Groups 1 & 2 (Edexcel A Level Chemistry): Flashcards
Exam code: 9CHO
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Define first ionisation energy.
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Define first ionisation energy.
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Why do both first and second ionisation energies decrease going down Group 2?
Both ionisation energies decrease going down Group 2 because, although nuclear charge increases, the greater shielding effect and increased distance between the outermost electrons and the nucleus outweigh the effect of the higher nuclear charge, making electrons easier to remove.
True or False?
Group 2 metals become less reactive going down the group.
False.
Group 2 metals become more reactive going down the group because the decreasing ionisation energies make it progressively easier to lose two electrons and form 2+ ions.
Going down Group 2, the increasing .......... and .......... between the outermost electrons and the nucleus cause ionisation energies to decrease.
Going down Group 2, the increasing shielding effect and distance between the outermost electrons and the nucleus cause ionisation energies to decrease.
What charge do Group 2 ions carry, and why?
Group 2 ions carry a +2 charge because Group 2 metals lose their two outermost electrons, acting as reducing agents and becoming oxidised.
Define shielding effect.
Shielding effect is the reduction in the attractive force experienced by outer electrons due to the repulsion from inner-shell electrons between them and the nucleus.
True or False?
Barium must be stored in oil to prevent it reacting with oxygen in air.
True.
Barium is so reactive that it must be stored in oil; its high reactivity is a consequence of its very low ionisation energies relative to the lighter Group 2 metals.
What evidence from reactions with dilute hydrochloric acid shows that Group 2 metals become more reactive down the group?
Reactions with dilute hydrochloric acid produce hydrogen gas more vigorously (faster bubble production) further down the group, indicating increasing reactivity.
What is the general equation for the reaction of a Group 2 metal with oxygen?
The general equation is: 2M (s) + O2 (g) → 2MO (s), where M is any Group 2 metal. Note that Sr and Ba also form a peroxide, MO2.
Magnesium reacts with cold water only .........., but reacts .......... with steam to form magnesium oxide and hydrogen gas.
Magnesium reacts with cold water only extremely slowly, but reacts vigorously with steam to form magnesium oxide and hydrogen gas.
True or False?
Beryllium reacts readily with water to form beryllium hydroxide.
False.
Beryllium does not react with water. All other Group 2 metals react with water to form the metal hydroxide and hydrogen gas, but Be is an exception.
What is limewater?
Limewater is a common name for an aqueous solution of calcium hydroxide, Ca(OH)2 (aq), formed when calcium oxide reacts vigorously with water.
Why is BeO described as amphoteric while other Group 2 oxides are basic?
BeO is amphoteric because it can act as both an acid and a base, whereas all other Group 2 oxides are purely basic and react with water to form alkaline solutions.
When a Group 2 oxide reacts with dilute sulfuric acid, an insoluble .......... forms at the surface of the oxide, which prevents further ...........
When a Group 2 oxide reacts with dilute sulfuric acid, an insoluble sulfate forms at the surface of the oxide, which prevents further reaction (neutralisation).
What products form when magnesium reacts with chlorine gas?
Magnesium reacts with chlorine gas to form magnesium chloride: Mg (s) + Cl2 (g) → MgCl2 (s).
True or False?
The alkalinity of Group 2 oxide solutions increases going down the group.
True.
Going down the group, hydroxide solubility increases, producing a higher concentration of OH- ions and therefore a more alkaline solution.
What is a peroxide?
A peroxide is a compound containing the O22- ion, in which oxygen has an oxidation state of −1. In Group 2, Sr and Ba form peroxides (MO2) in addition to the normal oxide (MO) when reacted with oxygen.
What is the trend in solubility of Group 2 hydroxides going down the group?
The solubility of Group 2 hydroxides increases going down the group, from the sparingly soluble Mg(OH)2 to the much more soluble Ba(OH)2.
What is the trend in solubility of Group 2 sulfates going down the group?
The solubility of Group 2 sulfates decreases going down the group, so barium sulfate is an insoluble white precipitate while magnesium sulfate is readily soluble.
True or False?
The alkalinity of Group 2 hydroxide solutions decreases going down the group.
False.
Alkalinity increases going down the group because solubility increases, producing a higher concentration of OH- ions and therefore a higher pH.
When a Group 2 oxide dissolves in water, the ionic equation is: O2- (aq) + H2O (l) → ..........
When a Group 2 oxide dissolves in water, the ionic equation is: O2- (aq) + H2O (l) → 2OH- (aq)
What is thermal stability (of a salt)?
Thermal stability is the resistance of a compound to decomposition when heated. In Group 2, thermal stability of hydroxides and sulfates increases down the group as the larger cations have lower polarising power.
