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A-Level Chemistry > Chapter 21 > Flashcards

Chapter 21 Flashcards

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1
Q

What is a buffer solution

A

System that minimises pH changes when small amounts of an acid or a base are added - buffer solutions contain two components to remove added acid or alkali ; a weak acid and its conjugate base

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2
Q

Weak acid role?

A

HA removes added alkali

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3
Q

Conjugate base role

A

A- removes added acid

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4
Q

When alkalis or acids are added to the buffer?

A

The two components in the buffer solution react and will eventually be used up - as soon as one component has all reacted the solution loses its buffering ability towards added acid or alkalis ; pH does change but only by a small amount (the pH does not stay completely constant)

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5
Q

How to prepare a buffer solution?

A

Needs a weak acid and it’s conjugate base

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6
Q

First method of preparing buffer solution?

A

Mixing a solution of ethanoic acid with a solution of one of its salts - sodium ethanoate (CH3COONa)
Ethanoic acid dissociates when added to water ; the acid partially dissociates and the amount of ethanoate ions in solution is very small, ethanoic acid is the source of the weak acid component of the buffer solution

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7
Q

CH3COOH dissociation

A

CH3COOH -> <- H+ + CH3COO-

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8
Q

Salts of weak acids?

A

Are ionic compounds and provide a convenient source of the conjugate base ; the salt completely dissolved when added to water - the salt is the source of the conjugate base component of the buffer solution
CH3COONa -> CH3COO- + Na+

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9
Q

Second way of preparing a buffer?

A

Partial neutralisation of the weak acid - adding an aqueous solution of an alkali like NaOH to an excess of the weak acid ; weak acid is partially neutralised by the alkali forming the conjugate base - some of the weak acid is left over unreacted ; the resulting mixture = salt of the weak acid + any unreacted weak acid

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10
Q

Ethanoic acid equilibrium

A

Equilibrium position lies well towards ethanoic acid - when CH3COO- ions are added to CH3COOH, the equilibrium position shifts even further to the left, reducing the already small concentration of H+ ions and leaving a solution containing mainly CH3COOH and CH3COO-

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11
Q

What’s so special about the ethanoic acid equilibrium?

A

CH3COOH and CH3COO- act as two reservoirs that are able to act independently to remove added acid and alkali ; achieved by shifting the buffer’s equilibrium system either to the right or left

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12
Q

Acid buffer solution

A

HA (aq) -> <- H+ + A-

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13
Q

Addition of an acid

A

Concentration of H+ increases
H+ react with A- (makes HA)
Equilibrium position shifts to the left, removing most of the H+ ions

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14
Q

Addition of an alkali?

A

Concentration of OH- increases
Small concentration of H+ ions reacts with the OH- ions (to create H2O)
HA dissociates, shifting the equilibrium position to the right to restore most of H+ ions

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15
Q

When is a buffer most effective?

A

When there are equal concentrations of the weak acid and it’s conjugate base ; HA = A-
ph of the buffer solution = pKa value of HA
Operating pH is typically over about two pH units, centred at the pH of the pKa value

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16
Q

pH of a buffer solution depends on

A

pKa value of the weak acid
Ratio of the concentrations of the weak acid and its conjugate base

17
Q

When calculating pH of a weak acid?

A

Make an approximation that H+ = A- (concentration wise) ; for a buffer solution this is false as A- has been added as one of the components of the buffer

18
Q

pH of buffer solution equation

A

Concentration of H+ = Ka * (HA/A-)
Ka * ratio of weak acid and it’s conjugate base

19
Q

If given pKa and concentrations of HA and A- are the same

A

pH = pKa

20
Q

pH control in the body

A

Role falls to buffer solutions - for example in the plasma of the blood ; blood plasma needs to be maintained at a pH between 7.35 and 7.45 ; carbonic acid-hydrogencarbonate buffer system being the most important

21
Q

If pH slips outside this range?

A

If below 7.34 then people can develop a condition called acidosis which can cause fatigue or shortness of breath and if the pH rises above 7.45 the condition is called alkalosis which can cause muscle spasms/nausea

22
Q

ph scale sensitivity?

A

Very sensitive - difference of 0.3 pH = two-fold difference in H+ concentration ; big difference in terms of acidity/alkalinity

23
Q

On addition of acid to carbonic acid-hydrogencarbonate system

A

Concentration of H+ increases
H+ ions react with the conjugate base HCO3-
Equilibrium position shifts to the left, removing most of the H+ ions

24
Q

In addition of an alkali to carbonic acid-hydrogencarbonate buffer system

A

Concentration of OH- increases
Small concentration of H+ ions react with the OH- ions to form water
H2CO3 dissociates, shifting the equilibrium position to the right to restore most of H+ ions

25
Q

Body produces…

A

Far more acidic materials than alkaline which the conjugate base HCO3- converts to H2CO3 ; body prevents build up of carbonic acid by converting it to carbon dioxide gas, which is exhaled by the lungs

26
Q

ph meter

A

Consists of an electrode that is dipped into a solution and connected to a meter that displays the pH reading - records it to two decimal places, it is much more accurate
pH titration curve could be plotted automatically using the data logger or appropriate software on a computer

27
Q

Stages of tirration curve?

A

When base is first added, the acid is in great excess so pH increases slightly - as vertical section is approached the pH starts to increase more quickly as the acid is used up more quickly
After vertical section - pH will rise very slightly as base is now in great excess

28
Q

Equivalence point

A

Volume of one solution that exactly reacts with the volume of the other solution - solutions have then exactly reacted with one another and the amounts used matching the Stoichometry of the reaction

29
Q

Where is the equivalence point on the curve?

A

Centre of the vertical section of the pH titration curve

30
Q

Acid base indicator?

A

Is a weak acid that has a distinctively colour from its conjugate base ; methyl orange for example
HA = red
Conjugate base = A- = yellow
AT END POINT THE INDICATOR CONTAINS EQUAL CONCENTRATIONS OF HA AND A- AND THE COLOUR WILL BE IN BETWEEN the two extremes

31
Q

When strong base added to strong acid?

A

Methyl red is originally red as the presence of H+ ions forces the equilibrium position to the left ; by adding OH- ions they react with H+ in the indicator and the weak acid dissociates, shifting the equilibrium position to the right - colour changes, first to orange at the end point and finally to yellow as the equilibrium position is shifted to the right

32
Q

If methyl orange is added to a basic solution and acid is added

A

H+ reacts with conjugate base
Equilibrium position shifts to the left
Colour changes ; first to orange at end point and finally to red when the equilibrium position has shifted to the left

33
Q

At the end point?

A

pH = pKa value of HA
Sensitivity of an indicator depends upon the indicator itself and eyesight : most indicators change colour over a range of about two pH units

34
Q

In a titration?

A

Must use an indicator that has a colour change which coincides with the vertical section of the pH titration curve ; ideally the end point and equivalence point would coincide ; but this may not always be possible and the end point may give a volume that is slightly different from the equivalence point but the difference will be very small

35
Q

For weak acid-weak base titrations

A

No indicator is suitable as there is no vertical section and even at its steepest, pH requires several cm^3 to pass through a typical pH indicator range of 2 pH units

A-Level Chemistry (28 decks)

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