Module 5: Physical Chemistry and Transition Elements Flashcards
What is heterogenous equilibrium and how will it affect a Kc expression?
An equilibrium which contains species that aren’t all in the same state, and as a result, only aqueous and gaseous species in the reaction are included in the Kc expression and solid and liquid species are omitted.
What is homogenous equilibrium and how will it affect a Kc expression?
An equilibrium which contains species that are all in the same state, and as a result, all species in the reaction are included in the Kc expression.
What is the mole fraction of a gas?
Proportion of moles of a particular gas out of all the moles of all the gases in the gas mixture.
Define partial pressure.
The contribution that a gas makes to the total pressure of a gas mixture.
What is the general expression for Kp ?
(products)/(reactants)
only includes gases
What is the only change that can result in a change in the value of K and why?
Change in temperature. This because if the change in temperature favours the forward reaction, yield of products will increase, and since K = products/reactants, K must change as well. The opposite is also true.
However, changes in concentration or pressure means a reaction mixture is no longer in equilibrium, and K has changed. As a result the system readjusts to restore the original value of K.
Define lattice enthalpy.
Enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
This is an exothermic process.
Define standard enthalpy of atomisation.
The enthalpy change that takes place when one mole of gaseous atoms are formed from an element in its standard state.
This is an endothermic process.
Define first electron affinity.
The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous -1 ions.
This is an exothermic process.
Why is first electron affinity exothermic, but all subsequent electron affinities endothermic?
For first electron affinity, the electron is being added to a neutrally charged atom, so it’s attracted to the nucleus.
For all subsequent electron affinities, the electron is being added to a negative ion, so they repel each other. This means energy needs to be put in to add the electron.
Define standard enthalpy change of solution.
Enthalpy change that occurs when one mole of a solute is dissolved in a solvent under standard conditions.
Define enthalpy change of hydration.
Enthalpy change that occurs when one mole of gaseous ions are dissolved to form one mole of aqueous ions under standard conditions.
What affect does increasing ionic radius have on lattice enthalpy?
Attraction between ions decreases, so lattice enthalpy is less negative.
What affect does increasing ionic radius have on enthalpy of hydration?
Attraction between ions and water decreases, so enthalpy of hydration is less negative.
What affect does increasing ionic charge have on lattice enthalpy?
Attraction between ions increases, so lattice enthalpy is more negative.
What affect does increasing ionic charge have on enthalpy of hydration?
Attraction between ions and water increases, so enthalpy of hydration is more negative.
Energy has a tendency to…
Spread out
Systems which are more chaotic…
Have a higher entropy value
what does ΔS represent?
Entropy change
If a system changes to become more disordered…
ΔS will be positive
If a system changes to become less disordered…
ΔS will be negative
ΔS =
ΣS(prod) - ΣS(react)
A reaction can only occur is ΣS(prod) < ΣS(react)
Why would the entropy change of a reaction be positive?
Disorder is increasing as particles gain more energy, move more freely
e.g. solid→liquid→gas
Why would the entropy change of a reaction be negative?
Disorder is decreasing as particles loose energy and move closer together
e.g gas→liquid→solid
Define entropy.
Measure of the disorder of a system.
Define feasibility.
Whether or not a reaction can occur.
What does ΔG represent?
Gibbs free entropy change
What is the equation for free entropy change?
ΔG = ΔH - TΔS
What are the conditions for a reaction to be feasible?
ΔG < 0
Define the order of a reactant
How changing the concentration of a reactant changes the rate of reaction.
What does it mean if the order of a reactant is zero?
Changing the concentration of that reactant has no effect on the rate.
What does it mean if the order of a reactant is one?
Rate ∝ [A]
What does it mean if the order of a reactant is two?
Rate ∝ [A]²
What is the rate equation.
Rate = k[A]ᵐ[B]ⁿ
How can orders be determined?
Experimentally, e.g. the initial rate method or continuous monitoring method
What is a clock reaction?
A more convenient way of obtaining the initial rate of a reaction by taking a single measurement; the time taken for an observable change to occur e.g. colour change, formation of a precipitate or effervescence.
This method is only accurate if the time is measured over a short time (less than 15% of the reaction has taken place).
What is the rate determining step of a reaction?
The slowest step in a reaction mechanism.
The reactants in this step appear in the rate equation, with the balancing numbers in the rate determining step being the orders of the reactants
What does the rate of a reaction depend on?
The value of k, and k depends on temperate and the Arrhenius constant.
What is the Arrhenius constant?
The number of particles with the correct orientation.
What is a Brønsted-Lowry acid?
A proton donor.
What is a Brønsted-Lowry base?
A proton acceptor.
What is an acid according to Arrhenius’s model?
A species that dissociates and releases H⁺ ions in aqueous solution.
What is a base according to Arrhenius’s model?
A species that dissociates and releases OH⁻ ions in aqueous solution.
Define a monobasic acid.
An acid which has one hydrogen that can be replaced per molecule.
Define a dibasic acid.
An acid which has two hydrogens that can be replaced per molecule.
Define a tribasic acid.
An acid which has three hydrogens that can be replaced per molecule.
What is hydronium ion?
H₃O⁺ (Water and a proton)
What two equations convert between pH and [H⁺]?
pH = -log([H⁺])
[H⁺] = 10⁻ᵖᴴ
This can be used to calculate pH of strong acids
What is the equation for Kₐ?
HA ⇌ H⁺ + A⁻
Kₐ= ( [H⁺][A⁻] ) / ( [HA] )
This is for weak acids
What is the ionic product of water?
=[H⁺][OH⁻]
At room temp, this equals 1x10⁻¹⁴
Define a buffer solution.
A buffer solution is defined as a system that minimises pH changes when small amounts of acid or bases are added.
Given an example of a buffer in the body.
H₂CO₃ ⇌ H⁺ + HCO₃⁻
This equilibrium exists in blood plasma, with the ideal pH 7.35-7.45
What is the equivalence point of a pH curve
The middle of the vertical section.
When using indicators, the pH range of the indicator should align with this vertical section.
Why is universal indicator not a good choice for any titration?
There is no sharp colour change.
Why is there no suitable indicator for the titration of weak base and weak acid?
pH changes gradually.
How can you construct redox equations?
Balance electrons or redox numbers.
What colour is potassium manganate?
Deep purple colour, so burette readings are read from the top of meniscus.
In manganate redox titrations, how can the end point be identified?
When potassium manganate is in excess, the reaction mixture turns colourless to pale pink.
