and its conjugate
acid in solution
What is oxidation?
Loss of electrons
Oxidising agent receives electrons so becomes reduced
What is reduction?
Gain of electrons
Reducing agent receives electrons so becomes oxidised
What are reactions involving electron transfer called?
Redox reactions
What is an oxidation number?
The charge the atom would have it the charge of the electrons binding the atoms together were redistributed to more electronegative atoms.
(the ionic charge in monatomic atoms)
What is the oxidation number for uncombined atoms or molecules?
= 0
What is the oxidation number for fluorine in all compounds?
-1
What is the oxidation number for oxygen? (when not in a peroxide)
-2
What is the oxidation number for oxygen in a peroxide?
-1
What is the oxidation number for hydrogen?
+1
What is the oxidation number for a hydride ion?
-1
How can the oxidation number be deduced in a compound or ionic species where an element is bonded to O, H or F?
The sum of the ONs (allowing for numbers of atoms) in a neutral compound or species must add up to 0.
The sum of the ONs (allowing for numbers of atoms) in an ionic species must add up to the charge on the ion.
What is a redox reaction?
The sum of an oxidation half equation and a reduction half equation
(H+, OH- or H2O can be used to balance in aqueous solution and e- to balance charge)
What do electrodes allow?
the passage of electrons into and out of a conducting solution or liquid
How will a potential difference be set up at an electrode?
Electrons reside in energy levels on the metal, depending on their equilibrium position the metal will acquire a positive or negative charge, and a potential difference will be set up.
How can electrons pass in an out of an electrode?
By using a piece of inert metal dipped into the solution e.g. platinum
In a reaction equilibria equation what do the solid lines represent?
Solid lines separate components in different physical states.
How is the electrode in an electrode equation written?
e.g.
“oxidised form”, “reduced form” │Pt.
What are brackets used for in an electrode equation?
[ ] brackets are used to enclose the oxidised and reduced forms where these involve species in more than one physical state or where they involve the use of H+ / H2O / OH− for balancing
What is the equation for the ultimate reference electrode the Standard Hydrogen Electrode (SHE)?
H+ (aq) + e- <—> 1/2 H2 (g)
temp of 25 degrees C
What is the potential of the standard hydrogen electrode?
The potential is defined to be zero at all temperatures.
In a cell which electrode does reduction always occur at?
the right hand electrode in a cell diagram
What is the equation for an electrode cell?
E cell = E right - E left
or
E cell = E cathode - E anode
What is the relationship between free energy change delta g and cell potential?
delta G = -nF Ecell
n = number of e- transferred in a cell reaction
F = faraday constant
