21 Acids, Bases and Buffers Flashcards
Symbol of concentration
Square brackets
pH
-log10(H+)
(H+)
10^-pH
How to find the alkaline solution
Calculate [H+]
Use kw = 1.00x10^-14 to calculate [H+]
Convert [H+] to pH using -log10[H+]
Difference between a strong and weak acid
Strong acid is fully desiccated
HCl — H+ + Cl-
NA ⇌ H+ + A-
What type of acid is ethanoic acid
Weak acid
Water
Little bit of water is ionised at any one time at a constant
(Water is slightly dissociated.)
pH scale
Made by Danish biochemist Soren sorenson ( 1909 )
Control of activity
Bronsted-lowry acid
An acid is a substance that can donate a proton and base can accept a proton
explain how a solution containing sodium hydrogen carbonate and sodium carbonate can act as a buffer as small amount of acid or alkali are added
small amount of acid
-the HCO3- reacts with the added H+, shifting the equilibrium shifts to the left
small amount of alkali
-H+ reacts with the OH- ions shifting equilibrium to the right
the overall amount of H+ stays pretty much the same
state the meaning of the term weak acid as applied to carboxylic acids
partially dissociated (partially ionised)
write an equations for the reaction of propanoic acid with sodium carbonate
2CH3CH2COOH + Na2CO3 —- 2CH3CH2COOHa + CO2 + H20
suggest what substance might be present in the air to cause the pH to change. Explain how and why the pH of the pH 9.00 solution
Carbon Dioxide
The pH was decrease as it has reacted with the H+ ions in solution
suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution. Explain why this apparatus is more suitable than a pipette for this purpose.
Buret
you can measure out exactly the right amount of liquid but also you can added it drop by drop
Explain why the end point of this reaction would be difficult to judge accurately using an indicator
the pH gradually increase so would be difficult to find the point at which it changes because the indicator could change over a range of values
suggest why choroethanoic acts is a stronger acid than ethnic acid
chlorine is more electronegative than hydrogen so withdrawn electrons which weaken the O-H bond
the dissociation of methanol acid in an aqueous solution is endothermic.
Deduce whether the pH of a solution of methanoic acid will increase, decrease or stay the same if the solution is heated. (explain your answer)
decrease the pH
if the forwards reaction is endothermic then more product is made to oppose the increase in temperature. the equilibrium shifts to the left.
buffer
a buffer solution resist the changes in pH when adding a small amount of acid and alkali
control the pH in living systems
made of weak acid and conjugate base
(A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.)
Acidic buffer
adding alkali
-removes the OH-
adding acid
-equilibrium shifts to the left
half-netrlisation
pH = pKa
measures of how strong a weak acid is
half way between the starting point and equivalence point
HA + OH- —- H2O + A-
acid-base titration
titration is used to find the concentration of a solution by gradually adding it to a second solution which it reacts with
titration curve
- the pH doesn’t change in the linear manner as a base is added
- each curve has almost horizontal sections where lots of base is added
- equivalence point = the point in a titration at which the reaction is just complete
Adding an acid (equations)
-calculate the pH of the buffer
-calculate the number of moles of acid added
-add the original number of moles of acid
-take this away from the original number of base
-calculate new concentration of acid and salt
-work out the pH
(ignore the small volume as it has little effect on the total volume)
Adding a base (equations)
-calculate the pH of the buffer
-calculate the number of moles of base added
-add the original number of moles of base
-take this away from the original number of acid
-calculate new concentration of acid and salt
-work out the pH
(ignore the small volume as it has little effect on the total volume)
weak acid
an acid that is only slightly dissociated into ions in solution
Ka is the dissociation constant for a weak acid
Weak acids and weak bases dissociate only slightly in aqueous solution
equivalence point
the point in a titration at which the reaction is just complete
end point
the point in a titration when the volume of reactant added just causes the colour of the indicator to change
a H+ in a aqueous solution is always H3O+
H+ very small
intenses electric field q
ionic product of water
little of the water is ionised at any one time
Kw = Kc x [H2O]
Kw = [H+][OH-]
acid and alkali
H+ + OH- —- salt and water
pH scale was inverted
1909
Danish biochemist soren Sorenson
control of acidity
increase pH
more alkali
how does a buffer act?
buffer solution minimises ph by
HA ⇌ H+ + A-
overall principle
(HA) weak cid removes added alkali
(A-) conjugate base moves added acid
addition of an acid
[H+] increase
conjugate base reacts with [H+] ions
equilibrium shifts to the left
HA ⇌ H+ + A-
addition of alkali
[OH-] increase
equilibrium shifts to the right
HA ⇌ H+ + A-
buffer in cosmetics
natural pH of the skin = 5.5 (protects the skin)
anti-age creams
lower ph than 5.5 - causing the skin to swell - hiding wrinkles
half neutralisation
pH = pKa
calculate buffer
Ka = [H+][A-] / [HA]
Ka = [H+]^2 / [HA]
only for a weak acid
does Kw increase with temperature ?
yes
acid
proton donator
base
proton accepter
acid- base equilibria
involves the transfer of protons
pH of hydrogen
The concentration of hydrogen ions in aqueous solution covers a very wide range. Therefore, a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration.
water pH
KW = [H+][OH–]
the value varies with temperature
pKa =
–log10 Ka
acidic buffer
solutions contain a weak acid and the salt of that weak acid
alkali buffer
solutions contain a weak base and the salt of that weak base
