Acids and Bases 8.4-

Question 1

What is pH?

Answer 1

pH is a measure of the concentration of H+ ions in a solution. The concentration can be stated in moldm-3 but the use of the log10 function simplifies the numbers involved.

pH is the negative logarithm to base 10 of the hydrogen ion concentration in an aqueous solution.
pH = -log10(H+)
So if the concentration of H+ ions in a solution is 1.57x10-3 so the pH is worked out as:
pH = -log10(1.57x10-3) = 2.8
Because pH is a log scale (to base 10), a 1 unit change in pH indicates a tenfold change in the H+ ion concentration.

Question 2

What does a 1 unit change in pH mean?

Answer 2

pH is the negative logarithm to base 10 of the hydrogen ion concentration in an aqueous solution. So a unit one change means a tenfold change in the H+ ion concentration.
H+/mol-3     pH
1                    0
0.1                  1
0.01               2
1x10-3            3
etc

Question 3

How do you calculate H+ concentration from pH?

Answer 3

(H+)=10-ph

Question 4

How does water dissociate?

Answer 4

H2O= H+ + OH-

Question 5

What is Kw?

Answer 5

This is the ionic product constant for water, it is calculated by multiplying the concentration of H+ ions by the concentration of OH- ions. It has a value of 1.0x10-14 at 298K.
Consider a 0.1 moldm-3 solution of HCl, it is a strong acid so will dissociate completely so the concentration of H+ ions will be 0.1, ANY SOLUTION IS 1x10-14 and so the OH- concentration must be 1x10-13.

Question 6

How do you work out concentrations of ions?

Answer 6

If a reaction is at equilibrium and in an aqueous solution the product of the concentration of the H+ ions and the OH- ions will always be equal to 1.0x10-14 at 298K.
So for example if you had 0.1moles of HCL, because it is a strong acid and therefore will fully dissociate, there will be 0.1 moles of HCL, then because the overall product is equal to 1x10-14, the concentration of the base must be 1x10-13.

Question 7

What are the concentrations like if a solution is acidic, neutral or alkaline?

Answer 7

Neutral - H+ = OH-
Acidic - H+>OH-
Alkaline - H+

Question 8

How do you calculate the pH solution of a strong acid?

Answer 8

Because strong acids can be assumed to dissociate fully in aqueous solution, the concentration of H+ ions is the same as the concentration of the acid.
For example, calculating the pH of a 0.00150 mol/dm-3 solution of hydrochloric acid. The acid is strong so full dissociation produces a H+ concentration of 0.00150mol/dm-3.
So the pH = -log10(H+)
-log10(0.00150) = 2.82

Question 9

If a 10cm solution of pH 1 is diluted with 90cm of water, what happens to the pH?

Answer 9

Because pH is a measure of concentration, and the concentration and we have increased the volume but not the H+ ions, the pH will increase. The solution has gone from 10 to 100 and has increased by tenfold, therefore the pH will increase by one, and the new pH is 2.

Question 10

If you dilute a solution of pH 2 by a factor of 10^6 what happens to the pH.

Answer 10

Increasing by a factor of 10 would increase the pH by 1, and therefore you would expect the solution to be pH 8, however you cannot dilute an acidic solution and make it alkaline, therefore the solution will get closer and closer to pH 7 but never reach it as there will always be that tiny concentration of H+ ions.

Question 11

How do you calculate the pH solution of a strong base?

Answer 11

The pH of an alkaline solution can be worked out by using the Kw constant 1.0x10-14 at 25 degrees.
For example, calculate the pH of a 0.250mol/dm-3 solution of potassium hydroxide at 25 degrees. Potassium hydroxide is a strong base and therefore the OH concentration is the same as the alkali. Then we know that at 25 degrees the concentration of OH- ions times the concentration of H+ ions equals 1x10-14. So you can divide 1x10-14 by 0.250 and get the H+ concentration. Then you get 4x10-14 and that is the H+ concentration. Then you -log10 that and get 13.4 which is the pH.

Question 12

What is a strong acid?

