Chemistry 12Acids, Bases and Salts
Metal, Non-metal and Metalloid Oxides
(also called Anhydrides)
Metal oxides act as BASES in aqueous solution.
Group 1 and Group 2 Oxides are ionic. They dissociate to form the oxide ion (O2-)
Eg. Na2O (s) 2 Na+(aq) + O2-(aq)
The oxide ( O2- ) ion then undergoes 100% hydrolysis (because it’s a strong base)
O2-(aq) + H2O (l) 2 OH-(aq)
example: BaO (s) Ba2+(aq) + O2-(aq)
We can also summarize the reactions of group 1 and group 2 metals with water in the form of formula equations:
Na2O + H2O 2 NaOH
BaO + H2O Ba(OH)2
Write a balanced formula equation for the overall reactions of the following oxides with water:
Calcium oxide:
______CaO + H2O Ca(OH)2______
Lithium oxide:
______Li2O + H2O 2 LiOH______
Non-Metal Oxides act as ACIDS in aqueous solution:
Some common examples of non-metal oxides:
NO2 , N2O5 , SO2 , SO3 , CO2 , Cl2O
These compounds react with water to form ACIDS.
The formula equations for some of these are:
SO2(g) + H2O(l) H2SO3(aq) (sulphurous acid)
2NO2(g) + H2O(l) HNO3 (aq) + HNO2 (aq)
(nitric nitrous acids)
Once these acids are formed, they can ionize (strong ones 100%, weak ones < 100%) to form H3O+ ions.
Eg. H2SO3(aq) + H2O(l) H3O+(aq) + HSO3-(aq) ( < 100% ionization since H2SO3 is a weak acid)
Eg. HNO3(aq) + H2O(l) H3O+(aq) + NO3-(aq) (100% ionization since HNO3 is a strong acid)
Metalloid Oxides (by staircase)
Eg. Al2O3 , Ga2O3 , GeO2 These compounds usually have LOW solubility so not many ions are able to undergo hydrolysis. So very little hydrolysis occurs so they do not act as acids or bases.
These compounds can react WITH acids or bases. Compounds that can do this are called amphoteric.
Anhydrides
Oxide compounds that react as BOTH acids and bases in aqueous solution are also called Anhydrides. (an-hydride translates to “without water”)
These are compounds that react WITH water to form acidic or basic solutions.
Acidic Anhydride—An oxide (“O” containing) compound which reacts with water to form an ACIDIC SOLUTION.
Acidic anhydrides are oxides of elements on the RIGHT side of the periodic table.
Some examples of acidic anhydrides are: SO2 , SO3 , Cl2O etc.
And some of their reactions with water are:
SO3(g) + H2O(l) H2SO4(aq) (sulphuric acid—a strong acid)
2 NO2(g) + H2O(l) HNO2 (aq) + HNO3(aq) (nitrous and nitric acids)
Cl2O (aq) + H2O(l) 2HClO (hypochlorous acid)
(NOTE: You should KNOW these equations!)
Basic Anhydride—An oxide (“O” containing) compound which reacts with water to form a BASIC SOLUTION.
NOTE: Basic Anhydrides are METALS (LEFT side of Periodic Table) oxides.
Some examples are: Na2O , CaO , MgO , CaO ….etc.
Formula equations for some Basic Anhydrides reacting with water:
Na2O + H2O 2 NaOH
CaO + H2O Ca(OH)2 (the base is called calcium hydroxide –
sometimes called “hydrated lime”)
Acid Rain
Since our atmosphere naturally contains CO2 (an acidic anhydride), some of this reacts with water (rain) to make the rain slightly acidic:
CO2(g) + 2H2O(l) H3O+(aq) + HCO3-(aq)
So natural rainwater (unaffected by human activities) can have a pH as low as 5.6 (due to the CO2 in air)
If rain has a pH < 5.6 it is called ACID RAIN.
Acid Rain is caused by Acidic Anhydrides (not counting CO2) in the air.
