Flashcards

buffers

megan trent A level Chemistry English 49 cards

How does a buffer work? Eg Ethanoic acid (acid) and Sodium ethanoate (salt)

Cards in this deck (49)

1A BUFFER SOLUTIONcan resist changes in pH despite the addition of small amounts of acid or alkali.
2A buffer is a solution containing:EITHER: A weak acid and the salt of that weak acid eg CH3COOH(aq) + CH3COONa(aq)
OR: A weak alkali and the salt of that weak alkali. eg NH4OH(aq) + NH4Cl(aq)
3Many biochemical reactions take place under exactpH conditions. Even small changes in pH for the fluids in living organisms can have serious consequences. For example, our blood is buffered at pH 7.4, and a change of only 0.5 pH units can be fatal.
4How does a buffer work?Buffers are examples of equilibrium systems. A typical buffer consists of a weak acid AND its salt, for buffers below 7 or a weak base AND its salt for buffers above 7
5How does a buffer work? Eg Ethanoic acid (acid) and Sodium ethanoate (salt)
6How does a buffer work? Eg Ethanoic acid (acid) and Sodium ethanoate (salt) If you add Acid to the system, What happens?A buffer contains a large reservoir of CH3COOH and CH3COO- . On addition of a small amount of acid (H+) the equilibria position shifts to the LHS the H+ reacting with the CH3COO- therefore the ration of [H+] / [CH3COO-] hardly changes and pH stays roughly constant.
7How does a buffer work? Eg Ethanoic acid (acid) and Sodium ethanoate (salt) If you add alkali to the system what happens?A buffer contains a large reservoir of CH3COOH and CH3COO- . On addition of a small amount of alkai (OH-) the H+ ions react with the OH- ions. the equilibria position shifts to the RHS, more acid then dissociates to maintain the concentration of H+ , therefore the ration of [H+] / [CH3COO-] hardly changes and pH stays roughly constant.
8What do you understand by a buffer?A solution of a weak acid and its salt or a weak base and its salt that resists change of pH on addition of a small amount of acid or base
9BUFFER CALCULATIONS Calculate the pH of a buffer solution which contains 0.1 mol dm-3ethanoic acid and 0.2 mol dm-3 sodium ethanoate. Ka for ethanoic acid = 1.7 x 10-5 mol dm-3. Assume:- (1) [CH3COOH(aq)]original = [CH3COOH(aq)]equilibrium Why can you make this assumption?Ethanoic acid is a weak acid with very little/ no dissociation, dissociation is further suppressed by the high concentration of CH3COO- from the salt.
10Calculate the pH of a buffer solution which contains 0.1 mol dm-3ethanoic acid and 0.2 mol dm-3 sodium ethanoate. Ka for ethanoic acid = 1.7 x 10-5 mol dm-3. Assume:- (2) [CH3COONa(aq)]original = [CH3COO-(aq)]equilibrium Why can you make this assumption?the salt is soluble and is completely dissociated and acid dissociated an insignificant amount.
We also assume that [H+(aq)] from water is negligible and can be ignored, but this is not one of the main assuptions.
11Write an equation to illustrate the dissociation of ethanoic acid:CH3COOH ⇌ CH3COO- + H+
CH3COOH + H2O ⇌ CH3COO- + H3O+
Leave the H2O out of the Ka expression as it is a solvent and also it never put in Ka
12Write an expression for Ka, for the dissociation of ethanoic acid: and rearrange the equation
13Substitute in values,
14the pH of a buffer solution depends on theratio of the concentrations of acid and salt and not on the actual concentrations.
15A special case arises if the
16USES OF BUFFERSIn healthy people blood pH lies in the narrow range of 7.35-7.45, a small shift from this can lead to serious complications.
Various process within the body tend to disrupt the normal pH balance, for example respiration which produces CO2, the CO2 reacts with the water to produce Carbonic acid.
Equ CO2 + H2O ⇌ H2CO3
However the blood pH stays constant because of the buffering action, in particular the buffer system based on the equilibrium between CO2, water and hydrogencarbonate ions.
Two major organs are involved in the buffering action, the lungs to remove CO2 and the kidneys to control the HCO3- ions.
The pH can be changed by a variety of respiratory and metabolic causes. For example overuse of diuretics increases the amount of urine excreted, if this contains large amounts of acid the blood pH will rise. If this happens the equilibrium shown above will move to the RHS
Any increase in the con H+ ions for example the production of lactic acid in the muscles will move the equilibrium to the LHS