Answer 12

Strong acids such as HCl (hydrochloric acid), H2SO4 (sulphuric acid) and HNO3 (nitric acid) dissociate completely in aqueous solutions.

Question 13

Is sulphuric acid a strong acid or a weak acid?

Answer 13

It is but only for the first dissociation. Sulphuric acid is a diprotic acid, meaning that it dissociates to make two protons. It is a strong acid for this reaction, when it dissociates for the first time:
H2SO4 + H2O = HSO4- + H30+ (imagine an arrow)
This arrow shows that the dissociation goes to completion, and this is a strong acid. Then for the second reaction:
HSO4- + H3O+ = SO4 2- + H3O+ (imagine a reversible arrow), this does not dissociate completely and therefore this reaction is not a strong acid reaction).

Question 14

What is a monoprotic acid?

Answer 14

An acid that forms one proton per molecule when it dissociates, for example HCl. A diprotic molecule would form two protons when it dissociates per molecule, for example H2SO4.

Question 15

What are weak acids?

Answer 15

Weak acids only dissociate partially in aqueous solutions. Examples of weak acids are carbonic acid (H2CO3) and carboxylic acids such as ethanoic acid (CH3COOH). The dissociation of a weak acid is represented by the equilibrium arrow to show that the reaction is reversible and does not go to completion. The dissociation of ethanoic acid is represented by:
CH3COOH = CH3COO- (imagine reversible arrow).

Question 16

How does carbonic acid dissociate?

Answer 16

Carbonic acid is formed when carbon dioxide dissolves in water. It is a diprotic acid and so it dissociates twice and releases two protons.
H2CO3 = HCO3- + H+ (imagine reversible arrow)
This produces HCO3-, which is actually amphiprotic, meaning that it can both donate hydrogens and accept them. It accepts a proton in the first reverse reaction (acting as a base) and then in the reaction below it donates one (acting as an acid).
It dissociates again like this:
HCO3- = CO3 2- + H+ (imagine reversible arrow).

Question 17

What is a strong base?

Answer 17

Strong bases ionise completely in aqueous solutions. For example sodium hydroxide,
NaOH = Na+ + OH- (imagine normal arrow). Strong bases include the group one hydroxides.

Question 18

What is a weak base?

Answer 18

Weak bases ionise only partially in aqueous solution. Ammonia is a typical weak base and ionises according to the equation:
NH3 + H20 = NH4+ + OH- (imagine reversible arrow)

In a 0.10 mol/dm-3 solution of ammonia at 25 degrees only about 1.3% of the molecules are ionised. Other weak bases are amines, such as ethylamine which ionises according to the equation:
CH3CH2NH2 + H20 = CH3CH2NH3+ + OH- (imagine reversible arrow). At 25 degrees about 7.1% of molecules ionise so ethylamine is a stronger base than ammonia.

Question 19

What is the relationship between the strength of an acid and the strength of it’s conjugate base?

Answer 19

The stronger an acid the weaker it’s conjugate base. A strong acid such as HCl dissociates completely in aqueous solution.
HCl = H+ + Cl- (imagine normal arrow)
The conjugate base of HCl is Cl-, which is a very weak base because it has virtually no tendency to react with H20 or H+ to reform HCl, this is why the above reaction basically goes to completion.
HCN on the other hand is an extremely weak acid and has very little tendency to dissociate:
HCN + H20 = CN- + H30+ (imagine reversible arrow).
The equilibrium lies a long way to the left. CN- has a much stronger tendency to react with H20 or H+, that is, to react as a base.
The conjugate base of HCN is much stronger than HCl and when it is added to water will react back to form the conjugate acid.
So the weaker an acid, the stronger its conjugate base.
The same is true of bases.
The stronger the base the weaker the conjugate acid.

Question 20

How do you distinguish between a strong acid and a weak acid?

Answer 20

The methods for distinguishing between acids rely on the fact that strong acids dissociate more than weak acids.

