Chapter 8 Reacitvity Trends Flashcards
What are the characteristic physical/chemical properties of the group 2 elements?
They are alkali metals because they form alkaline metal hydroxides
Highly reactive and do not occur as metals in nature
Form stable compounds like calcium carbonate
Why are the group 2 elements considered as reducing agents?
Each metal is oxidised losing 2 electrons to form a 2+ ion
Another species will gain these electrons and be reduced, therefore they are reducing agents
What occurs during a redox reaction with group 2 elements and oxygen? Give an example and equation with Magnesium
They form a metal oxide with the general formula MO made of M2+ and O2- ions
For example:Mg burns with a brilliant white light to form MgO
2Mg(s) + O2(g) -> 2MgO(s)
Oxygen is reduced
What occurs during a redox reaction with a group 2 element and water? Give an equation with Strontium
Form an alkaline hydroxide with the general formula M(OH)2 and H2(g)
Water and Mg react slowly but the reaction becomes more vigorous down the group as the reactivity increase down the group
Sr(s) + 2H2O(l) -> Sr(OH)2(aq) + H2(g)
most hydrogen is reduced but the H in Sr(OH)2 doesn’t have a change in oxidation number
What occurs in a redox reaction with group 2 elements and dilute acids? Give an equation with Magnesium and Hydrochloric Acid
Form a salt and hydrogen gas
metal+acid->salt+hydrogen
Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)
Hydrogen is reduced
Describe the trends in reactivity and ionisation energy for group 2 elements
Ionisation energies decrease down the group because the attraction between the nucleus and outer shell electrons decreases
This is a result of increasing atomic radius and electron shielding from the increased electron shells
This makes it easier (require less energy) to lose electrons
Reactivity increases down the group for the same reasons
Describe the reaction between group 2 oxides and water. Use calcium oxide as an example for equations
They react to release hydroxide ions which can react with group 2 metal actions to make metal hydroxides
CaO(s) +H2O(l) -> Ca2+ (aq) + 2OH-
Group 2 hydroxides are only slightly soluble in water. When the solution becomes saturated metal ions and hydroxide ions will form a solid ppt.
Ca2+(aq) + 2OH-(aq) -> Ca(OH)2(s)
Explain the trend in solubility of group2 hydroxides down the group.
Solubility of group 2 hydroxides in water increases down the group, so the resulting solutions have a higher concentration of OH- ions making them more alkaline ( higher pH )
Mg(OH)2 is slightly soluble in water with a low OH- concentration and a pH~ 10
Ba(OH)2 is much more soluble in water with a higher concentration of OH- and a pH ~13
How can group 2 compounds be used in agriculture?
Ca(OH)2 is added to fields as lime by farmers to increase the pH of acidic soils
Ca(OH)2(s) + 2H+(aq) -> Ca2+(aq) + H2O(l)
How can group 2 compounds be used in medicine?
Often used as antacids for treating acid indigestion
Many tablets use MgCO3 and CaCO3
Milk of magnesia is a suspension of white Mg(OH)2 in water
Stomach acid is mostly HCl
Mg(OH)2 (aq) + 2HCl(aq) -> MgCl2(aq) + 2H2O(l)
CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) CO2(g)
Carbonates decompose at higher temperatures down the group
How can calcium carbonate be used in power stations?
Sprayed down power station chimneys to neutralise sulphuric dioxide as a scrubber, to prevent acid rain
What are the trends down the group of the halogen boiling points?
Boiling point increases down the group because each diatomic particle has more electrons than the previous halogen
This means that the induced dipole-dipole forces are stronger which require more energy to break
What happens to the oxidising power of the halogens down the group?
Oxidising power (reactivity) deacreases down the group because:
Atomic radius increases
Electron shielding increases (outweighs increase in nuclear charge)
Attraction to the nucleus is lowered
Ability to gain an electron decreases
What are halogens considered as oxidising agents? Use Chlorine to wire a half equation
Each halogen atom is reduced, gaining 2 electron to form a 1- halide ion
Therefore they oxidise another species in the reaction
Cl2 + 2e- -> 2Cl-
Describe the halogen displacement reactions of Chlorine, Bromine and Iodine as well as the colour changes seen and the colour change with a suitable organic compound such as cyclohexane.
A solution of each halogen is added to an aqueous solution of a halide and another compound.
If the halogen is more reactive than the halide present:
-the halogen will displace the halide
-the solution changes colour
Chloride + bromine or Iodine = no reaction
Bromide+ chlorine= displacement to form an orange solution (Br2) which remains orange in cyclohexane
Bromide + Iodine= no reaction
Iodide + chlorine or bromine= displacement to form an orange brown solution which turns violet in cyclohexane
