Acids and bases

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  • Created by: Massu786
  • Created on: 09-11-17 18:32
define PH
-log(H+)
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what is an acid and a base?
acid is a proton donor and base is a proton accepter
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what can u assume when working out PH of a strong acid?
the concentration of the acid and the H+ ions are equal because the acid fully dissociates
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What is the Kw expression?
Kw= [OH] x [H+]
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Why isnt water included in the Kw expression?
It is said to be at a constant rate as there is a high concentration of water compared to OH and H+, the equilibrium is far to the left
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What is Pka?
-log ka
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What do we have to assume when calculating PH of a weak acid?
That the concentration of the salt and H+ is equal because they have dissociated in a 1:1 ratio. The initial concentration of the undissociated acid has remained constant.
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How do you work out PH of a neutralisation reaction involving strong acid + strong base?
work out moles of acid and base and work out which one is in excess. work out new concentration of 1) excess [H+] or if base if excess then excess base, you do moles in excess over total volume then either use Ka or Kw expression.
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In a weak acid strong base neutralisation reaction what do you do if there is excess acid to work out PH?
work out the new concentration of excess acid ,using moles in excess over total vol, work out concentration of salt formed using moles of base over total vol, substitute these into Ka expression then rearrange to find H+, and work out PH.
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what is the half equivalence point?
when the moles of base added is exactly half the moles of acid originally present
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What can we assume at the half equivalence point?
we can assume ka= H+ which means PH=Pka as Pka is equal to PH-log[Ka]
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What is a buffer solution?
a solution that resists changes in PH when small amounts of acid or alkali are added to it. Made from a weak acid and its soluble salt or a weak base and its soluble salt
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What is an acidic buffer?
It is a buffer made from a weak acid and a salt of that weak acid, it works because the dissociation of a weak acid is an equilibrium reaction.
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What happens when u add acid to the buffer?
the equilibrium shifts to the left to remove H+ ions by combining with the salt. Moles of buffer salt decrease by number of moles of acid added.
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What happens when alkali is added to the buffer?
The OH- ions from the alkali will react with H+ to form water and this removes H+ ions so the PH first slightly increases, but then the equilibrium shifts to the right to oppose the change and replace the H+ ions lost.
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How do you calculate the PH of buffer solutions?
you work out the moles of both solutions and because they have the same final volume you enter moles straight into KA expression. rearrange Ka to get [H+]= (ka x acid conc) / salt conc
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How do you choose a suitable indicator?
The PH range of the indicator must lie on the steep part of the titration curve as this is when the indicator will change colour as this is the neutralisation/end point
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How do you work out Ka from the graph?
You use Pka which is the half equivalence point when half the bases needed for neutralisation is added. Do 10 to the power of minus pka to get Ka.
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What is an important characteristic, for a solution to be an acidic buffer?
both the acid and its salt must be in large concentrations relative to any amounts of added OH- and H+. The buffer must contains a reservoir of the acid to remove OH added and salt to remove H+.
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What is the concentration of the acid and salt at neutralisation point?
the concentration of the salt is half the initial concentration of the acid
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where is the equivalence point on the graph?
midway on the steep vertical part of the graph
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Other cards in this set

Card 2

Front

what is an acid and a base?

Back

acid is a proton donor and base is a proton accepter

Card 3

Front

what can u assume when working out PH of a strong acid?

Back

Preview of the front of card 3

Card 4

Front

What is the Kw expression?

Back

Preview of the front of card 4

Card 5

Front

Why isnt water included in the Kw expression?

Back

Preview of the front of card 5
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