1. Solutions of strong acids conduct electricity better than solutions of weak acids, because a strong acid dissociates fully there are more ions in solution to carry charge. A weak acid does not dissociate fully and has much less ions in solution to carry charge and therefore carries it much less effectively. The conductivity of solutions can be measured using a conductivity meter or by looking at the brightness of a bulb, the bulb will glow more brightly in solution of a strong acid than it will of a weak one. Strong acids may be described as strong electrolytes whereas weak acids can be describes as weak electrolytes. This also applies to weak and strong bases.
2. Strong acids have a lower pH than weak acids. This can be tested using a pH meter or a universal indicator. This can also be used for distinguishing between strong and weak bases.
3. Strong acids react more violently with metals or carbonates. This is because they have a higher concentration of free H+ ions and therefore react more rapidly with a metal such as magnesium to form hydrogen. This can be shown by a rapid bubbling when the metal is added. A similar effect is seen when a carbonate/hydrogen carbonate is added. Stronger acids would also have a faster rate of reaction but this is harder to observe visually.

Question 21

What is the difference between strength and concentration?

Answer 21

The concentration of an acid refers to the number of moles of an acid in a certain volume. The strength of an acid refers to how much it dissociates in aqueous solution. No matter how concentrated a weak acid is, it will never be a strong one because it wont dissociate fully.

Question 22

What can a titration not do?

Answer 22

A titration cannot distinguish between a strong and a weak acid, because a highly concentrated weak acid would have the same end point as a low in concentration strong one. pH can only be used to compare acid strength if equal concentrations of acids are being compared.

Question 23

What can pH not do?

Answer 23

pH is simply a measure of the concentration of H+ ions, it cannot measure how weak or strong an acid is. Because a high concentration of a weak acid would have more H+ ions even though it is weak than a low concentration of strong acid. It can only be used as an indicator if the concentrations are definitely the same.

Question 24

Are oxides of non-metals acidic or basic?

Answer 24

Oxides of non-metals are acidic, and dissolve in water to form acidic solutions.

Question 25

Is rain acidic or basic?

Answer 25

Rain is naturally acidic because of the dissolved carbon dioxide in it. It forms carbonic acid.
H2O + CO2 = H2CO3 (imagine reversible arrow)
Carbonic acid is a weak acid and dissociates partially according to the equation:
H2CO3 = H+ + HCO3-
Because of this reaction the pH of rain is about 5.6. This is natural and rain with acidity between 5.6 and 7 is not considered to be acid rain.

Question 26

What is the pH of acid rain?

Answer 26

Rain is naturally acidic because carbon dioxide from the air dissolves in it and forms carbonic acid which can dissociate to donate hydrogens. This means that naturally rain has a pH of around 5.6 and rain between 7 and 5.6 is not considered to be acid rain. Rain with a pH lower than 5.6 however, is considered to be acid rain.

Question 27

What is acid deposition?

Answer 27

Acid deposition is a more general term than acid rain. It refers to any process in which acidic substances (particles, gases and precipitation) leave the atmosphere to be deposited on the surface of the earth. It can be divided into wet deposition (acid rain, fog and snow) and dry deposition (acidic gases and particles).

Question 28

Give some examples of acidic pollutants?

Answer 28

Oxides of sulphur and nitrogen. Sulfur dioxide.

Question 29

How is sulphur dioxide harmful?

Answer 29

Sulfur dioxide can dissolve in water and make it acidic, or turn into sulphur oxide and dissolve in water, forming sulfuric acid.

Question 30

How are nitrogen oxides formed?

Answer 30

Nitrogen oxides are formed in internal combustion engines, coal burning, gas, or oil fuelled power stations. The production temperatures are very high and the oxidation of atmospheric nitrogen occurs forming NO.
N2 + O2 = 2NO
This can then be oxidised to N2O which can then react with a hydroxyl free radical to form nitric acid.
N20 + OH* = HNO3

Question 31

What are the problems with acid deposition?

