Chapter 8: Reactivity trends

metal + oxygen –> metal oxide metal + water –> metal hydroxide + hydrogen metal + acid –> salt + hydrogen

• becomes more reactive DOWN the group • when the metals react they lose their 2 outer shell electrons • down the group, atoms become bigger (atomic radius increases) and there is more shielding • therefore there is a weaker attraction between the nucleus and the outer shell electrons and so the outer shell electrons are more easily lost

• increases DOWN the group therefore • solutions contain more OH- ions and are more alkaline • Mg(OH)2 (s) is only very slightly soluble in water. The solution has a low OH- concentration and a pH ~10 • barium hydroxide is much more soluble in water. the solution has a greater OH- concentration and a pH~13.

Splitting compounds using heat

Group 2 carbonates become more stable to thermal decomposition going down the group.

Metal ions become larger down group 2 but they have the same charge. So, charge density is reduced. A metal ion with a high charge density has a strong polarizing power, it can therefore polarize the carbonate ion, making it more likely to split into o2- and co2 when heated. A metal ion with less charge density has weak polarizing power, so carbonate ion is less polarized and therefore more thermally stable.

The ability of a cation to distort an anion i.e. how strong the cation can attract the anion’s electron cloud to itself.

Each metal atom is oxidised, losing 2e- to form 2+ ion Another species will gain these 2 electrons and be reduced group 2 element is called a reducing agent as it reduces another species

Calcium hydroxide is added to fields as lime to increase pH of acidic soils. Calcium hydroxide neutralises acid in soil, forming neutral water. Ca(oh)2 (s) + 2h+ (aq) –> ca2+ (aq) + 2h2o (l) Acidic soils = bad because of nutrient deficiency

Often used as antiacids to treat acid indigestion. Many use magnesium and calcium carbonates as main ingredients, and “milk of magnesia” is a suspension of white Mg(OH)2 in water. Acid in stomach is mainly HCl, which is neutralised.

fluorine - pale yellow gas chlorine - pale green gas Bromine - red brown liquid Iodine - shiny grey-black solid Astatine - never been seen At RTP, all halogens exist as diatomic molecules, X2.

• More electrons down the group • Stronger London forces • More energy required to break intermolecular forces • Boiling point increases

Each halogen as s2p5 at the end - each gain one electron in a redox reaction. Each halogen is reduced - electron configuration of nearest noble gas. Halogen is an oxidising agent because it oxidises other species (the halogen itself is reduced).

When a halogen acts as an oxidising agent, it gains electrons (taken from the oxidised species). Down the group it becomes harder to gain an electron because: - atoms are larger (increased atomic radius) and more shielding due to extra electron shell - Oxidising power gets weaker down the group, as there is less nuclear attraction to capture electron from another species

A more reactive halogen will replace a less reactive one. The reactivity of halogens decreases down the group.

Chlorine - pale green Bromine - Orange Iodine - Brown

chlorine - pale green Bromine - orange Iodine - violet

Redox reaction in which same species is both oxidised and reduced.

Cl2 (aq) + H2O (L) –> HClO (aq) + HCl (aq)

Chloric (I) acid and hydrochloric acid: - bacteria killed by chloric acid and chlorate (i) ions, ClO- rather than chlorine. - Chloric (i) acid also acts as weak bleach. Indicator paper first turns red due to the presence of 2 acids. Then the colour disappears as chloric (i) acid bleaches acid.

Cl2 (aq) + 2NaOH(aq) –> NaCl (aq) + NaClO (aq) + H2O (l) Reaction of chlorien and water is limited by low solubility of chlroine in water. Much more chlorine dissolves in aqueous sodium hydroxide - disproportionation reaction takes place.

Resulting solution has a large concentration of chlorate (i) ions from sodium chlorate (i) NaClO, that is formed. Solution is used as a household bleahc.

• kills microorganisms (like bacteria) • prevents algae growth, eliminates bad tastes and smells, removes discolouration caused by organic compounds • chlorine can irritate respiratory system if breathed in • liquid chlorine can cause severe chemical burns • chlorine can also react with organic compounds (decomposing plant material for example) to form chlorinated hydrocarbons which can cause cancer (are carcinogenic).

Quantitative - technique with numerical results Qualitative - relies of observations rather than measurements