Karteikarten
Buffers and Neutralisation Flashcards
Karten in diesem Stapel (30)
- 1What is a buffer?A buffer is a solution which RESISTS CHANGES IN pH when small quantities of acid or alkali are added, or on dilution
- 2What two components do buffer solutions contain to remove added acid or alkali?- Component 1 = A WEAK ACID, HA (removes the added alkali)
- Component 2 = A CONJUGATE BASE, A- (removes the added acid) - 3What are some uses of buffer solutions?- Standardising pH meters
- Buffering biological systems eg. the blood
- Maintaining the pH of shampoos - 4How do buffer solutions work and what are they designed to act like?A buffer solution is designed to keep the concentrations of H+ and OH- ALMOST UNCHANGED
It has to contain things that will remove any H+/OH- ions that you might add to it
Otherwise, the pH will change
They are based on EQUILIBRIUM REACTIONS
The pH doesn't change by very much but you should NOT assume that the pH stays completely constant - 5What are the two types of weak acid buffer solutions?- A weak acid and a SALT of the weak acid (salts of weak acids are IONIC COMPOUNDS and provide a convenient source of the conjugate base)
- Excess of weak acid and strong alkali (partial neutralisation) - 6Show how a buffer solution can be formed from a weak acid and a salt of the weak acidEg. a mixture of ethanoic acid (weak acid) and sodium ethanoate (salt of the weak acid)
Ethanoic acid is a weak acid and PARTIALLY ionises in solution
CH3COOH (aq) ---> CH3COO- (aq) + H+ (aq) (reversible)
Sodium ethanoate will ionise COMPLETELY in water:
CH3COONa + (aq) ---> Na+ (aq) + CH3COO- (aq)
In the buffer solution, there exists the equilibrium:
CH3COOH (aq) ---> CH3COO- (aq) + H+ (aq) - 7So, what 3 things does the solution contain in weak acid/salt of the weak acid buffer solutions contain, using the example of CH3COOH/CH3COONa?- Lots of un-ionised CH3COOH
- Lots of ethanoate ions (CH3COO-) from the SODIUM ETHANOATE
- Enough H+ ions to make the solution ACIDIC - 8How does the buffer solution in a weak acid/salt of the weak acid solution control pH, explained by Le Chatelier's principle?HA (aq) ---> A- (aq) + H+ (aq) (reversible)
When a small amount of ACID is added:
- [H+] increases
- H+ ions react with A- to make HA
- The equilibrium shifts to the LEFT, producing more HA and thus reducing the amount of H+ ions
Due to the relatively large conc. of HA and A- ions, the ratio of conc. of HA to A- remains the SAME. Thus, the pH remains FAIRLY CONSTANT, but will change slightly
When a small amount of ALKALI is added:
- [OH-] increases
- The small concentration of H+ ions reacts with the OH- ions
H+ + OH- ---> H2O
- HA dissociates, shifting the equilibrium position to the RIGHT, to restore most of the [H+] ions - 9How can a buffer solution be formed from an excess of weak acid and strong alkali? (partial neutralisation method)Buffers can also be prepared by the PARTIAL NEUTRALISATION of a weak acid, by a STRONG BASE, like NaOH
To do this, the weak acid must be present in EXCESS so [OH-] < [HA]
The weak acid is partially neutralised by the alkali, forming a CONJUGATE BASE
Eg. CH3COOH + OH- ---> CH3COO- + H2O
Some of the weak acid is left over, unreacted
The resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid - 10Different weak acids result in buffer solutions that operate over different pH ranges When is a buffer most effective? How do you know which weak acid to use?A buffer is most effective at removing either added acid or alkali when there are EQUAL CONCENTRATIONS of the weak acid and its conjugate base
When [HA] = [A-]
- The pH of the buffer solution is the same as the pKa value of HA
- The operating pH is typically over about two pH units, centred at the pH of the pKa value
eg. CH3COOH pKa = 4.76, so, pH operating range = 3.76 - 5.76
The ratio of the concentrations of the weak acid and its conjugate base can then be adjusted to fine-tune the pH of the buffer solution - 11To calculate the pH of a buffer solution, work out [H+] using the Ka expression What is the formula for calculating Ka of a buffer solution and why is this different to calculating Ka of weak acids?When you calculate the pH of a weak acid, you assume [H+] = [A-]