The main human sources of acid rain are:
1. Burning fuels containing sulphur.
2. Car exhaust
- When burning coal or other fuels containing sulphur, the sulphur burns to form sulphur dioxide:
S(s) + O2(g) SO2(g) (an acidic anhydride)
In the atmosphere further oxidation can occur producing sulphur trioxide:
2 SO2(g) + O2(g) 2 SO3(g) (an acidic anhydride)
In rainwater or cloud droplets two reactions can then occur:
SO2(g) + H2O(l) H2SO3(aq) (sulphurous acid – a weak acid)
SO3(g) + H2O(l) H2SO4(aq) (sulphuric acid – a strong acid)
2. In the hot cylinders in an internal combustion engine, N2 from the air and O2 from the air react to form nitrogen oxides:
N2(g) + O2(g) 2NO (g) (nitrogen monoxide – an acidic anhydride)
N2(g) + 2O2(g) 2NO2(g) (nitrogen dioxide – an acidic anhydride)
Nitrogen monoxide can further oxidize in the air to produce nitrogen dioxide:
2NO(g) + O2(g) 2 NO2(g)
Nitrogen dioxide reacts with rain water or cloud droplets to produce both nitrous and nitric acids:
2 NO2(g) + H2O(l) HNO2 (aq) + HNO3(aq)
Some natural sources of Acid Rain
Volcanoes can produce SO2 into the atmosphere (which produces both H2SO3 and H2SO4 )
Lightning can provide enough energy to cause nitrogen and oxygen in the air to react and form NO2 (which produces HNO2 and HNO3 ).
Natural Protection
Some areas are not as sensitive to acid rain as others — even when there are major sources of acid rain present! This is because these areas have rocks and soils that are high in “carbonates” or compounds containing carbonate (CO32-). The CO32- acts as a weak base and neutralizes the acid rain to a certain extent:
Eg.) H2SO4(aq) + CaCO3(s) CaSO4(s) + CO2(g) + H2O(l) (This neutralizes the H2SO4 caused by acid rain)
In some cases, powdered limestone (CaCO3(s)) is dumped into lakes that are too acidic and this helps neutralize the acid rain. But the process is expensive.
Problems Associated with Acid Rain (see p. 187-188 in SW.)
- Aquatic life is affected (from bottom of food chain up)
- Forests weakened and killed
- Minerals are leached out of the topsoil to lower levels. Al3+ ions are released which are very toxic to fish and plants
- Metal and stone buildings and statues (especially marble (CaCO3)) are damaged.
- Acid Rain is carried over large distances (due to high smoke stacks)
Possible Solutions to the Problem
- International conferences and agreements to limit sulphur in fuels and NOx in car exhaust.
- Alternate, less polluting energy sources being used. (geothermal, solar, wind etc.)
- Industrial processes are being modernized (eg. new process for pulp mills involving H2O2 instead of sulphites.)
- Devices to remove gases like SO3 from smoke stacks. (scrubbers). SO3 + CaO CaSO4(s). The
CaSO4 is removed by electrostatic precipitation. - Recycling
Metal, Non-metal and Metalloid Oxides
(also called Anhydrides)
Metal oxides act as BASES in aqueous solution.
Group 1 and Group 2 Oxides are ______They dissociate to form the ______
Eg. Na2O (s) 2 Na+(aq) + O2-(aq)
The oxide ( O2- ) ion then undergoes 100% hydrolysis (______)
______
Ex: BaO (s) ______
We can also summarize the reactions of group 1 and group 2 metals with water in the form of formula equations:
Na2O + H2O 2 NaOH
BaO + H2O ______
Write a balanced formula equation for the overall reactions of the following oxides with water:
Calcium oxide: ______
Lithium oxide: ______
Non-Metal Oxides act as ACIDS in aqueous solution:
Some common examples of non-metal oxides:
______
These compounds react with water to form ______.
The formula equations for some of these are:
SO2(g) + H2O(l) ______(sulphurous acid)
2NO2(g) + H2O(l) ______+ ______(nitric & nitrous acids)
Once these acids are formed, they can ionize (strong ones 100%, weak ones < 100%) to form H3O+ ions.
Eg. H2SO3(aq) + H2O(l) ______( < 100% ionization since H2SO3 is a weak acid)
Eg. HNO3(aq) + H2O(l) ______(100% ionization since HNO3 is a strong acid)
Metalloid Oxides (by staircase)
Eg. Al2O3 , Ga2O3 , GeO2 These compounds usually have ______
______. So very little hydrolysis occurs so they do not act as acids or bases. These compounds can react WITH acids or bases.
