4.1 groups 1, 2 & 7 Flashcards
explain why ionisation energy decreases down group 2.
- each element down group 2 has an additional energy level compared to the element above.
- this additional energy level shields the outer electrons from the attraction of the nucleus.
- the electrostatic attraction between the nucleus and outer electrons decrease down the group, as the additional energy level causes the outer electrons to move further away from the nucleus.
ionisation energy decreases down group 2. use this trend to explain the trend in the reactivity of group 2 elements.
- reactivity increases down group 2.
- most group 2 elements react by losing their outermost two electrons.
- the higher the first and second ionisation energies, the more likely group 2 elements are to lose these electrons.
group 2 elements react with water, oxygen and chlorine. give the product(s) formed in each of these reactions, including their state symbols.
group 2 element + water → metal hydroxide (aq) + hydrogen (g)
group 2 element + oxygen → oxide (s)
group 2 element + chlorine → chloride (s)
the oxides of group 2 elements react readily with water to form metal hydroxides, which dissolve. how do these dissolved OH⁻ ions affect the pH of the resulting solution?
OH⁻ ions make the resulting solution strongly alkaline.
give the two oxides of group 2 which do not react with water, and explain why they are unable to react with water to form metal hydroxides.
beryllium oxide - does not react with water due to its small size and high ionisation energy compared to other elements in the group.
magnesium oxide - reacts very slowly with water due to its high lattice enthalpy.
why do group 2 oxides form more strongly alkaline solutions down the group?
because the hydroxides become more soluble as you progress down the group.
solubility trends of group 2 elements depend on the compound anion. explain the solubility trend of group 2 hydroxides, and how this is different from group 2 sulphates. provide an example for each.
- the solubility of group 2 hydroxides increases down the group as the compounds contain singly charged negative ions, such as OH⁻.
- in contrast, the solubility of group 2 sulphates decreases down the group as the compounds contain doubly charged negative ions, such as SO₄²⁻
with reference to the ions involved, explain how distortion can affect the stability of group 2 carbonates and nitrates.
- the carbonate and nitrate ions are anions, and can be made unstable by the presence of a cation.
- the cation polarises the anion, which causes it to distort.
- the greater the distortion, the less stable the compound is.
explain why large cations cause less distortion than small cations, and how this can affect the stability of group 2 carbonates and nitrates.
- large cations cause less distortion than small cations as they have a lower charge density.
- the larger the cations, the lower the charge density, so the less distortion caused and the more stable the carbonate / nitrate.
give the thermal stability trends of group 1 and 2 compounds.
- group 1 carbonates are thermally stable - they do not decompose.
- group 1 nitrates decompose to form the nitrate and oxygen.
- group 2 carbonates decompose to form the oxide and carbon dioxide.
- group 2 nitrates decompose to form the nitrate, nitrogen dioxide, and oxygen.
give one way in which you could test for the decomposition of a nitrate in the lab.
by measuring the time taken for a certain amount of oxygen to be produced, for example, by measuring how long it takes for a glowing splint to be relit.
give one way in which you could test for the decomposition of a carbonate in the lab.
by measuring the time taken for a certain amount of carbon dioxide to be produced, for example, by measuring how long it takes for a solution of limewater to turn cloudy.
give the flame colours produced during a flame test for the following group 1 elements: Li, Na, K, Rb, Cs.
- lithium - red flame.
- sodium - yellow/orange flame.
- potassium - lilac flame.
- rubidium - red/purple flame.
- caesium - blue/violet flame.
explain how to carry out a flame test in the lab.
- mix a small amount of the element to be tested with a few drops of HCl.
- heat a piece of nichrome wire loop in a bunsen burner flame to sterilise the wire.
- dip the wire loop into the compound / acid mixture.
- hold the wire loop in a very hot flame and note the flame colour produced.
give the flame colours produced during a flame test for the following group 2 elements: Mg, Ca, Sr, Ba.
- magnesium - bright white flame.
- calcium - red/orange flame.
- strontium - crimson flame.
- barium - pale green flame.
with reference to electron activity, explain why group 1 and 2 elements burn with distinctive flame colours.
- the thermal energy absorbed from the flame causes the electrons to move to higher energy levels within the atom.
- these distinctive colours are produced when the electrons fall back down to lower energy levels, releasing the energy previously gained in the form of light.
- the colour of the flame produced is dependent on the wavelength of light released, which is determined by the difference in energy between the higher and lower energy levels within the atom.
halogens in their natural state exist as what type of covalent molecules?
covalent diatomic molecules.
explain why halogens have low solubility in water.
halogens have low solubility in water because they are non-polar molecules.
halogens are oxidising agents. explain what is meant by an oxidising agent.
- an oxidising agent is a substance that oxidises another species by causing it to lose electrons.
- an oxidising agent gains electrons, so itself is reduced.
explain why the reactivity of the halogens decreases down the group.
- the atoms of group 7 halogens become larger as you progress down the group.
- the outer electrons move further away from the nucleus, and are shielded from the positive attraction of the nucleus due to an increased number of inner electrons.
- this makes it more difficult for large atoms to attract the electron needed to form an ion, so reactivity decreases down the group.
explain why the electronegativity of the halogens also decreases down the group.
- as you progress down group 7, there is an increase in the number of electron shells, and an increase in the distance between the nucleus and the bonding electrons.
- this makes it more difficult for an atom to attract an electron in a covalent bond, so the electronegativity decreases down the group.
explain why the melting and boiling points of group 7 elements increase down the group.
- as you progress down the group, there is an increase in electron shells, and therefore electrons.
- this causes an increase in the London forces present between the halogen molecules.
- this increase in London forces means that more energy is required to overcome the strong intermolecular forces present, which causes the melting and boiling points of the halogens to increase down the group.
the oxidising power of the halogens can be seen in their displacement reactions with halide ions. explain the trend in the oxidising power of the halogens down the group.
- the oxidising power of the halogens decreases down the group.
- as the halogen atoms become larger, they accept electrons less easily, which causes the oxidising power to become weaker.
when a displacement reaction between a halogen and halide occurs, a colour change will be observed. give the colour change that would be observed if bromide is displaced and bromine is formed.
an orange colour change would be observed.