For a buffer solution, this is no longer true as A- has been added as one of the components of the buffer
Ka = [H+] x [salt]/[acid]
So, to calculate pH:
[H+] = Ka x [acid]/[salt]
pH = -log[H+]
When the concentrations of [HA] and [A-] are the same:
Ka = [H+]
So, pH = pKa - 12Name two assumptions for buffer calculations1. All the anions come from the SALT
2. Almost all the acid molecules put into the buffer remain UNCHANGED - 13What are the steps for calculating the pH of a buffer solution from a weak acid/salt of the weak acid buffer, given the value of Ka?1. Find the moles acid and salt of the acid
2. Work out the total volume
3. Work out the concentration of each, using c = n/v
4. Put into the formula with Ka, to work out [H+]
5. Work out pH - 14What are the steps for calculating the pH of a buffer solution from excess acid/strong alkali buffer from partial neutralisation, given the value of Ka?1. Calculate the moles alkali in the buffer solution
2. Calculate the moles of acid used
3. Do moles acid used - moles alkali in buffer solution (added) to find the moles of acid in the buffer solution
4. Then, do the normal steps 1-5 from the previous flashcard, working out concentrations, putting into the formula and working out pH - 15Buffers in the body: How does a buffer control blood pH?Buffers are found in many biological systems
The human body relies on precise pH controls and different parts of the body needing specific pH values eg. due to different enzymes having different optimum pH's
Blood plasma needs to be maintained at a pH between 7.35 - 7.45
Normal, healthy blood is at a pH of 7.40
Blood plasma contains CARBONIC ACID, from dissolved CO2 and HYDROGENCARBONATE IONS (H2CO3/HCO3-)
This mixture of a weak acid and a sodium salt (blood contains sodium hydrogencarbonate) BUFFERS the pH of the blood
The same chemicals are also part of the waste transport system - 16What happens if blood pH gets too high/low?When the pH of blood falls below 7.35, people can develop ACIDOSIS
(can cause fatigue, shortness of breath etc.)
When the pH of blood rises above 7.45, people can develop ALKALOSIS
(can cause muscle spasms, nausea etc.) - 17Describe specifically how the carbonic acid/hydrogencarbonate buffer system works- Carbonic acid, H2CO3, is a WEAK ACID
- Hydrogencarbonate ion, HCO3-, is the CONJUGATE BASE of carbonic acid
H2CO3 (aq) ---> HCO3- (aq) + H+ (aq)
(reversible)
ADDING H+:
- [H+] increases
- H+ ions react with the conjugate base, HCO3-
- The equilibrium shifts to the left, removing the H+ ions
ADDING OH-:
- [OH-] increases
- OH- ions react with the small concentration of H+ ions to produce H2O
- H2CO3 dissociates, shifting the equilibrium to the right, to restore the H+ ions - 18Define a neutralisation reaction How does pH link to neutralisation in acid-base titrations?Neutralisation reaction = a reaction between an acid and an alkali
The CONCENTRATION of an acid can be determined by titrating it against a STANDARD SOLUTION of an alkali, using a suitable INDICATOR
Similarly, the concentration of an alkali can be determined by titrating it against a standard acid solution - 19Describe how to use a pH meterA pH meter consists of an ELECTRODE that is dipped into a SOLUTION and connected to a meter, what displays the pH reading
A pH meter typically records pH values to 2 decimal places
Indicator paper is usually matched from colour charts to the nearest whole number, so a pH meter is more accurate
pH meters can be used to determine pH values during an acid-base titration, by measuring the pH of alkali/acid in a conical flask after each 1cm3 addition (and swirl of contents) of an acid/alkali from the burette
When the pH starts to change more rapidly, add the aqueous base drop-wise for each reading, until the pH changes less rapidly. Then, add the base 1cm3 at a time again until an excess has been added and the pH hasn't changed much, for several additions