Answer 31

• It has a bad effect on vegetation, it is not even necessarily the acid, the H+ ions can displace some of the metals in the soil that are then washed away. Particularly magnesium, calcium and potassium are needed for crops to grow, Mg2+ ions are needed to produce chlorophyll, so plants could therefore be prevented from photosynthesising properly if magnesium is washed away. The acid rain also causes aluminium ions to dissolve from rocks which damages plant roots and prevents water uptake. This can cause stunted growth and the thinning and or yellowing of leaves.
• Lakes and rivers can become acidic and the aquatic life is very sensitive to pH falling below 6. Insect larvae, fish and invertebrates, among others, cannot survive below pH 5.2 and below 4 basically no organisms will survive. Acid rain also dissolves harmful substances that can accumulate in lakes and damage life. In particular Al3+ ions damage fish gills.
• Limestone and marble are eroded by acid rain and dissolve away exposing a fresh surface to react with more acid.
  CaCO3 + H2SO4 = CaSO3 + CO2
• Acids irritate mucous membranes and cause respiratory illnesses such as asthma and bronchitis. Acidic water can dissolve heavy metal compounds releasing poisonous ions such as Cu2+, Pb2+ and Al3+ which may be linked to alzheimer’s disease.

Question 32

What are the methods of dealing with acid deposition?

Answer 32

• Improving the design of vehicle engines
• Using catalytic coverters
• removing sulfur before burning fuels
• using renewable power supplies
• making greater use of public transport
• designing more efficient power stations
• ‘liming’ of lakes - calcium oxide or hydroxide neutralises acidity
  For example:
  CaO + H2SO4 = CaSO4 + H2O

Question 33

What are pre- and post - combustion methods of reducing sulfur dioxide emissions?

Answer 33

Sulfur is present in fossil fuels such as coal and various fuels obtained from crude oil, such as gasoline (petrol). When these fuels are burnt sulfur dioxide is produced, which can contribute towards acid rain. To produce more environmentally friendly fuel the sulfur can either be removed from these fuels before they are burned or after the fuel has been burnt.
The regulations governing how much sulfur can be present in gasoline are quite strict and therefore sulfur is removed from the fuel before it is burned. Hydrodesulfurisation can be used - this involves heating crude oil fractions with hydrogen in the presence of a catalyst. This converts the sulphur to hydrogen sulfide, which can be removed from the solution by bubbling it through an alkaline solution. The H2S can then be converted back into sulfur and sold to companies wanting to make sulfuric acid. That is a pre combustion method.
Post combustion methods are commonly used in coal-fired power stations and involve passing the exhaust gases from the furnace through a vessel where the sulfur dioxide can react with alkalis/bases such as calcium oxide, calcium carbonate or calcium hydroxide. Calcium carbonate reacts with sulfur dioxide to form calcium sulfite.
CaCO3 + SO2 = CaSO3 + CO2

Question 34

What is Ka?

Answer 34

Ka is the acid dissociation constant. It is the equilibrium constant (concentration of products/concentration of reactants) times the concentration of water. The acid dissociation constant is a measure of extent to which an acid dissociates. The stronger the acid the more it dissociates, the higher the number on top, the higher the value of Ka.

Question 35

How do you calculate pKa?

Answer 35

Acid dissociation constants can be expressed in a more convenient form by taking the negative logarithm to the base 10 of Ka, like you do of concentration to make pH. A lower value of Ka (a weak acid) makes a higher pKa value,

Question 36

Calculate the Ka value for HCN at 25 degrees if a 0.5moldm solution dissociates to give a hydrogen ion concentration of 1.41x10^-5?

Answer 36

So you would use the Ka expression which is the concentration of the products over the concentration of the reactants. You know that one of the products is 1.41x10^-5 because it says in the question, and you know that the concentration of CN- must be the same as H+ because they are in the same ratio in the element. Therefore you know the two top concentrations and the bottom concentration is the original one in the question minus the ones that have changed into the products because this is it at equilibrium. The formula is at equilibrium.
1.41x1.41/0.5-1.41

Question 37

What is Kb?

Answer 37

Kb is the base ionisation constant. It is basically the same as Ka but for bases.

Question 38

What is different about Ka and Kb to Kw?

Answer 38

Kw is the constant of water, Ka and Kb are the constants of acids and bases, they do not include the concentration of water as it is very much higher than the other concentrations and it is essentially constant.

