1.6. Chemical Equilibrium Flashcards
What is K?
The equilibrium constant.
What are the units of K?
There are none.
What is the general equation for K?
For the reaction:
aA + bB <=> cC + dD
K = [C]^c [D]^d / [A]^a [B]^b
What do the capital letters in the square brackets symbolise?
The concentrations of the letter inside it.
What is homogenous and heterogenous equilibrium?
Homogenous equilibrium is an equilibrium reaction in which all the reactants and products are in the same state. Heterogenous has different states.
What is the value of the concentration of a pure liquid or solid in the equilibrium constant equation?
1 as the concentration is taken as a constant.
What is the only thing that can affect the value of K?
Temperature.
If you increased the temperature in an ENDOthermic reaction, what would happen?
The products would be favoured so the P:R ratio would increase. K would also increase as a result as the equilibrium constant equation would be top heavy. The opposite would happen if you decreased the temperature.
If you increased the temperature in an EXOthermic reaction, what would happen?
The reactants would be favoured so the P:R ratio would decrease. K would also decrease as a result as the equilibrium constant equation would have a higher denominator. The opposite would happen if you decreased the temperature.
What is the water equilibrium equation?
H20 (l) + H20 (l) <=> H30+ (aq) + OH- (aq)
or
H20 (l) <=> H+ (aq) + OH- (aq)
What is a hydronium ion?
A hydrated proton.
What is Kw and what is its formula?
Kw is the ionic product of water (the equilibrium constant of water) and has a formula of Kw = [H+][OH-].
What is the value of Kw at room temperature?
1x10^-14
Water can act as both an acid and a base. What is this characteristic called?
Amphoteric.
How can you calculate the pH knowing just the H+ concentration?
Through using the formula:
pH = -log10 [H+]
This can be rearranged to find the H+ concentration:
[H+] = 10 ^ -pH
For every change in 1 in the pH scale, what factor does the [H+] change by?
A factor of 10.
pH change of 1 = [H+] change of 10
pH change of 2 = [H+] change of 100
and so on…
How can you calculate the pOH?
Using the formula:
pOH = -log10 [OH-]
If you knew the pH, how could you work out the pOH?
By subtracting the pH from 14. This goes for finding the pH from the pOH too.
What classifies a strong acid?
The fact that it can completely ionise in solution.
An example would be HCl
HCl (aq) -> H+ (aq) + Cl- (aq)
A solution of a strong acid exists only as its dissociated ions. Other molecules include nitric and sulfuric acid.
What classifies a weak acid?
The fact that it only partially dissociates in solution.
An example would be CH3COOH.
CH3COOH (aq) <=> CH3COO- (aq) + H+ (aq)
A weak acid dissociation is an equilibrium reaction.
What position does the equilibrium lie in weak acid dissociations?
It lies over to the left so there will be much more undissociated acid molecules than its ionic components.
What is the value of K in weak acid equilibria?
K < 1.
Define a diprotic acid.
An acid that has two hydrogen atoms that can become hydrogen ions.
Define a monoprotic acid.
An acid that has only one hydrogen atom that can ionise.
What classifies a strong base?
The fact that it can fully dissociate in solution.
An example would be NaOH.
NaOH (s) -> Na+ (aq) + OH- (aq)
Oxides and hydroxides are all strong bases.
What classifies a weak base?
The fact that it can only partially dissociate in solution.
An example is NH3.
NH3 (aq) + H20 (l) <=> NH4 (aq) + OH- (aq)
The equilbrium lies over to the left again meaning there are fewer products than reactants.
Amines are weak acids.
Why do strong acids and bases make good conductors?
Since they fully ionise in water their ions are all dissociated and can conduct much easier. There are more H+ ions and OH- ions.
What is the pH of a salt of a strong acid and strong base?
Neutral as they are both strong and both fully ionise in water meaning the equilibrium is unaffected.
What is the pH of a salt of a strong acid and weak base and why?
Acidic.
Take the example of the salt of HCl and NH3. The salt of this is NH4+Cl-.
In solution the NH4+Cl- is acidic and therefore reacts with some of the OH- ions in the water equilibrium. This reaction between NH4+ and OH- form NH3 and H2O. As ammonia is a weak base there will be more reactants than products.
For any salt of a strong acid and weak base the solution is acidic due to the positive ions of the salt removing the OH- ions.