From this data, pH-titration curves can be drawn! - 20When a simple acid-base titration is carried out, an INDICATOR is used to show when the acid and alkali have been mixed in exactly the right proportions to 'neutralise' each other What is the end point vs equivalence point of a titration?End point = when the indicator changes colour
Equivalence point = the volume of one solution that EXACTLY reacts with the volume of the other solution. The solutions have exactly reacted with one another and the amounts used match the stoichiometry of the reaction
So, the 'end-point' of a titration isn't necessarily the same as the equivalence point - 21What is on the x and y axes of pH titration curves? How is the shape of pH titration curves different when you measure volume of base added compared to volume of acid added?y axis = pH
x axis = volume of acid/base added
When measuring BASE ADDED to acid, the curve is 'S' direction shaped
When measuring ACID ADDED to base, the curve is flipped vertically
Look at folder pictures for the differences! - 22What are some of the key features of pH titration curves?- Vertical section = when the pH changes RAPIDLY on addition of a very small volume of acid/base, acid and base concentrations are SIMILAR
- Equivalence point on a pH titration curve = the CENTRE of the vertical section of the pH titration curve - 23What are the 4 possible combination of acid and base, assuming that an aqueous acid is delivered from a burette to an aqueous base in a conical flask?- Strong acid into strong base
- Strong acid into weak base
- Weak acid into strong base
- Weak acid into weak base
These each produce DIFFERENT pH CURVES! - 24How do the pH titration curves and equivalence points differ for the 4 combinations (look in folder for actual shapes)- Strong acid, strong alkali equivalence point = 7
- Strong acid, weak alkali equivalence point = less than 7
- Weak acid, strong alkali equivalence point = greater than 7
- Weak acid, weak base - pH at equivalence point depends on RELATIVE strength of acid and base - NO CLEAR VERTICAL SECTION, instead, there is a 'point of inflection' - the lack of a steep bit makes this titration hard to carry out - 25What are indicators?Indicators are WEAK ACIDS which have a DIFFERENT COLOUR to their conjugate base
HA ---> A- + H+ (reversible)
The HA will be one colour and the A- will be another
In a LOW pH, the equilibrium is pushed to the LEFT, forming the HA colour
In a HIGH pH, the equilibrium is pushed RIGHT, forming the A- colour - 26How do you know which indicator to choose for which of the 4 combinations of reactions?For an indicator to work in a titration, it must have a colour change which COINCIDES WITH THE VERTICAL SECTION of the pH titration curve
Ideally, the end point and the equivalence point would coincide, however, this may not be possible and the end point may give a volume that is slightly different from the equivalence point
Any difference may be very small = 1-2 drops - 27What are the 2 indicators to know about and what are their colours at low/high pH's?METHYL ORANGE:
- RED at low pH
- ORANGE at high pH
- pH range = 3.2 - 4.4
PHENOLPHTALEIN:
- COLOURLESS at low pH
- PINK at high pH
- pH range = 8.2-10.0 - 28Indicators and Ka values (are weak acids!)Different indicators have different Ka values and change colour over different pH changes
At the end point, [HA] = [A-]
So, Ka = [H+]
So, pKa = pH - 29Which indicator(s) should be used in which titrations, based on coinciding pH ranges and vertical sections?- Strong acid/strong base = EITHER methyl orange or phenolphthalein
- Strong acid/weak base = methyl orange
- Weak acid/strong base = phenolpthalein
- Weak acid/weak base = no suitable indicator (need to use a pH meter) - 30—
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- A level Chemistry Karteikarten am Buffers and Neutralisation Flashcards, erstellt von Rhea Sekhon am 22/04/2022.
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