Topic 8 : Acids and Bases Flashcards
1
Q
The Brønsted-Lowry Theory
A
- The Brønsted-Lowry Theory defines acids and bases in terms of proton transfer between chemical compounds
- A Brønsted-Lowry acid is a species that gives away a proton (H+)
- A Brønsted-Lowry base is a species that accepts a proton (H+) using its lone pair of electrons
- The Brønsted-Lowry Theory is not limited to aqueous solutions only and can also be applied to reactions that occur in the gas phase
2
Q
Conjugate acid-base pairs
A
- A conjugate acid-base pair is two species that are different from each other by a H+ ion
- Conjugate here means related
- In other words, the acid and base are related to each other by one proton difference
3
Q
Amphiprotic Species
A
- Species that can act both as proton donors and acceptors are called amphiprotic
- Eg. water as a Brønsted-Lowry acid
4
Q
amphiprotic and amphoteric
A
- A compound that is amphoteric means it has both basic and acidic character. When the compound reacts with an acid, it shows that it has basic character. When it reacts with a base, it shows that it’s acidic
- An example of amphoteric is aluminium oxide
- When a compound is amphiprotic, it means it can act as a proton donor and as a proton acceptor
- Aluminium oxide is not amphiprotic, even though it is amphoteric
5
Q
Difference between amphiprotic and amphoteric
A
- Amphiprotic describes a substance that can accept and donate a proton or H+ while amphoteric means the ability to act as both an acid and a base
- All amphiprotic substances are amphoteric but not all amphoteric substances are amphiprotic
6
Q
A
7
Q
Reaction between metals and acids
A
- acid + metal → salt + hydrogen
- the extent of reaction depends on the reactivity of the metal and the strength of the acid
- Metals low in reactivity do not react at all, for instance copper does not react with dilute acids
- Stronger acids will react more vigorously with metals than weak acids.
- Using stronger acids will make the reaction more exothermic and the metal will dissolve faster
8
Q
Reaction between metals and oxides
A
- acid + metal oxide → salt + water
9
Q
Reaction between metals and hydroxides
A
acid + metal hydroxide → salt + water
10
Q
Reaction between metals and carbonates
A
- acid + metal carbonate → salt + water + carbon dioxide
11
Q
Reaction between metals and hydrogen carbonates
A
acid + metal hydrogencarbonate → salt + water + carbon dioxide
12
Q
Neutralization Reaction
A
- A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt
- The proton of the acid reacts with the hydroxide of the base to form water
- The spectator ions which are not involved in the formation of water, form the salt
Acid + Base (alkali) → Salt + Water
S
13
Q
Salt produced in neutralization reaction with hydrochloric acid
A
a chloride
14
Q
Salt produced in neutralization reaction with nitric acid
A
A nitrate
15
Q
Salt produced in neutralization reaction with sulfuric acid
A
a sulfate
16
Q
Acid-Base Titration
A
- Acid-base titrations are used to find the unknown concentrations of solutions of acids and bases
17
Q
Acid-Base Indicators
A
- Acid-base indicators give information about the change in chemical environment. They change colour reversibly depending on the concentration of H+ ions in the solution
- Indicators are weak acids and bases where the conjugate bases and acids have a different colour
- Many acid-base indicators are derived from plants, such as litmus
- A good indicator gives a very sharp colour change at the equivalence point
18
Q
Litmus paper in acid and alkali
A
Acid - Pink
Alkali - Blue
19
Q
Methyl Orange in acid and alkali
A
Acid - Red
Alkali - Yellow
20
Q
Phenolphthalein in acid and alkali
A
Acid - colourless
Alkali - Pink
21
Q
pH Scale
A
- The acidity of an aqueous solution depends on the number of H+ (H3O+) ions in solution
- pH is defined as pH = -log10[H+] where [H+] is the concentration of H+ in mol dm–3
- The pH scale is a logarithmic scale with base 10
This means that each value is 10 times the value below it. For example, pH 5 is 10 times more acidic than pH 6. - It is a numerical scale that shows how acidic or alkaline a solution is
- The scale goes from 0-14 (extremely acidic substances have values of below 0)
22
Q
Acids and Alkalis in the pH scale
A
- All acids have pH values of below 7, all alkalis have pH values above 7
- The lower the pH then the more acidic the solution is
- The higher the pH then the more alkaline the solution is
23
Q
pH of acids
A
- Acidic solutions (strong or weak) always have more H+ than OH- ions
- Since the concentration of H+ is always greater than the concentration of OH- ions, [H+] is always greater than 10-7 mol dm-3
- Using the pH formula, this means that the pH of acidic solutions is always below 7
- The higher the [H+] of the acid, the lower the pH