Question 39

What is pKb?

Answer 39

pKb, is the negative logarithm to the base 10 of the base ionisation constant. The higher the Kb the lower the value of pKb, so the lower the pKb the stronger the base.

Question 40

What do you have to be careful with, with pKb?

Answer 40

The lower the pKb value the stronger the base. This is annoying because the high value of pH14 would be a strong acid but the lower the value of pKa the closer it is to pH14.

Question 41

How do you calculate the pH solution of a weak acid?

Answer 41

A weak acid dissociates only partially and therefore in order to calculate the pH we need to know how much the acid dissociates. This can be worked out using the Ka or pKa value of the acid.
For example:
Calculate the pH of a 0.1mol solution of ethanoic acid at 25 degrees. The pKa of ethanoic acid at 25 degrees is 4.76.

1) Use the reverse equation
10^(pKa)
10^-4.76
Ka=1.73x10^-5

2) Then you use the equation to work out the concentrations
1.73x10^-5= x^2/0.1
x^2=0.1 x 1.73x10^-5
x^2=1.73x10^-6
x=1.315x10^-3

3) then pH is the concentration of hydrogen to the negative logarithm of base 10.
- log1.315= 2.88

Question 42

How do you calculate the Ka for a weak acid from it’s pH?

Answer 42

You have to reverse the pH equation to get hydrogen concentration and then use that to work out the Ka.
For example:
Calculate the value of the acid dissociation constant for benzoic acid if a 0.250moldm solution has a pH of 2.4?

1) Reverse the pH equation
10^-2.4 = 3.98x10^-3

2) Then use that concentration to work out the pH
(3. 98x10^-3) x (3.98x10^-3) / 0.250 = 6.3395x10^-5

That is the answer!

Question 43

What is the pOH?

Answer 43

This is the negative logarithm to the base 10 of the concentration of OH- ions.
pOH = -log10(OH-)

Question 44

What is the relationship between pOH and pH and Kw?

Answer 44

pOH and pH are the negative logarithms to the base 10 of the concentrations. Kw is the concentration of hydrogen x hydroxide ions, therefore because with logs adding is multiplying, if you then turn Kw into a p, but negative logarithming it, then it is the same as adding the pOH and the pH.
pKw= pH + pOH

Question 45

What is special about pH and pOH?

Answer 45

pH+pOH =14
This is useful when we have questions like this:
Calculate the pH of 0.05moldm sodium hydroxide solution at 25 degrees:
-log(0.05) = 1.3
14-1.3=12.69

Question 46

If pH

Answer 46

The solution is acidic, because if the pOH is big then it is a weak base, or an acid.

Question 47

If pOH

Answer 47

Then the solution is alkaline because the pH is big.

Question 48

How do you calculate pH or pOH for a weak base?

Answer 48

Because a weak base ionises only partially in aqueous solution, we must use Kb or pKb to work out the concentration of OH- ions present in a solution.
Calculate pH and pOH of a 0.120moldm-3 solution of ammonia, given that its pKb is 4.75 at 25 degrees:

1) 10^-4.75 = 1.778x10^-5 is the Kb

2) 1.778x10^-5 = x^2 / 0.120
x^2 = 2.1336
x= 1.46x10^-3

3) -log(1.46x10^-3) = 2.835 is the pH
the pOH is therefore
14-2.835 = 11.16

Question 49

What happens to Kw with different temperatures?

Answer 49

Kw increases with temperature, but remember at 25 degrees it is 1x10^-14

Question 50

What is pH 7?

Answer 50

pH 7 is the hydrogen concentration to the negative logarithm of base 10, but as Kw equals 1x10^-14 only at 25 degrees we can see that pH7 is only neutral at 25 degrees.

Question 51

What is the relationship between Ka and Kb?

Answer 51

If you times Ka by Kb, because they are equilibrium products over reactants equations and they are the reverse of each other they cancel, meaning that the equation is basically the concentration of H+ times the concentration of OH- which is the equation to calculate Kw and therefore:
Ka x Kb = Kw

And this therefore means that
pKa + pKb = pKw
Because logs times one another are added.