Predict the observations when barium hydroxide solution is mixed with dilute sulfuric acid.
A white precipitate of barium sulfate forms because BaSO4 is insoluble. The reaction is: Ba(OH)2 (aq) + H2SO4 (aq) → BaSO4 (s) + 2H2O (l).
True or False?
Group 1 hydroxides are less soluble than Group 2 hydroxides.
False.
Group 1 hydroxides are more soluble than Group 2 hydroxides. For example, KOH has a solubility of 121 g per 100 cm3, far greater than Ba(OH)2 at 4.68 g per 100 cm3.
Why does the alkalinity of the solution increase going down Group 2 when metal oxides react with water?
Going down Group 2, hydroxide solubility increases, so more OH- ions are released into solution, raising the pH and increasing alkalinity.
Define thermal decomposition.
Thermal decomposition is the breakdown of a compound into two or more different substances using heat.
What are the products of thermal decomposition of a Group 2 carbonate?
A Group 2 carbonate decomposes on heating to form a metal oxide and carbon dioxide gas: XCO3 (s) → XO (s) + CO2 (g), where X is the Group 2 element.
What are the products of thermal decomposition of a Group 2 nitrate?
A Group 2 nitrate decomposes on heating to form a metal oxide, nitrogen dioxide gas (brown, toxic), and oxygen gas: X(NO3)2 → XO + ½O2 + 2NO2.
True or False?
The thermal stability of Group 2 carbonates increases going down the group.
True.
Going down the group, the larger cations have lower polarising power, so they distort the carbonate ion less, making the carbonate more resistant to thermal decomposition.
The thermal stability of Group 1 and 2 carbonates and nitrates .......... down the group because smaller cations have greater .......... and polarise anions more strongly.
The thermal stability of Group 1 and 2 carbonates and nitrates increases down the group because smaller cations have greater polarising power and polarise anions more strongly.
Which Group 1 carbonate decomposes on heating, and what are the products?
Lithium carbonate (Li2CO3) is the only Group 1 carbonate that decomposes at Bunsen temperatures, forming lithium oxide (Li2O) and carbon dioxide (CO2).
How do Group 1 nitrates (sodium to caesium) decompose on heating, and how does this differ from Group 2 nitrates?
Group 1 nitrates (Na to Cs) decompose to the metal nitrite and oxygen (no nitrogen dioxide): 2XNO3 (s) → 2XNO2 (s) + O2 (g). Group 2 nitrates decompose more completely to the metal oxide, NO2, and O2.
True or False?
Nitrogen dioxide produced during nitrate decomposition is a colourless, non-toxic gas.
False.
Nitrogen dioxide (NO2) is a brown, toxic gas. Because of this, thermal decomposition of nitrates is carried out in a fume cupboard.
What is polarising power?
Polarising power is the ability of a cation to distort the electron cloud of a nearby anion. It increases with higher charge and smaller ionic radius. High polarising power weakens covalent bonds in anions such as carbonate and nitrate, lowering thermal stability.
What is a flame test?
A flame test is a qualitative analytical technique used to identify metal ions by the characteristic colour they produce when heated in a Bunsen flame.
Why must the wire loop be cleaned before each flame test?
The wire loop must be cleaned to remove contaminating ions. If more than one type of ion is present, the flame colours will mix, making identification unreliable. The loop is dipped in concentrated acid and held in the blue flame until no colour change is seen.
In a flame test, heat causes electrons to move to a .......... energy level. As they fall back down, energy is emitted as .......... light.
In a flame test, heat causes electrons to move to a higher energy level. As they fall back down, energy is emitted as visible light.
What flame colours are produced by lithium, sodium, potassium, calcium, strontium, and barium ions?
Li+ — red; Na+ — yellow/orange; K+ — lilac; Ca2+ — brick red; Sr2+ — crimson; Ba2+ — green.
True or False?
Magnesium ions produce a visible colour in a flame test.
False.
Mg2+ does not produce an observable colour in a standard flame test because the energy emitted during the electron transition is outside the visible spectrum.
Why do different metal ions produce different flame colours?
Different metal ions produce different colours because their electrons transition between energy levels that are specific to each element. The wavelength of light emitted corresponds to the energy gap between those levels, which differs for each metal ion.
True or False?
A nichrome or platinum wire is used in a flame test because it is unreactive.
True.
An unreactive wire such as nichrome or platinum is used to avoid contributing its own ions to the flame and introducing contamination that would affect the result.
To carry out a flame test, dip the wire into the .......... sample and hold it at the edge of the .......... Bunsen flame.
To carry out a flame test, dip the wire into the solid sample and hold it at the edge of the blue Bunsen flame.
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