Compounds that can do this are called ______
Anhydrides
Oxide compounds that react as acids or bases in aqueous solution are also called Anhydrides. (an-hydride translates to “without water”). These are compounds that react WITH water to form acidic or basic solutions.
Acidic Anhydride—
Acidic anhydrides are oxides of elements on the ______.
Some examples of acidic anhydrides are: SO2 , SO3 , Cl2O etc.
And some of their reactions with water are:
SO3(g) + H2O(l) ______(sulphuric acid—a strong acid)
2 NO2(g) + H2O(l) ______(nitrous and nitric acids)
Cl2O (aq) + H2O(l) ______(hypochlorous acid)
(NOTE: You should KNOW these equations!)
Basic Anhydride—
NOTE: Basic Anhydrides are METALS (______) oxides.
Some examples are: Na2O , CaO , MgO , CaO ….etc.
Formula equations for some Basic Anhydrides reacting with water:
Na2O + H2O ______(the base is called sodium hydroxide)
CaO + H2O ______(the base is called calcium hydroxide – sometimes called “hydrated lime”)
Read p. 184-185 in SW.
Do Ex. 144-145 on p. 185 of SW.
Acid Rain
Since our atmosphere naturally contains CO2 (an acidic anhydride), some of this reacts with water (rain) to make the rain slightly acidic:
CO2(g) + 2H2O(l) ______
So natural rainwater (unaffected by human activities) can have a pH as low as ______(due to the CO2 in air)
If rain has a ______it is called ACID RAIN.
Acid Rain is caused by Acidic Anhydrides (not counting CO2) in the air.
The main human sources of acid rain are:
1. ______
2. ______
Here’s why…
1. When burning coal or other fuels containing sulphur, the sulphur burns to form sulphur dioxide:
______(an ______)
In the atmosphere further oxidation can occur producing sulphur trioxide:
______(an ______)
In rainwater or cloud droplets two reactions can then occur:
______(sulphurous acid – a weak acid)
______(sulphuric acid – a strong acid)
2. In the hot cylinders in an internal combustion engine, N2 from the air and O2 from the air react to form nitrogen oxides:
______(nitrogen monoxide – an acidic anhydride)
______(nitrogen dioxide – an acidic anhydride)
Nitrogen monoxide can further oxidize in the air to produce nitrogen dioxide:
2NO(g) + O2(g) 2 NO2(g)
Nitrogen dioxide reacts with rainwater to produce both nitrous and nitric acids:
2 NO2(g) + H2O(l) HNO2 (aq) + HNO3(aq)
Some natural sources of Acid Rain
Volcanoes can produce SO2 into the atmosphere (which produces both H2SO3 and H2SO4 )
Lightning can provide enough energy to cause nitrogen and oxygen in the air to react and form NO2 (which produces HNO2 and HNO3 ).
Natural Protection
Some areas are not as sensitive to acid rain as others — even when there are major sources of acid
rain present! This is because these areas have ______
______or compounds containing carbonate (CO32-). The CO32- acts as a
______and neutralizes the acid rain to a certain extent:
Eg.) H2SO4(aq) + CaCO3(s) CaSO4(s) + CO2(g) + H2O(l) (This neutralizes the H2SO4 caused by acid rain)
In some cases, powdered limestone (CaCO3(s)) is dumped into lakes that are too acidic and this helps neutralize the acid rain. But the process is expensive.
Problems Associated with Acid Rain (see p. 187-188 in SW.)
- Aquatic life is affected (from bottom of food chain up)
- Forests weakened and killed
- Minerals are leached out of the topsoil to lower levels. Al3+ ions are released which are very toxic to fish and plants
- Metal and stone buildings and statues (especially marble (CaCO3)) are damaged.
- Acid Rain is carried over large distances (due to high smoke stacks)
Possible Solutions to the Problem
- International conferences and agreements to limit sulphur in fuels and NOx in car exhaust.
- Alternate, less polluting energy sources being used. (geothermal, solar, wind etc.)
- Industrial processes are being modernized (eg. new process for pulp mills involving H2O2 instead of sulphites.)
- Devices to remove gases like SO3 from smoke stacks. (scrubbers). SO3 + CaO CaSO4(s). The CaSO4 is removed by electrostatic precipitation.
- Recycling