Question 52

What does pKa + pKb = pKw actually mean?

Answer 52

This means that the lower the value of pKa for example, the stronger the acid, and therefore the higher the value of pKb and therefore the weaker the base. So it means that the stronger the acid the weaker its conjugate base or viseversa.

Question 53

What is the pH curve for a strong acid and a strong base?

Answer 53

The titration curve for adding 0.1 mol of sodium hydroxide to 0.1 mol of hydrochloric acid, it starts at pH 1, and increases slowly and extremely gradually with a very very small gradient until it hits the equivalence point at pH7 where the curve increases with such a high gradient it is almost vertical in this region.
The very tiny increase at the start is due to the fact that there is such a high H+ ion concentration that adding NaOH has very very little effect on the pH. It also arises as a result of the nature of logarithms, because even if the concentration decreases from 0.1 to 0.01 that is a 10 fold change but only correlates to a change of 1 pH unit.
In the vertical change region the H+ ion concentration is already low and so adding NaOH causes a bigger change in pH. The log scale magnifies this, in this period it only decreases by 0.00031 but because it decreases from 3.15x10^-4 to 3.15x10^11 the pH changes by about 7 units.

Question 54

What is the equivalence point?

Answer 54

The equivalence point is the point at which equivalent numbers of moles of acid and base have been added. The pH at the equivalence point for a strong acid-base titration is pH 7. If the concentrations of both are equal then the equivalence point should be when the volumes of both are equal.

Question 55

If you doubled the amount of one of the acid or alkali in a titration what would happen to the pH curve?

Answer 55

If you got 0.1mol of HCl, and added 0.2mols of NaOH, they are both strong acids and strong bases. It would be the same shape as the 0.1 to 0.1 graph however the volume needed would be halved and so the equivalence point would occur earlier, but the pH at the equivalence point would be the same, it would just occur at an earlier point.

Question 56

What is the weak acid - strong base titration curve?

Answer 56

Let us consider adding 0.1moldm NaOH to 25cm^3 of 0.1moldm CH3COOH. Use the pKa value of ethanoic acid to work out the original Ka so you can work out the hydrogen ion concentration. The equivalence point occurs when the same number of moles of acid and alkali have been added, but the pH at this point will not be 7 but higher than 7 because the acid is weak. If you compare the weak acid curve with the strong acid curve they are the same sort of shape but the weak acid curve is a lot higher up as it starts at a higher pH. Note that the strength of the acid does not effect the amount needed to reach the equivalence point, this is purely based on volume alone, because the equivalence point IS NOT when it is neutral but just when there are equal volumes. In the weaker acid curve the line is also steeper because the acid only partially dissociates and therefore the concentration of H+ ions is slower and more easily changed.

Question 57

What is a strong acid - weak base titration curve?

Answer 57

In a similar way to the weak acid - strong base, the equivalence point will be at the same point, but the pH will be lower, this is because the weak base does not dissociate and therefore the acid does not move up much.

Question 58

What is the weak acid - weak base titration curve?

Answer 58

There is no very steep (almost vertical) part in this titration curve, and the change in pH throughout the titration is more gradual than in the other titrations we have considered. The pH at the equivalence point may be lower or higher than 7 depending on the relative strength of the acid and the base.

Question 59

In maths terms, what is the equivalence point?

Answer 59

The equivalence point is the point of inflexion on the curve. A point of inflection is where the gradient (slope) of the curve stops increasing (or decreasing) and starts decreasing (or increasing).

Question 60

What is the difference between end point and equivalence point?

Answer 60

The equivalence point of a titration is the point at which equivalent numbers of moles of acid and alkali have been added. The end point of a titration is the point at which the indicator changes colour - these are not necessarily the same.

Because only strong acid - strong base titrations have pH =7 at the end point, it is important to remember that the indicator is being used to determine the point at which equivalent numbers of moles of acid and alkali have been added and not the point at which pH=7.

Question 61

Why are titrations carried out?

