Similarities & Trends in the Properties of the Group 2 Metals, Magnesium to Barium, & Their Compounds (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Write the general equation for the reaction of a Group 2 metal M with water.
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Write the general equation for the reaction of a Group 2 metal M with water.
M (s) + 2H2O (l) → M(OH)2 (s) + H2 (g)
Exceptions: Be does not react with water; Mg reacts with steam (not cold water) to give MgO (s) and H2 (g).
True or False?
Group 2 metals react vigorously with dilute H2SO4 to produce hydrogen gas in all cases.
False.
Ca, Sr and Ba react with dilute H2SO4 but the reaction slows or stops because an insoluble sulfate layer (CaSO4, SrSO4 or BaSO4) forms on the metal surface, blocking further reaction. Only Mg reacts vigorously throughout.
Magnesium reacts vigorously with .......... to form MgO (s) and H2 (g), but reacts very slowly with .......... water.
Magnesium reacts vigorously with steam to form MgO (s) and H2 (g), but reacts very slowly with cold water.
What is a Group 2 element?
A Group 2 element is a metal in the second group of the periodic table (e.g. Mg, Ca, Sr, Ba) characterised by having two valence electrons, forming 2+ ions and reacting with oxygen, water and acids.
Why does barium react more vigorously with water than magnesium?
Going down Group 2, the metals become more reactive. Barium has a larger atomic radius and its outer electrons are further from the nucleus with more shielding, so they are more easily lost. This makes Ba react vigorously with cold water, while Mg reacts only very slowly.
True or False?
Strontium reacts with oxygen to form the oxide SrO.
True.
Strontium reacts with oxygen to form strontium oxide (SrO), a basic oxide. It reacts with water to form strontium hydroxide, Sr(OH)2, giving an alkaline solution.
The reaction of Group 2 metals with dilute HCl follows the general equation:
M (s) + 2HCl (aq) → .......... (aq) + .......... (g).
The reaction of Group 2 metals with dilute HCl follows the general equation:
M (s) + 2HCl (aq) → MCl2 (aq) + H2 (g).
What is observed when calcium reacts with water, and what is the product?
Calcium reacts moderately with cold water, producing bubbles of hydrogen gas. The product is calcium hydroxide, Ca(OH)2 (s):
Ca (s) + 2H2O (l) → Ca(OH)2 (s) + H2 (g).
Write the general equation for the reaction of a Group 2 metal M with oxygen.
2M (s) + O2 (g) → 2MO (s)
Note that Sr and Ba also form peroxides (MO2) and that Mg burns with a bright white flame, while Ca, Sr and Ba produce a red or green flame.
What products form when CaO reacts with water, and what observation is made?
CaO (s) + H2O (l) → Ca(OH)2 (s)
The reaction is vigorous, releases a lot of heat and causes some water to boil off. The solid lump appears to expand. The solution formed (limewater) has pH ~12.
True or False?
All Group 2 oxides are basic.
True.
All Group 2 oxides covered by the CIE specification (MgO, CaO, SrO, BaO) are basic — they react with acids to form a salt and water, and with water to give alkaline solutions.
When Group 2 carbonates react with dilute HCl, they form a soluble .......... salt, water and .......... gas.
When Group 2 carbonates react with dilute HCl, they form a soluble chloride salt, water and CO2 gas.
Define limewater.
Limewater is a solution of calcium hydroxide, Ca(OH)2, in water. It is used as a test for carbon dioxide gas, which turns limewater milky due to the formation of calcium carbonate.
Why does the reaction between BaCO3 and dilute H2SO4 stop quickly after initial effervescence?
The reaction initially produces CO2 gas (effervescence), but insoluble BaSO4 quickly forms on the surface of the carbonate. This layer prevents the acid from reaching the carbonate beneath, stopping further reaction.
True or False?
Group 2 hydroxides react with dilute HCl to produce a salt and water only (no gas).
True.
Group 2 hydroxides react with dilute HCl by a neutralisation reaction: M(OH)2 + 2HCl → MCl2 + 2H2O. No gas is produced, unlike the reaction of metals or carbonates with acids.
Going down Group 2, Group 2 hydroxides become more .......... in water, so the solutions formed become more .......... .
Going down Group 2, Group 2 hydroxides become more soluble in water, so the solutions formed become more alkaline.
Write the equation for the reaction of Mg(OH)2 with dilute H2SO4.
Mg(OH)2 (s) + H2SO4 (aq) → MgSO4 (aq) + 2H2O (l)
MgSO4 is soluble, unlike CaSO4, SrSO4 and BaSO4 which are sparingly soluble or insoluble.
What is the general equation for the reaction of a Group 2 oxide with dilute hydrochloric acid?
Metal oxide + dilute HCl → salt + water
For example: MgO (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l). If H2SO4 is used instead, an insoluble sulfate layer can form on the oxide surface and slow the reaction.
Define thermal decomposition.
Thermal decomposition is the breakdown of a compound into two or more simpler substances when it is heated.
Write the general equation for the thermal decomposition of a Group 2 carbonate (XCO3).
XCO3 (s) → XO (s) + CO2 (g)
For example: MgCO3 (s) → MgO (s) + CO2 (g). The solid metal oxide remains and carbon dioxide gas is given off.
True or False?
Thermal stability of Group 2 carbonates increases going down the group.
True.
Going down Group 2, the metal ions become larger and have lower polarising power. They polarise the carbonate ion less, making the carbonate more stable and requiring more heat to decompose.
When Group 2 nitrates decompose on heating, the products are the metal .......... , brown fumes of .......... gas and oxygen gas.