What is the pH of a salt of a weak acid and a strong base and why?
Basic.
Take the example of CH3COOH and NaOH and their salt Na+CH3COO-.
The negative ions in the sodium ethanoate react with the H+ ions in the water equilibrium and form ethanoic acid molecules. This is due to ethanoic acid being a weak acid so it’s equilibrium is on the left hand side meaning there is more reactants than products.
Due to the negative ethanoate ions removing H+ ions, the solution will be alkaline.
What is the pH of a soap?
Basic as soaps are made of fatty acids (weak) and strong bases. A typical salt is sodium stearate.
What did Brønsted and Lowry define acids and bases as?
Acids = Proton donors.
Bases = Proton acceptors.
What is a conjugate base and a conjugate acid?
A conjugate base is a base which is formed after an acid donates a proton.
A conjugate acid is an acid which is formed after a base accepts a proton.
What is Ka?
The acid dissociation constant.
How do we know the concentration of a strong acid?
It is equal to the concentration of H+ ions.
What is the equilibrium equation for the dissociation of a weak acid?
HA (aq) + H2O (l) <=> H3O+ (aq) + A- (aq)
HA is an acid and A- is its conjugate base.
This equation can be simplified to:
HA (aq) <=> H+ (aq) + A- (aq)
What is the formula for finding out Ka?
Ka = [H+][A-] / [HA]
How can the Ka equation be simplified to Ka = [H+]^2 / c
[HA] is taken to be the same value as the unionised acid concentration given the symbol c.
When one molecule of HA ionises, one H+ ion and one A- ion are created meaning that [H+] = [A-] so we can write [H+][A-] as [H+]^2.
What is the formula for working out the pKa of something?
pKa = -log10 [Ka]
What is the equation for working out the pH of a weak acid using pKa?
pH = 1/2 pKa - 1/2 log10c
What is a buffer solution?
A solution in which the pH remains around the same when small amounts of acid and alkali are added.
What is an acid buffer?
An acid buffer is a buffer which consists of a weak acid and one of its salts.
What happens when you add an acid to an acid buffer solution?
Look at the equilibrium for CH3COOH
CH3COOH (aq) <=> CH3COO- (aq) + H+ (aq)
When an acid is added to this buffer solution the newly added H+ (aq) ions react with the ethanoate ions from the salt which forms more ethanoic acid molecules to counteract the addition of the H+ ions so the pH stays the same.
What happens when you add a base to an acid buffer solution?
CH3COOH (aq) <=> CH3COO- (aq) + H+ (aq)
The new OH- ions from the base react with the H+ ions from the ethanoic acid. More ethanoic acid molecules dissociate to account for these lost H+ ions so the pH stays the same.
What happens when you add an acid to a basic buffer?
NH3 (aq) <=> NH4+ (aq) + OH- (aq)
When an acid is added to this buffer solution the H+ ions can react with the ammonia molecules to form more NH4+, or they can react with the OH- ions to form ammonia and water.
What happens when you add a base to a basic buffer?
NH3 (aq) <=> NH4+ (aq) + OH- (aq)
When you add a base to this basic buffer solution the newly added OH- ions react with the positive ammonium ion to form ammonia and water.
What is the formula for finding out the pH of a buffer solution?
pH = pKa - log10 [acid/base]/[salt]
What are indicators?
Indicators are weak acids which have a different colour than their conjugate bases.
What is the equilibrium equation for the dissociation of an indicator?
HIn (aq) + H20 (l) <=> H3O+ (aq) + In- (aq)
What is KIn?
The equilibrium constant for an indicators dissociation equilibrium.
What is the formula for KIn?
KIn = [H3O+][In-] / [HIn]
This can be rerranged to:
[In-]/[HIn] = KIn/[H3O+]
How do we know that pKIn = pH?
As in the rearranged equation if [In-][HIn] = 1, then KIn/[H3O+] = 1 too. This means they must be the same value. [H3O+] is another way of saying pH. Therefore pKIn = pH.
We can deduce that a colour change occurs when pKIn = pH, or when concentrations of the weak acid and conjugate base are the same.
The colour change is only distinguishable when [HIn] and [In-] are different by a factor of ~10. What does this mean?
The pH range over which the colour change of an indicator occurs is approx. pH = pKIn +/- 1.
Where does the pH change occur most rapidly?
Around the end point of the reaction.