17Buffer calculations What is the pH of a buffer solution made by making up 1.00 dm3 of solution containing 1.00 mole of ethanoic acid and 1.00 mole of sodium ethanoate.
18pH TITRATION CURVES Experiment 1 STRONG ACID / STRONG BASE (MANUAL) diagram
19pH TITRATION CURVES Experiment 1 STRONG ACID / STRONG BASE (MANUAL) what happensWhen an alkali is added to an acid the pH changes rapidly at neutralisation. By drawing a graph of pH (y axis) against volume of alkali added (x axis), the change in pH during the titration can be monitored and the equivalence point found.
20The equivalence pointis the point at which: no. moles H+(aq) = no. moles OH-(aq)
21how to work out equilivence pointDraw tangent to steepest portion of graph = region of rapid change.
Equivalence point = halfway on vertical portion.
In this case it corresponds to pH 7
22Equivalence point is whenmoles acid=moles base
23pH TITRATION CURVES Experiment 2 WEAK ACID / STRONG BASE On which two factors does the starting pH depend?Concentration of the acid in mol dm-3
Degree of dissociation, Ka
24pH TITRATION CURVES Experiment 2 WEAK ACID / STRONG BASE Lower region of graph. Comment on the shape of the curve and on the species present which are responsible for this shape.Rapid increase as OH- added.
After sufficient salt has been formed there is enough HCOO- and HCOOH to act as a buffer so additional OH- causes relatively little increase = buffering action.
Eventually the HCOOH is used up so excess OH- cannot be buffered  pH ↑
255) CALCULATION OF THE ACID DISSOCIATION CONSTANT Ka FOR ETHANOIC ACID FROM pH TITRATION CURVE. no need to make associations whyno need to make the assumptions as values are being taken from the graph
26What condition could be controlled to improve the accuracy of your value for Ka. ExplainEnsure that there are no temperature changes – unlikely a major factor as solutions were dilute however the experiment may not be being conducted at 298K used in quoted value
dissociated.
27An indicatoris a weak acid or a weak base,
which has one colour in the undissociated form and a different colour as a dissociated ion.
28Consider a weak acid, HIn which satisfies these conditions. HIn(aq) ⇌ H+(aq) + In-(aq) red blue Apply Le Chatelier's Principle. If excess H+(aq) ions are added, i) what will happen to the equilibrium position? ii) What colour will you see?Shifts to LHS
red
29Consider a weak acid, HIn which satisfies these conditions. HIn(aq) ⇌ H+(aq) + In-(aq) red blue Apply Le Chatelier's Principle. If excess H+(aq) ions are added, If excess OH-(aq) ions are added, What will happen to the equilibrium position? What colour will you see?Shifts to RHS
blue
30End point of an indicator is when[In] = [HIn]
31Is the end point of a titration when the acid and the alkali used have reacted exactly?Yes
32The ideal indicator for a given acid - base titration is one that isin the middle of its colour change at the pH of the equivalence point of the titration. i.e. end point = equivalence point
33This pH depends on the acid and base being titrated. Write an expression for the acid dissociation constant, KIn:
34Colour changes of some common indicators
35At the equivalence point of a titration, the pH must changesharply by several units if the end point of the titration is to coincide accurately with the equivalence point.
36pH titration curve
37Equivalence point explanationsWhen a salt is dissolved in water, the resulting solution may be acidic, alkaline or neutral. Acidic or alkaline solutions are produced when the salts are hydrolysed by water. This is important as it has an effect at the equivalence point of titrations when there is only salt and water present.
381) SALT OF A STRONG ACID AND A STRONG BASE e.g. NaCl
392) SALT OF WEAK ACID AND A STRONG BASE e.g. CH3COONa
403) SALT OF A STRONG ACID AND A WEAK BASE e.g. NH4Cl
414) SALT OF A WEAK ACID AND A WEAK BASE e.g. CH3COONH4
42WEAK ACID / WEAK BASEThe pH changes slowly around the equivalence point.
No indicator will change colour clearly when a small volume of alkali is added at the equivalence point. The solution is buffered throughout this titration A pH meter can be used to detect the equivalence point.
43acidic buffers contain a weak acid and its conjugate base
44acidic buffers contain a weak acid and its conjugate base 2
45alkaline buffers are made from a weak base and one of its salts
46buffer action can be seen on a titration curve
47buffer solutions are important in blood
48how to calculate the pH of a buffer solution
49you need to be able to calculate concentration