Further Aspects Of Equilibria Flashcards
pH is defined as….
The negative logarithm to the base 10 of the hydrogen ion concentration. ( pH = -log₁₀[H] ).
For a strong acid or base, the concentration of H or OH in a solution of the acid or base is….
(Approximately)The same as the concentration of the acid or the base.
formula for ionic product of water. (Kw).
Kw = [H] [OH].
What is Ka?
Acid dissociation constant. (Ka of HA–> Ka = [H][A] / [HA]. )
A high value of Ka indicates that…..
The acid is almost completely ionised and exists mostly as the ions of the acid.
A low value of Ka indicates…..
That the acid is only slightly ionised and exists mostly as the molecules of the acid.
pKa values are used to compare strengths of acids. What is the equation to find it?
pKa = -log₁₀Ka.
When calculating the value of Ka for a weak acid. (Assumptions).
- We ignore the Conc. of the H-ions produced by the ionisation of the water molecules present in the sol. As the Kw of water is negligible compared with most weak acids.
- We assume the ionisation of the weak acid is so small that the Conc. of undissociated HA molecules in eq. is approx. the same as that of the unaq. acid.
What is an acid-base indicator?
This is a dye that changes colour over a specific PH range.
On the graph of reaction of acid and base the end point is where?
At the midpoint of the sharp fall.
Range of sharp fall in reaction between strong acid and strong base.
pH between 10.5 and 3.5.
Range of sharp fall in reaction between a weak base and strong acid.
pH between 7.5 and 3.5.
Range of sharp fall in reaction between a strong base and weak acid.
pH between 11 and 7.5.
What is a buffer solution?
Is a solution in which the pH does not change significantly when small amounts of acids or alkalis are added. E.g a mixture of a weak acid and its salts (ethanoic acid & sodium ethanoate)
How does a buffer solution work?
- If H(plus ions) are added to the solution the pos. of equ. shifts to the left until equilibrium is established.
- the large reserve supply of the anion (c.base)from the dissociation of the salt ensures the conc.of the anion doesn’t change significantly.
- The large reserve supply of the acid ensures conc.of the acid doesn’t change significantly.
For an increase in OH ions what happens in a buffer solution?
- This reduces the conc. Of Hplus ions and the pos. of equ. shifts to the right.
- So the acid ionises to form the ions to restore equilibrium.
- The large reserve supply of the acid ensures there is no significant change in conc. of the acid.
- The large reserve supply of the anion doesn’t change significantly.
In polluted areas rainwater that has CO₂ dissolved in it acts as a buffer solution as it forms carbonic acid (H₂CO₃).
But this is not so for areas polluted by acidic oxides of N and S , why?
The concentrations of the carbonic acid and its conjugate base are not high enough to cope with the large amounts of acidic pollution involved.
Equations to calculate pH of buffer solution.
[H] = Ka × [Acid] / [Salt].
&
pH = pKa + log([salt] / [Acid])
Uses of buffer solution.
- Many industrial processing like electroplating.
- Leather treatment & dye manu.
- Used to keep pH of some animals constant e.g in human blood.
What is solubility?
This is the number of grams or moles needed to saturate 100g of water at a given temp.
What is solubility product? (Ksp).
This is the product of the concentrations of each ion in a saturated solution of a sparingly soluble salt at 298K, raised to the power of their relative concentrations.
Equation for solubility product.
E.g for AgCl
Ksp = [Ag(aq)][Cl(aq)]. ( it is raised to the power of their moles and their oxidation numbers are put on top of their formulas).
How do you know if a precipitate is formed?
Let’s say compound is HA.
If [H][A] is greater than the value of Ksp given.
What is the common ion effect?
The reduction in solubility of a dissolved salt achieved by adding a solution of a compound which has an ion in common with the dissolved salt. Often results in precipitation.