Answer 61

To establish the equivalent amounts of acid and base that react with each other, and hence the concentration of the acid or the alkali.

Question 62

What are indicators?

Answer 62

Either weak acids or weak bases. For example, if it is a weak acid for example, Hln, the indicator dissociates according to the equation:
Hln (a red colour) = H+ + Ln- (a blue colour) (imagine reversible arrow)

The blue ionised Ln- and the un ionised HLn forms must have different colours for the substance to function as an indicator. If we add acid to the solution, we are adding H+ ions and therefore the equilibrium will shift to the left to use up the H+ that has been added, the solution will turn red and we will see that acid has been added.
If we add some base to the solution we are adding OH- ions and as Chatelier’s principle, equilibrium shifts to the right to use up the OH- ions that have been added. This restores the values of Kc.

If we consider an indicator that is a weak base, it will split like this:
Ln- (red) + H2O = Hln (blue) + OH-
If you add an acid to this, equilibrium will shift to the left to react with the OH- ions. If a base is added it will react with the H+ ions to form water.

Question 63

What is the pH range of an indicator?

Answer 63

The pH range of an indicator is the pH values between which the indicator has intermediate colours because comparable amounts of the un-ionised and ionised forms are present. Where possible, an indicator should be chosen so that the equivalence point of the titration occurs within the pH range of the indicator. Basically the indicator must be chosen to give a sharp end point, so if it goes gradually from yellow to blue passing through green for example, this gradual change needs to occur very very quickly and so need to be exactly over the vertical part of the curve, so when the equivalence point is.

Consider a strong acid - strong base titration. In the vertical region of the curve, approximately one drop of alkali causes the pH to change by about 6-7 units. Therefore, if an indicator has its range in this part of the curve, adding one drop of alkali will cause the indicator to change from the acid colour to the alkali colour.

If you use an indicator that has too early a pH range then it will change colour before the equivalence point, and so you will not be able to detect it.

DAISY’S EXPLANATION = You need to pick an indicator from a table with a pH range vaguely in the right pH where you think the equivalence point is going to be. Because at the equivalence point the pH changes by about 5 pH units anyway, an indicator vaguely in this range will change colour at this point and as there is only 1cm3 difference lots of indicators will do. For example if you have a weak acid and a strong base you know the equivalence point will occur above 7, and so you pick an indicator above this point, and then in that 1cm3 difference where it changes by 5pH units the colour will change.

Question 64

How do you work out the pKa value of an indicator?

Answer 64

Consider the dissociation of an indicator that is a weak acid.
HLn = H+ + ln-
The Ka expression for this acid is
(H+)(ln-)/(Hln)
At the midpoint, half of the Hln will have dissociated into ln-, so you can cancel those.
So at the midpoint Ka = (H+)
so pKa = pH ONLY AT THE MIDPOINT!
Therefore this sort of works out the pH of an indicator. Therefore we can see if an indicator is suitable by seeing if the pKa value would be near the midpoint of the reaction.

Question 65

How is a salt formed?

Answer 65

A salt is formed when an acid reacts with a base.

Question 66

What will the salt of a weak acid and a strong base do in water?

Answer 66

The base is stronger than the acid and therefore when the salt is split by water molecules, when it dissolves (salt hydrolysis), it will be basic.

Question 67

What is the salt of a strong acid and a strong base?

Answer 67

Consider sodium chloride, NaCl. This is made from the strong acid HCl and the strong base NaOH. These are both fully dissociated in aqueous solution, so a solution of NaCl is neutral (pH 7 at 25 degrees).

Question 68

What is the pH of a weak acid and a weak base’s salt?

Answer 68

It is not possible to predict whether it will be acidic or basic in water unless you can look at the pKa values. For example if the acid has a pKa value of 9.24 and the alkali has a value of pKa being 9.25. As the acid has a slightly lower value it will be slightly acidic. But because they are so close together it will be basically pH 7.

Question 69

What is the affect of positive ions in solution?