24
Q
pH of bases
A
- Basic solutions (strong or weak) always have more OH- than H+ ions
- Since the concentration of OH- is always greater than the concentration of H+ ions, [H+] is always smaller than 10-7 mol dm-3
- Using the pH formula, this means that the pH of basic solutions is always above 7
- The higher the [OH-] of the base, the higher the pH
25
Q
The pH of water
A
- Water at 298K has equal amounts of OH- and H+ ions with concentrations of 10-7 mol dm-3
- water has a pH of 7 at 298 K
- An equilibrium exists in water where few water molecules dissociate into proton and hydroxide ions
- Since the concentration the H+ and OH- ions is very small, the concentration of water is considered to be a constant
- The product of the two ion concentrations is always 10-14 mol2 dm-6
26
Q
Measuring pH
A
- Most accurate way is through a pH meter
- Universal Indicator
27
Q
pH meter
A
- The pH meter is connected to the pH electrode which shows the pH value of the solution
28
Q
Universal Indicator
A
- The universal indicator paper is dipped into a solution of acid upon which the paper changes colour
- The colour is then compared to those on a chart which shows the colours corresponding to different pH values
29
Q
Strong acids
A
- A strong acid is an acid that dissociates almost completely in aqueous solutions
- e.g. HCl (hydrochloric acid), HNO3 (nitric acid) and H2SO4 (sulfuric acid)
- The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction
- The solution formed is highly acidic due to the high concentration of the H+/H3O+ ions
30
Q
Weak Acids
A
- A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
- Eg. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)
- The position of the equilibrium is more over to the left and an equilibrium is established
- The solution is less acidic due to the lower concentration of H+/H3O+ ions
31
Q
Strong Bases
A
- A strong base is a base that dissociates almost completely in aqueous solutions
- E.g. group 1 metal hydroxides such as NaOH (sodium hydroxide)
- The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction
- The solution formed is highly basic due to the high concentration of the OH- ions
32
Q
Weak Bases
A
- A weak base is a base that partially (or incompletely) dissociates in aqueous solutions
- e.g. NH3 (ammonia) and amines
- The position of the equilibrium is more to the left and an equilibrium is established
- The solution is less basic than in strong bases due to the lower concentration of OH- ions
33
Q
Conjugate Pairs & Acid-Base Strength
A
- In general strong acids produce weak conjugate bases and weak acids produce strong conjugate bases
34
Q
Strong vs Weak Acids
A
Can be distinguished by their
1. pH value (using a pH meter or universal indicator)
2. Electrical conductivity
3. Reactivity
35
Q
pH value of strong vs weak acids
A
- An acid dissociates into H+ in solution according to:
HA → H+ + A- - The stronger the acid, the greater the concentration of H+ and therefore the lower the pH
36
Q
electrical conductivity of strong vs weak acids
A
- Since a stronger acid has a higher concentration of H+ it conducts electricity better
- Stronger acids therefore have a greater electrical conductivity
- The electrical conductivity can be determined by using a conductivity meter
- Like the pH meter, the conductivity meter is connected to an electrode. The conductivity of the solution can be read off the meter
37
Q
reactivity of strong vs weak acids
A
- Strong and weak acids of the same concentrations react differently with reactive metals
- This is because the concentration of H+ is greater in strong acids compared to weak acids
- The greater H+ concentration means that more H2 gas is produced in a shorter time
38
Q
Acid Rain
A
- Rain is naturally acidic because of dissolved CO2 which forms carbonic acid
H2O (l) + CO2 (g) ⇌ H2CO3 (aq) - Carbonic acid is a weak acid and dissociates in the following equilibrium reaction giving a pH of 5.6
H2CO3 (aq) ⇌ H+ (aq) + HCO3- (aq) - For that reason acid rain is defined as rain with a pH of below 5.6
39
Q
Acid Deposition
A
- Acid deposition includes all processes by which acidic components leave the atmosphere
- This could be gases or precipitates
- There are two types of acid deposition: wet acid deposition and dry acid deposition
- Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3
40
Q
Wet acid deposition
A
Wet acid deposition refers to rain, snow, sleet, hail, fog, mist and dew
41
Q
Dry acid deposition
A
Dry acid deposition refers to acidic particles and gases that fall to the ground as dust and smoke
42
Q
Formation of sulfur based acids
A
- Fossil fuels are often contaminated with small amounts of sulfur impurities
- When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulfur dioxide
