Chapter 21 Flashcards by Ishir Sharma

What is a buffer solution

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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Weak acid role?

HA removes added alkali

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Conjugate base role

A- removes added acid

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When alkalis or acids are added to the buffer?

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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How to prepare a buffer solution?

Needs a weak acid and it’s conjugate base

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First method of preparing buffer solution?

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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CH3COOH dissociation

CH3COOH -> <- H+ + CH3COO-

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Salts of weak acids?

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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Second way of preparing a buffer?

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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Ethanoic acid equilibrium

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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What’s so special about the ethanoic acid equilibrium?

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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Acid buffer solution

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

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Addition of an acid

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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Addition of an alkali?

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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When is a buffer most effective?

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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pH of a buffer solution depends on

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

When calculating pH of a weak acid?

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

pH of buffer solution equation

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

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

pH = pKa

pH control in the body

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

If pH slips outside this range?

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

ph scale sensitivity?

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

On addition of acid to carbonic acid-hydrogencarbonate system

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

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

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

Body produces…

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

ph meter

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

Stages of tirration curve?

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

Equivalence point

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

Where is the equivalence point on the curve?

Centre of the vertical section of the pH titration curve

Acid base indicator?

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

When strong base added to strong acid?

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

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

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

At the end point?

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

In a titration?

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

For weak acid-weak base titrations

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