Answer 69

Positive ions in solutions are hydrated to form aqueous ions, eg. Fe3+, [Fe(H2O)6 ]3+
This dissociates to form [Fe(H20)6]2+ + H+, and therefore adds acidity to the solution. All 3+ ions in solutions are acidic, this comes up a lot in exams!
The reason this occurs is because of the high charge density (or charge to radius ratio) of the ion which causes the water molecules to be sufficiently polarised for H+ to dissociate. The Fe3+ ion pulls electrons away from the H2O molecules which increases the positive delta charge on the hydrogen and therefore it is more easily lost as H+. The larger an ion is and the lower it’s charge, then the smaller the charge density of the ion and the lower its tendency to polarise water molecules. So the Fe2+ ion will cause less polarisation of water molecules and, although a a solution of Fe2+ will still be acidic, it will have a higher pH than a solution of Fe3+ of the same concentration.

Question 70

What is a buffer solution?

Answer 70

A solution that resists changes in pH when small amounts of acid or alkali are added. A buffer solution consists of two components and must always contain something to react with any acid added and something to react with any base added. In other words, a buffer solution always contains an acid and a base.

Question 71

Why are buffer solutions important?

Answer 71

They are important for lots of industrial processes where pH remaining the same is important, for example in the blood if it rises or decreases by 0.5 you die.

Question 72

What does an acid buffer solution contain?

Answer 72

An acid buffer solution consists of a weak acid and a salt of that weak acid. So it is in equilibrium moving from the salt to the acid + Hydrogen ions. If some acid is added to the solution the added H+ reacts with the salt in the solution CH3COO-, to make the acid. If some alkali is added to the solution the added OH- reacts with the acid to make the salt and water instead of the salt and hydrogen.

The H+ added is basically ‘mopped up’ by reaction with the ethanoate ion (a base). The OH- added is ‘mopped up’ by reaction with the ethanoic acid.

Question 73

Why does the pH in a buffer solution change so little?

Answer 73

If we look at the expression for Ka we can understand why the pH changes so little.
Ka = (CH3COO-) X (H+) / (CH3COOH)
This can be rearranged as:
(H+) = (CH3COOH)/(CH3COO-) x Ka
Because Ka is a constant, it means that the H+ concentration is proportional to the ratio of the ethanoic acid concentration to the ethanoate ion concentration. For the solution to function as a buffer, both CH3COOH and CH3COO- must be large so that any changes in their concentrations that occur when an acid or a base are added are small compared with these concentrations. This means that the value of the ratio changes very little and the H+ concentration changes very little.

Question 74

What is in a basic buffer solution?

Answer 74

It consists of a weak base and a salt of that weak base. So the equilibrium is like:

NH3 + H2O = NH4+ OH-

If some hydrochloric acid is added then it reacts with the NH3 in the solution to make NH4+. If some alkali is added it reacts with the NH4+ in the solution to make water and NH3.

Question 75

How do you make a buffer solution by partial neutralisation of a weak acid/base?

Answer 75

A buffer solution can be made by partial neutralisation of a weak acid with a strong base - for example adding sodium hydroxide to ethanoic acid. When they are added together some of the ethanoic acid will be converted to sodium ethanoate and some will remain as ethanoic acid. So the solution contains both ethanoic acid and sodium ethanoate which are components of a buffer solution. As long as the number of moles of sodium hydroxide are less than the number of moles of ethanoic acid present in the original solution then it can remain as a buffer.
You can also make it with a strong acid and a weak base, as long as the strong acid never has more moles than the weak base it will act as a buffer.

Question 76

According to the Bronsted Lowry theory how does each species act in the equilibrium below?
CH3COOH + H2SO3 = CH3COOH2+ + HSO3-

Answer 76

This was an exam question and is confusing because CH3COOH is a carboxylic acid, but here because the Bronsted Lowry theory says that an acid is a proton donator and a base is a proton acceptor, CH3COOH is actually a base, because it accepts a proton. Then H2SO3 looses a proton and therefore is a proton donator and therefore an acid.
CH3COOH2+ has the ability to now donate a proton and therefore is a Bronsted Lowry acid, and HSO3- will accept a proton and therefore be a Bronsted Lowry base.