S (s) + O2 (g) → SO2 (g) - Sulfur dioxide may be further oxidised to sulfur trioxide
2SO2 (g) + O2 (g) ⇌ 2SO3 (g) - The sulfur dioxide and sulfur trioxide then dissolve in rainwater droplets to form sulfurous acid and sulfuric acid
SO2(g) + H2O (l) → H2SO3 (aq)
SO3 (g) + H2O (l) → H2SO4 (aq) - These acids are components of acid rain which has several damaging impacts on the environment
43
Q
Formation of acid rain by nitrogen oxides
A
- The temperature in an internal combustion engine can reach over 2000 °C
- Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide:
N2 (g)+ O2 (g) ⇌ 2NO (g) - Nitrogen monoxide reacts further forming nitrogen dioxide:
2NO (g) + O2 (g) ⇌ 2NO2 (g) - Nitrogen dioxide gas reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain:
2NO2 (g) + H2O (l) → HNO2 (aq) + HNO3 (aq) - Lightning strikes can also trigger the formation of nitrogen monoxide and nitrogen dioxides in air
- Nitrogen dioxide gas reacts with rain water and more oxygen to form nitric acid
4NO2 (g) + 2H2O (l) + O2 (g)→ 4HNO3 (aq) - When the clouds rise, the temperature decreases, and the droplets get larger
- When the droplet containing these acids are heavy enough, they will fall down as acid rain
44
Q
Effects of acid deposition
A
- Affects materials
- Affects Plants
- Affects Water
- Affects Human Health
45
Q
How acid deposition affects materials
A
- Acid deposition can react with metals and rocks (such as limestone) causing buildings and statues to get damaged
46
Q
How acid deposition affects plants
A
- Apart from acid deposition directly falling on leaves and killing plants, acid particulates can block stomata ( plant pores) and prevent gaseous exchange
- Acid rain can fall on soils and release important minerals such as magnesium, calcium and potassium which are leached (washed out) from soils and are therefore unavailable to plants
- Aluminium ions released from rocks are toxic to many plants and damage their roots
47
Q
How acid deposition affects water
A
- When acid rain falls on rivers and lakes the pH can fall to levels that are unable to support life
- Below about pH 4, aluminium ions are released from rocks when they are held as aluminium hydroxide
- Alumnium ions are toxic to fish as they damage the gills and prevent fish from efficiently absorbing oxygen
- Nitrate ions from nitric acid in acid rain can contribute to over-fertilization of waterways and lead to eutrophication. Eutrophication is excessive algal growth that results in oxygen depletion and stagnation of waterways
48
Q
How acid deposition affects human health
A
- Although acid rain is too dilute to cause any direct impact on the skin, acidic particulates in the air can increase the risk of respiratory diseases such as bronchitis, asthma and emphysema
- When acid rain comes into contact with metal pipes there is an increased risk that toxic metal ions will be released into the water supply such as Cu2+, Al3+ and Pb2+
49
Q
Reducing Sulfur Oxide Emissions
A
- The removal of sulfur from fossil fuels can either take place pre-combustion or post-combustion
- The oxides of sulfur, SO2 and SO3, are both acidic and toxic gases
- Sulfur dioxide is produced naturally during volcanic eruptions, but large quantities have been and continue to be emitted by burning coal, oil and natural gas
50
Q
Pre-combustion removal of sulfur oxide
A
- Pre-combustion of sulfur takes place for coal and petroleum, although it is expensive to remove all the sulfur, so a small percentage often remains
- It is essential to remove most of the sulfur as it damages the workings of internal combustion engines
- The sulfur is removed by reacting it with hydrogen in a process called hydrodesulfurization
- The sulfur is recovered and used in the manufacture of sulfuric acid
51
Q
Post-combustion removal of sulfur oxide
A
- Post-combustion is carried out on in coal-fired power stations
- The waste gases from burning the coal contain sulfur dioxide
- The waste gases are passed through a wet slurry of calcium oxide and calcium carbonate which react with the SO2 and produce calcium sulfate
CaO (s) + SO2 (g) + ½O2 (g) → CaSO4 (s)
CaCO3 (s) + SO2 (g) + ½O2 (g) → CaSO4 (s) + CO2 (g)
- The calcium sulfate is also known as gypsum and is used to make plasterboard and other useful building materials
52
Q
Global Policies in Sulfur Oxide
A
- Global policies working towards combating global warming will have the additional benefit in reducing acid deposition
- As we switch away from burning fossil fuels for energy there will be a fall in the emission of oxides of sulfur and nitrogen, which can only be a good thing for the environment
- Ultimately reducing the emission of primary pollutants is achieved by greater use of renewable energy sources, greater use of public transport and more efficient energy transfer systems
