# Chemistry- Unit 4- Acids,bases and buffers

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What is an acid?
Substance that can donate a proton (H+ ion) - Proton donor
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What is a base?
Substance that can accept a proton - Proton acceptor
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What does water act as? What is it in terms of ions?
It can be both an acid and a base! It is also slightly ionised
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Equation for water (to do with ions? )
H20 H+ + OH- OR H20 + H20 H3O+ + OH-
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Equilibrium equation for this?
Kc = (H+)(OH-)/ (H20)
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Equation for Kw? Why can we achieve this?
Kw= (H+)(OH-) and its because the conc of water is constant and is kw= kc vs H20
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Value for Kw? Units?
298k equal to 1 x 10-14 (DONT NEED TO LEARN THIS) mol2 dm-6
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How can we rearrange that equation? why can we do this?
Because each H20 dissosiates into 1 H+ ion and one OH- ion, we can write it as Kw= [H+]2
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Definition of ph?
-log10[H+(aq)]
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The smaller the ph...?
The greater the concentration of H+
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Diffrence of one ph...
Equals a tenfold difference in H+ concentration
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How does ph get effected by temperature? (related to neutrality)
If its an endothermic reaction then increasing the temperature will move the equilibrium to the right, therefore resulting in greater conc of [H+] so the ph will decrease. But will remain neutral because [H+] = [OH-]
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At 298k, what is acidic,neutral and base?
Less then 7 acid, more then 7 its a base.
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How do we find [H+] from ph?
Press inv on the log10 button and add a minus sign before the ph value. (known as antilogging)
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Finding [OH-] from ph?
Calculate the [H+] from the method above and then ,using the value for kw, divide that by [H+] because kw= [H+][OH-]
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What is a strong acid?
An acid which fully dissociates in dilute aqueous solution.
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Examples of these?
HCL, H2s04, HN03
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What can we do with strong acids?
Simply -log10 their concentration to find ph because their conc =[H+]
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Finding the ph of alkaline solutions?
Calculate the concentration of [OH-} then use [H+][OH-]= 1 x 10-14
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What are strong bases?
Bases which completely disassociate into ions in aqueous solutions
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Examples?
NaOH and KOH
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What are weak acids and bases?
Ones which do not fully dissociate into ions in aqueous solutions.
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Examples of each?
acids- ethanoic acid (carboxylic acids)
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What is the equation for a weak acid dissociation?
HA H+ + A-
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Ka equation? What does Ka stand for?
Ka= [H+][A-]/[HA]
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How does the value of Ka relate? Units?
Larger the Ka value, the further to the right the equlibrium is, the more acid dissosiated and the stronger it is. Units can be calculated by dividing and cancelling.
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What cant we assume now? What must we do instead?
That their concentration = [H+]. Instead we must use Ka.
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How do we do this?
Since the degree of dissociation is so small, [H+] is so small and therefore 1- [H+] is around 1 still. So therefore [H+]2/[HA] = ka. We can then use the ka value and the Ha value to calculate [H+]2, then square root, then log10 it.
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What is pka?
It is -log10Ka. It is useful for measuring how stroke a weak acid is. The smaller the value, the stronger the acid.
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How can we use a ph meter in a tritration?
Can be used to tell us the ph of the solution, effectively acting as an indicator.
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Titration curves examples- base being added to known volume of acid
N/A
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what is the equivalence point?
Where sufficient base has been added to just neutralize the acid (and vice versa)
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strong acid, strong base?
very large equivalence point, between 3-11 ph.
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weak acid, strong base?
smaller equivalence point, between 7 and 11.
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strong acid, weak base?
smaller equivalence point between 3 and 7
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weak acid, weak base?
very small equivalence point between 6 and 8.
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the curve will flip.
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What does a suitable indicator need to meet?
Colour change must be sharp rather then gradual at the end point, end point must be the same as the equivalence point, distinct colour change.
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What is the half neutralization point? why is it useful?
it halfway between the zero and the equivalence point. Its useful as it tells us we can add acid or base up to this point knowing it will make very little change to the ph.
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At this point, what is the equation?
[HA]= [A-] so therefore Ka = [H+] so therefore -log10ka = pka.
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What is a buffer? How do they work?
a solution that resists changes of acidity or alkaninity. Therefore when small amounts of either are added to it, their ph remains almost constant. Designed to keep the ions concentrations unchanged by equilibriu moving in either direction to oppose.
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What are acidic buffers made of? Why do they work?
weak acid and a soluble salt of that acid - work because the dissociation of a weak acid is an equilibrium reaction.
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What happens if you add alkali to an acidic buffer?
HA + OH- ---> H20 + A-. This removes the added OH so therefore we see very little ph change.
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Adding acid to an acidic buffer?
If H+ is added then the equilibrium shifts to the left. Since the [A-] is so small the supply of A- soon runs out and there's none left, hence no buffer.
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How can we solve this?
add to the solution a supply of extra A- by adding soluble salt of HA (eg: Na+A-).
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So what are the two equations?
HA + OH- ---> A- + H20 AND A- + H+ ---> HA
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How to calculate the ph change when acid is added to a buffer?
when we add an acid the moles of acid is increased (add moles of acid added) and the moles of the salt will decrease by the amount.
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why?
Because all the H+ ions will react with the A- ions to form HA.
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How to calculate the ph change when base is added to a buffer?
the acid will decrease and the salt will increase.
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why?
They will react with the H+ and therefore cause more HA to be ionised, decreasing its conc.
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## Other cards in this set

### Card 2

What is a base?

#### Back

Substance that can accept a proton - Proton acceptor

### Card 3

#### Front

What does water act as? What is it in terms of ions?

### Card 4

#### Front

Equation for water (to do with ions? )

### Card 5

#### Front

Equilibrium equation for this?