1.12 Acids and bases Flashcards
What is an acid?
A proton donor
What is a base?
A proton acceptor
What is an alkali?
A soluble base
How do you find the pH of a solution? (equation)
-log₁₀[H⁺]
What is [H⁺]?
The concentration of hydrogen ions in the solution.
What is a strong acid?
When H⁺ fully dissociates from the acid.
What is a weak acid?
When H⁺ only partially dissociates from the acid.
What is a monoprotic acid?
Releases one H⁺ per molecule of acid.
What is a diprotic acid?
Releases two H⁺ per molecule of acid.
What is the equation used to find concentration or volume post-dilution?
C₁V₁ = C₂V₂
What is the ionic product of water, based on?
The fact that water only dissociates partly.
What is the dissociation of water equation?
H₂O ⇌ H⁺ + OH¯
H₂O + H₂O ⇌ H₃O⁺ + OH¯
What is the ionic product of water formula?
Kw = [H⁺][OH¯]
Why is the concentration of water not in the ionic product of water?
It is in so much excess, that the water concentration is pretty much constant.
What is Ka?
The weak acid dissociation constant.
What is the weak acid dissociation formula?
[HA]
What is pKa?
The pH of a weak acid solution.
How do you find pKa?
-log₁₀Ka
When does pKa = pH?
When the weak acid is at the half neutralisation point where [A¯] = [HA]
What is Ka when the concentration of [H⁺] = [A¯]?
[HA]
Why does Kw vary with temperature?
- The forward’s reaction is endothermic.
- So increasing temperature shifts the equilibrium in favour of the forward’s reaction.
- This, therefore, increases the Kw.
How do you find the pH of a reaction between a weak acid & strong base? (when acid is in excess)
- Find acid (HA) moles and OH¯ (base) moles.
- Find out how much HA is in excess?
- Find A¯ moles and concentration.
- Find the excess HA concentration.
- Use Ka to find [H⁺] and then pH.
How do you find the pH of a reaction between a weak acid & strong base? (when the base is in excess)
- Find acid (HA) moles and OH¯ (base) moles.
- Find out how much OH¯ is in excess.
- Use Kw to find [H⁺] and then find the pH.
Outline what a titration reaction is?
- Used to find the concentration of a solution by gradually adding a second solution to react with.
- You need to know the equation of the reaction.
- You need to know the concentration of the second solution.
- It is all based on neutralisation between the two solutions.
How do you know which indicator to use?
- Must show the endpoint of the reaction with a distinct colour change.
- The colour change must be rapid.
- Must lie between the vertical part of the acid/base graph.
What is the equivalence point between an acid and a base?
When [H⁺] = [OH¯]
When is half neutralisation?
When the reaction is halfway between zero and the equivalence point.
What does a buffer do?
Maintains an approximately constant pH, and combats the addition/dilution of small amounts of acid & base.
What is an acidic buffer made of?
- A weak acid
- A salt of that same weak acid
How do you make an acidic buffer? (2 ways)
- Add a weak acid and a weak acid salt.
2. The half neutralisation point of a reaction between a weak acid & alkali.
What is the pH of a buffer made from half neutralisation?
Because [HA] = [A¯]
pH = pKa
How does the acid in an acidic buffer oppose the change and keep the pH constant?
The acid, HA, opposed changes when OH is added. The HA reacts with OH to form H₂O + A¯.
How does the weak salt in an acidic buffer oppose the change and keep the pH constant?
The initial weak acid dissociation provides a small amount of A¯ so can’t oppose all the change from extra H⁺. The salt provides enough A¯, so when excess H⁺ is added it can react with the A¯ and use it up (by forming HA).
How do you find the pH of an acidic buffer? (half neutralisation)
- Find HA & OH¯ moles
- Find HA & A¯ moles
- Find [HA] & [A¯]
- Use Ka to find [H⁺] and the pH.
How do you find the pH of an acidic buffer? (acid + salt)
- Find HA & A¯ moles
- Find [HA] & [A¯]
- Use Ka to find [H⁺] and the pH.
How do you find the pH after adding some acid/base to the buffer solution?
- Find original HA & A¯ moles
- Find the moles of what is added.
- If you add acid (H⁺), add these moles to HA, and take them away from A¯.
- If you add a base (OH¯), take these away from A¯, and take them away from HA.
