Group 2 Flashcards
What are the colours of the flames of group 2 elements when burning in oxygen?
In this deck, only magnesium to barium will be assessed; berrylium is very unreactive
- Magnesium - white flame
- Calcium - red flame
- Strontium - red flame
- Barium - green flame
Magnesium burns readily while the other three are difficult to ignite; this is an exception to the usual reactivity trends in group 2 and is because of the lattice enthalpies associated with the reactions
What is the general equation for the reaction between a group 2 element and oxygen?
2M (s) + O₂ (g) –> 2MO (s)
Strontium and barium can react this way, but have an alternative reaction pathway where the general equation is M (s) + O₂ (g) –> MO₂ (s)
What is the trend in rate of reaction with water down group 2?
- Reactivity increases
- Magnesium reacts slowly, calcium reacts moderately, strontium reacts rapidly and barium reacts vigorously
- This is because ionisation energy decreases
What is the general equation for the reaction between a group 2 element and water?
M (s) + 2H₂O (l) → M(OH)₂ (s) + H₂ (g)
Magnesium reacts rapidly with steam with the equation Mg (s) + 2H₂O (g) → MgO (s) + H₂ (g)
What is the general equation for the reaction between a group 2 element and hydrochloric acid and what is the trend in reactivity?
- M (s) + 2HCl (aq) → MCl₂ (aq) + H₂ (g)
- Reactivity increases from magnesium to barium, though all react fairly vigorously
What is the general equation for the reaction between a group 2 element and sulfuric acid and what is the trend in reactivity?
- M (s) + H₂SO₄ (aq) → MSO₄ (aq) + H₂ (g)
- Unusually, reactivity decreases down the group as the rate at which an insoluble sulfate layer coats the metal increases
What is the general equation for the reaction between a group 2 oxide and water?
MO (s) + H₂O (l) → M(OH)₂ (s)
The alkalinity of the solutions formed increases down the group as hydroxide solubility increases
What is the general equation for the reaction between a group 2 oxide and hydrochloric acid?
MO (s) + 2HCl (l) → MCl₂ (aq) + H₂O (l)
- This is a neutralisation reaction
- The solutions formed in these reactions are all colourless
What is the general equation for the reaction between a group 2 oxide and sulfuric acid?
MO (s) + H₂SO₄ (l) → MSO₄ (aq) + H₂O (l)
An insoluble sulfate layer (which becomes more insoluble down the group) often forms around the oxide, though using powdered oxides and stirring can prevent this
What is the general equation for the reaction between a group 2 hydroxide and water?
There is no reaction, just dissolution of the metal and hydroxide ions
What is the general equation for the reaction between a group 2 hydroxide and hydrochloric acid?
M(OH)₂ (s) + 2HCl (l) → MCl₂ (aq) + 2H₂O (l)
What is the general equation for the reaction between a group 2 hydroxide and sulfuric acid?
M(OH)₂ (s) + H₂SO₄ (aq) → MSO₄ (aq) + 2H₂O (l)
What is the general equation for the reaction between a group 2 carbonate and hydrochloric acid?
They do not react with water as they are insoluble
MCO₃ (s) + 2HCl (aq) → MCl₂ (aq) + H₂O (l) + CO₂ (g)
What is the general equation for the reaction between a group 2 carbonate and sulfuric acid?
MCO₃ (s) + H₂SO₄ (aq) → MSO₄ (aq) + H₂O (l) + CO₂ (g)
What is the general equation for the thermal decomposition of group 2 carbonates?
MCO₃ (s) –> MO (s) + CO₂ (g)
What is the general equation for the thermal decomposition of group 2 nitrates?
M(NO₃)₂ (s) –> 2MO (s) + 4NO₂ (g) + O₂ (g)
What is the trend in thermal stability of group 2 carbonates and nitrates down the group and what is the explanation for this trend?
- Thermal stability increases
- This is because the smaller ions at the top of the group polarise the carbonate or nitrate ion more strongly, pulling electron density towards itself
- This polarisation distorts and destabilises the carbonate or nitrate ion, weakening the C-O or N-O bonds and allowing the ions to break apart more readily
Thermal stability dictates how much energy is required for decomposition to occur
How does melting point change down the group and what is the explanation for this trend?
- It decreases
- This is because the ionic radii of the metal cations in the metallic lattices increases down the group, decreasing the electrostatic forces of attraction between metal cations and the delocalised electrons
What is the trend in the water solubility of group 2 hydroxides down the group?
The explanation is optional but interesting
- It increases
- This is because the lattice enthalpy of dissociation decreases more sharply than hydration enthalpy, meaning the energetic favourability of dissolution increases overall as one descends the group
- Lattice enthalpy of dissociation and hydration enthalpy decrease as the atomic radius of the group 2 cations increases
This increasing solubility means the alkalinity of solutions formed increases
What is the trend in the water solubility of group 2 sulfates down the group?
- It decreases
- This is because the decrease in hydration enthalpy outweighs the decrease in lattice enthalpy, making dissolution less energetically favourable down the group
- Both enthalpies decrease as the atomic radius of the cations increases, but the relative rate of decrease of the two enthalpies is different when compared with the hydroxides (as described above)
