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Chemistry AS Level CIE > Chapter 13 Nitrogen > Flashcards

Chapter 13 Nitrogen Flashcards

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1
Q

Reactivity of Nitrogen

A
  • Nitrogen is a diatomic molecule and the main unreactive gas in air
  • 78% of air is nitrogen gas
  • The lack of reactivity of nitrogen gas can be explained by looking at its intramolecular bonds
  • Intramolecular bonds are the bonds within a molecule
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2
Q

Bonding in nitrogen

A
  • The electron configuration of a nitrogen atom is 1s^2 2s^2 2p^3
  • To achieve a full outer shell of electrons, it needs to gain three electrons
  • Nitrogen atoms therefore form a triple covalent bond between two nitrogen atoms in which they share three electrons with each other
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3
Q

The bond enthalpy of the nitrogen triple bond is

A
  • 1000 kJ mol-1
  • This means that 1000 kJ of energy is needed to break one mole of N2 triple bond
  • As it is so difficult to break the nitrogen triple bond, nitrogen and oxygen gas in air will not react with each other
  • Only under extreme conditions will nitrogen gas react (eg. during a thunderstorm)
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4
Q

Polarity of nitrogen

A
  • The electrons in a nitrogen molecule are shared equally between the two nitrogen atoms
  • Therefore, nitrogen molecules are nonpolar molecules
  • Due to the lack of polarity, nitrogen gas is not attracted to or likely to react with other molecules the way polar molecules would
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5
Q

Properties of Ammonia

A
  • Ammonia is a compound of nitrogen and will turn damp red litmus paper blue as it is an alkaline gas
  • Ammonia is made on a large scale in industry using the Haber process:

N2(g) + 3H2(g) ⇌ 2NH3(g)

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6
Q

Basicity of ammonia

A

-Ammonia can act as a Brønsted–Lowry base by accepting a proton (H+) using the lone pair of electrons on the nitrogen atom to form an ammonium ion:

NH3(aq) + H+(aq) → NH4+(aq)

-In an aqueous solution of ammonia, an equilibrium mixture is established

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq)

  • Since the position of the equilibrium lies well over to the left the ammonia solution is only weakly alkaline
  • There is a higher concentration of ammonia molecules than hydroxide ions in solution
  • Ammonia is therefore a weak base
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7
Q

formation of ammonium ion

A

-The ammonium ion is formed by an acid-base reaction of ammonia with water:

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq)

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8
Q

Structure of ammonium ion

A
  • The ammonium ion has a tetrahedral shape in which all bonds have the same length
  • The nitrogen in ammonia is covalently bonded to three hydrogen atoms and has one lone pair of electrons causing the ammonia molecule to have a pyramidal shape

-The nitrogen atom in ammonia uses its lone pair of electrons to form a dative bond with a proton to form the ammonium ion

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9
Q

Preparation of ammonia gas from an ammonium salt

A

-Ammonia gas can be prepared from an ammonium salt and a base in an acid-base reaction:

Ammonium chloride (NH4Cl) and calcium hydroxide (Ca(OH)2) are mixed together and then heated

  • NH4+ acts as an acid (proton donor) and OH– acts as a base (proton acceptor)
  • This acid-base reaction can be used to test if an unknown solution contains ammonium ions
  • If the unknown solution does contain ammonium ions, it will react with calcium hydroxide to form ammonia gas
  • This ammonia gas will turn damp red litmus paper blue
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10
Q

Nitrogen oxide as a catalyst

A
  • Acid rain also contains dilute sulfuric acid (H2SO4)
  • Sulfur(IV) oxide (SO2) is another pollutant found in the atmosphere
  • When SO2 is oxidised, it forms SO3 which reacts with rainwater to form dilute sulfuric acid as follows:

SO3(g) + H2O(l) → H2SO4(aq)

  • Nitrogen oxides can directly cause acid rain but can also act as catalysts in the formation of acid rain
  • NO2 catalyses the oxidation of SO2 to SO3:

NO2(g) + SO2(g) → SO3(g) + NO(g)

-The formed NO gets oxidised to regenerate NO2:

NO(g) + ½ O2(g) → NO2(g)

-The regenerated NO2 molecule can get again oxidise another SO2 molecule to SO3 which will react with rainwater to form H2SO4

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11
Q

Formation of acid rain by nitrogen oxides

A

-As mentioned earlier, lightning strikes trigger the formation of nitrogen(II) and nitrogen (IV) oxides in air:

2NO(g) + O2(g) ⇌ 2NO2(g)

  • The air also contains oxygen and tiny droplets of water that make up clouds
  • The nitrogen(IV) oxide (NO2) dissolves and reacts in water with oxygen as follows:

NO2(aq) + H2O(l) + 1½O2(g) → 2HNO3(aq)

  • When the clouds rise, the temperature decreases, and the droplets get larger
  • When the droplet containing dilute nitric acid are heavy enough, they will fall down as acid rain
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12
Q

Peroxyacetyl nitrate (PAN, CH3CO3NO2)

A

is one of the harmful pollutants found in photochemical smog

-PAN affects the lungs and eyes and in high concentrations plant-life

secondary pollutant

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13
Q

volatile organic compound (VOCs)

A

-Exhaust fumes contain another primary pollutant

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14
Q

nitrogen oxides (NO/NO2)

A
  • Nitrogen oxides are extra dangerous as they can react with substances in the air to make secondary pollutants
  • These are pollutants that are not given off directly into the air from human activity

eg: lightning, car exhausts, power plants

is primary

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15
Q

primary pollutants

A

because they are given off directly into the air from the source of pollution

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16
Q

Catalytic removal of nitrogen oxides

A
  • The nitrogen oxides released through cars’ exhaust fumes pollute the atmosphere
  • Many car exhaust systems are therefore fitted with catalytic converters to reduce the pollutants from motor vehicles
  • The nitrogen oxides are reduced on the surface of the hot catalyst (eg. platinum) to form the unreactive and harmless nitrogen gas which is then released from the vehicle’s exhaust pipe into the atmosphere
  • The chemical reaction for the reduction of nitrogen oxide to nitrogen gas by the catalyst is as follows:

2CO(g) + 2NO(g) → 2CO2(g) + N2(g)

17
Q

Man-made occurrence of nitrogen oxides

A
  • In the engine of a car, a mixture of air and fuel is compressed and ignited by a spark
  • Air consists of 78% of nitrogen and 21% of oxygen
  • Under this high pressure and temperature inside a car engine, nitrogen can react with oxygen to form nitrogen oxides
  • These nitrogen oxides are released into the atmosphere through the car’s exhaust fumes
18
Q

Natural occurrence of nitrogen oxides

A
  • Due to its lack of reactivity, only under extreme conditions will nitrogen react with oxygen to form gaseous nitrogen oxides
  • An example of these extreme conditions is lightning which can trigger the formation of nitrogen(II) and nitrogen(IV) oxides (NO and NO2 respectively)

-The chemical equations for these reactions are:

N2(g) + O2(g) → 2NO(g)

N2(g) + 2O2(g) → 2NO2(g)

Chemistry AS Level CIE (21 decks)

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