When Group 2 nitrates decompose on heating, the products are the metal oxide, brown fumes of NO2 gas and oxygen gas.
Write the equation for the thermal decomposition of Ca(NO3)2.
2Ca(NO3)2 (s) → 2CaO (s) + 4NO2 (g) + O2 (g)
The brown fumes of nitrogen dioxide produced are toxic and are the characteristic observation in this reaction.
True or False?
Magnesium carbonate is more thermally stable than barium carbonate.
False.
BaCO3 is more thermally stable than MgCO3. Ba2+ is larger and has lower polarising power than Mg2+, so it polarises the carbonate ion less, making BaCO3 harder to decompose.
Smaller Group 2 ions have higher .......... power, causing them to .......... the carbonate ion more and weakening its bonds.
Smaller Group 2 ions have higher polarising power, causing them to polarise the carbonate ion more and weakening its bonds.
Why do smaller Group 2 cations cause greater thermal decomposition of carbonates and nitrates?
Smaller cations (e.g. Mg2+) have higher charge density and greater polarising power. They attract the delocalised electrons in the carbonate or nitrate ion towards themselves, weakening the C–O or N–O bonds and making decomposition easier.
What is the general equation for the thermal decomposition of a Group 2 nitrate X(NO3)2?
2X(NO3)2 (s) → 2XO (s) + 4NO2 (g) + O2 (g)
This can also be written with a half equation: X(NO3)2 (s) → XO (s) + 2NO2 (g) + ½O2 (g).
Why does the reactivity of Group 2 metals increase going down the group?
Going down Group 2, the outer electrons are in a higher principal quantum shell, further from the nucleus. Increased shielding and greater distance outweigh the higher nuclear charge, so ionisation energies decrease and electrons are lost more easily, increasing reactivity.
True or False?
Group 2 metals act as reducing agents when they form ionic compounds.
True.
Group 2 metals donate their two outer electrons to form M2+ ions. In doing so, they are oxidised (lose electrons) and act as reducing agents.
Going down Group 2, atomic radius .......... and melting point generally .......... due to the outer electrons being further from the nucleus.
Going down Group 2, atomic radius increases and melting point generally decreases due to the outer electrons being further from the nucleus.
Define first ionisation energy.
A first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Predict the formula of the ion formed by radium (Ra, Group 2) and explain your reasoning.
Radium forms Ra2+. Like all Group 2 metals, radium has two valence electrons which it donates to form a 2+ ion. The pattern is consistent across the entire group.
True or False?
The density of Group 2 metals decreases going down the group.
False.
The density of Group 2 metals increases going down the group. Molar mass increases more rapidly than atomic volume, so the metals become denser from Be to Ba.
Barium is stored in .......... to prevent it reacting with .......... in the air, since it is so reactive.
Barium is stored in oil to prevent it reacting with oxygen in the air, since it is so reactive.
Predict whether radium sulfate is more or less soluble than strontium sulfate, and explain why.
Radium sulfate is less soluble than strontium sulfate. The solubility of Group 2 sulfates decreases going down the group, so RaSO4 (below BaSO4 in the group) will be even less soluble.
Why does the melting point of Group 2 metals generally decrease going down the group?
Going down Group 2, the outer bonding electrons are further from the nucleus and in a higher principal quantum shell. The attraction between the nucleus and the delocalised electrons in the metallic lattice weakens, reducing the melting point.
How does the solubility of Group 2 hydroxides change going down the group?
The solubility of Group 2 hydroxides increases going down the group. Mg(OH)2 is sparingly soluble, while Ba(OH)2 is soluble. As solubility increases, more OH- ions are released, making the solution more alkaline.
True or False?
BaSO4 is more soluble than MgSO4.
False.
MgSO4 is highly soluble, while BaSO4 is effectively insoluble. The solubility of Group 2 sulfates decreases going down the group.
Going down Group 2, hydroxide solubility .......... and the pH of the resulting solution ...........
Going down Group 2, hydroxide solubility increases and the pH of the resulting solution increases.
Define sparingly soluble.
A sparingly soluble compound is one that dissolves only to a very small extent in water. Mg(OH)2 is a sparingly soluble Group 2 hydroxide with a solubility of approximately 2.0 × 10-5 mol per 100 g of water.
Write the ionic equation showing how a Group 2 oxide dissolves in water to produce OH- ions.
XO (s) + H2O (l) → X(OH)2 (aq)
The hydroxide then dissociates: X(OH)2 (aq) → X2+ (aq) + 2OH- (aq). As more OH- ions are released, the solution becomes more alkaline.
True or False?
The alkalinity of solutions formed from Group 2 oxides reacting with water increases going down the group.
True.
Going down Group 2, hydroxide solubility increases, releasing more OH- ions. Higher OH- concentration means a higher pH and a more alkaline solution.
The solubility of Group 2 sulfates .......... going down the group, so .......... sulfate is the most soluble.
The solubility of Group 2 sulfates decreases going down the group, so magnesium (Mg) sulfate is the most soluble.
What observation would you expect when Ba(OH)2 solution is mixed with dilute H2SO4?
A white precipitate of BaSO4 would form. Barium sulfate is insoluble in water. The solution would also become less alkaline as OH- ions are neutralised by the acid.
Explain why solutions of Group 2 oxides dissolved in water become more alkaline going down the group.
Going down the group, Group 2 hydroxides become more soluble. More M(OH)2 dissolves, releasing a greater concentration of OH- ions. A higher [OH-] means a higher pH and a more alkaline solution.
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