Gases in the air
Composition of Air:
- Nitrogen (N2) = 78%
- Oxygen (O2) = 21%
- Carbon dioxide (CO2) = 0.04%
- Argon (Ar) = 0.9%
- Other gases (including water vapour and other noble gases) = 0.06
Experiment to Determine % of Oxygen in the Air:
Appearance change:
Shiny pink-brown → Black
- Equations: 2Cu (s) + O2(g) → 2CuO (s)
Copper + Oxygen → Copper (II) oxide
- Volume: (Initial)
When heated, the gas initially rises as it expands.
Starts to decrease whilst heating with a Bunsen burner as the oxygen reacts with copper.
When oxygen reacts with copper syringes to allow oxygen to pass through, a 'lump' of copper is presented.
- Gases are passed down the tube over oxide copper as copper has a higher SA, speeding up the reaction.
- Copper turnings are in excess to ensure all oxygen has reacted.
- Cu is used, not Mg, as it is very reactive.
- Carbon/Charcoal can't be used as CO2 will just add more gas.
- All the oxygen from the air in the gas syringes has reacted when the reading on the gas syringe stays the same.
- The apparatus should be cooled to room temperature. Before taking the final volume reading, make sure the gas wasn't expanded when it was hot.
Experimental Results:
- The final volume may be 79 cm³. ∴ there could be a leak where the O₂ escaped.
- Final volume may be 79cm3 ∴ ① Not all oxygen reacted with copper and ② the reading was taken when the gas hadn't cooled.
Carbon Dioxide:
- CO2 is a colourless, odourless gas, denser than air, slightly soluble in water, and doesn't allow substances to burn in it.
Uses:
- Putting out fires
- Carbonated drinks
- Dry ice
Remember: Acid + Metal Carbonate → Salt + Water + Carbon dioxide
CO2 can be made by:
1. Reacting metal carbonates with acids
- CaCO3(s) + 2HCl (aq) → CaCl2(aq) + H2O (l) + CO2(g)
2. Thermal decomposition of a metal carbonate
- When copper (II) carbonate was heated, it turned into a black solid.
- The limewater turned cloudy, showing the presence of CO₂.
- With limewater:
CO2 (g) + Ca(OH)2(aq) → cloudy ppt.
→ can also do for:
- H2
- CO2
- NH3 It's insoluble in water.↑ collection over water.
Gas Molar Volume:
- 1 dm³ = 1000 cm³
- The molar gas volume is occupied by one mole of any gas at a stated temperature and pressure.
- At room temperature and atmospheric pressure (r.t.p.): 24 dm³/mol.
- Avogadro's Law states that equal volumes of all gases under the same conditions of t + p contain the same number of moles/molecules: the majority of the volume of a gas is empty space - if you have heavier gas molecules, there's just a little less room between particles.
Examples:
1. Calculate the number of moles: 234 cm³ of SO₂.
2. Calculate mass: 226 cm³ of HBr.
3. Calculate Mr: 0.373 g of gas occupies 56 cm³.
Burning Sodium:
- Sodium burns in air with a bright orange/yellow flame to form a white solid.
- 4Na(s) + O₂ → 2Na₂O(s)
- Sodium oxide formed is soluble in water and reacts in water to make a solution.
- Na2O (s) + H2O(l) → 2NaOH(aq)
- (OH)- ions make the solution alkaline. Adding UI to this solution would cause it to turn blue or purple.
- The Na₂O is termed a base oxide: it dissolves in water to form an alkaline solution, and the oxide reacts with an acid.
Burning Magnesium:
- Mg burns in the air with a bright, white light to form a white solid.
- 2Mg(s) + O2 → 2MgO(s)
- MgO (s) formed is very sparingly soluble to form Mg(OH)2 which releases hydroxide ions.
- MgO + 2H2O → Mg(OH)2 + H2
- The (OH)- ions make the solution alkaline. If UI were added to the solution, it'd turn blue.
- The sodium oxide formed a 'basic oxide': it dissolves in water to form an alkaline solution, and the oxide reacts with a base.
Burning Carbon:
- Carbon burns in the air and glows orange to form a colourless gas.
- C + O2 → CO2
- Carbon dioxide can dissolve in water to form an acidic carbonic acid solution.
- CO2(g) + H2O(l) → H2CO3(aq)
- The acid releases H+ ions, making the solution acidic. If UI were added to this solution, it would turn yellow/orange.
- The CO2 formed an 'acidic oxide': it dissolves in water to form an acidic solution and the oxide would react with a base.
Burning Sulphur:
- Sulphur burns in air to form a colourless gas (SO2) with a blue flame.
- S + O2 → SO2
- SO2 can dissolve in water to form an acidic solution of sulphur acid.
- SO2(g) + H2O(l) → H2SO3(aq)
- The acid releases H+ ions, making the solution acidic. If UI were added, it'd turn red.
- The SO₂ gas formed is classified as an acidic oxide: it dissolves in water to form an acidic solution, and the oxide reacts with a base.
Summary:
- When nonmetals burn in oxygen, they form nonmetal oxides.
- When they dissolve in water, they become acidic oxides.
- When metals burn in oxygen, they form metal oxides.
- When they dissolve in water, they become alkaline.
- You can only say a metal oxide is an alkali if it can dissolve in water. Not all metal oxides are water-soluble, if they don't dissolve in water, they're called 'bases' instead of 'alkalis'; they will dissolve in acids.
Greenhouse Gases:
- CO2 is a greenhouse gas; increasing its emission levels contributes to the increasing levels of global warming/climate change.