reactivity trends Flashcards
where are the two electron of the outer shell in a group 2 element found?
outer s sub shell
what is the most common reaction in group 2 elements?
redox reactions, each metal ion is oxidised forming a 2+ ion. Another species will gain 2 electrons and become reduced therefore group 2 elements are also known as a reducing element.
describe the redox reaction between group 2 elements and oxygen
metal oxide is formed e.g MgO is formed when using magnesium, made up go Mg2+ and O2- ions.
what can you see when magnesium reacts with oxygen?
brilliant white light which forms white magnesium oxide
describe the change in oxidation number when magnesium reacts with oxygen
as two magnesiums are needed to react with oxygen and each magnesium increases by +2, the total increase is +4. As two oxygen are in O2 (diatomic) and each oxygen decreases by 2 the total decrease is -2.
Describe the redox reaction between group 2 elements and water
an alkaline hydroxide is formed with the general formula M(OH)2 and hydrogen gas.
Describe the redox reaction between group 2 elements and dilute acids.
salt and hydrogen gas in formed
why does reactivity increase going down the group 2 elements?
ionisation energies decrease down the group because the attraction between the nucleus and outer electron decreases as a result of its increasing atomic radius and shielding
In group 2 elements what takes up most of the energy input?
first and second ionisation energy.
Describe how group 2 oxides react with water?
hydroxide ions(OH-) are released therefore solution becomes more alkaline and filled with group 2 hydroxides. Group 2 hydroxides are only slightly soluble in water, they form a solid precipitate.
Describe the trend in solubility of group 2 hydroxides
group 2 hydroxides become more soluble going down the group so the solutions more hydroxide ions (OH-) and are more alkaline.
How are group 2 compounds used in agriculture?
Calcium hydroxide, Ca(OH)2 is added to fields to increase the pH of acidic soil. The calcium hydroxide neutralises the acid forming neutral water
How are group 2 compounds used in medicine?
they are used as antacids to neutralise stomach acid to relieve indigestion.
why does boiling point increase going down group 7 (halogens)?
- higher electron density
- stronger London forces
- more energy required to break the intermolecular forces
- boiling point increases
where are the outer electrons found in halogens?
two electrons are in the outer s-sub shell and 5 in the outer p sub shell.
what is the most common reaction in halogens?
redox reactions, each halogen atom gains one electron to form a 1- halogen ion. I becomes reduced however the other species becomes oxidised therefore halogens are known as a oxidising agent
how can you test the trend in reactivity of halogens?
do a displacement reaction, a colour change indicates a displacement.
what colour is chlorine and bromine?
chlorine is a pale green and bromine is orange
what is the colour of iodine?
brown but for a positive result of iodine from a displacement reaction it is the colour, violet
what is the colour of fluorine?
pale yellow gas that reacts with almost any substance
what is the colour of astatine?
rare decays rapidly, has never been seen. Least reactive halogen
why do halogens become less reactive going down the group?
- atomic radius increases
- sheilding increases
- less nuclear attraction to capture an electron
- reactivity decreases
what is disproportionation?
a redox reaction where the same element is oxidised and reduced.
how is the reaction of chlorine and water a disproportionate reaction?
in the chlorine molecule, one atom is oxidised and the other is reduced. Chloric (I) acid, HClO, and hydrochloric acid are made.
why is chlorine commonly added to water?
to purify the water, the bacteria are killed by chloric acid.
How can another disproportionate reaction occur?
if the water has sodium hydroxide, more chlorine dissolves. Sodium chlorate (I) is formed which is used as household bleach.
what are the risks of chlorine?
chlorine gas is very irritant and can be fatal in large concentrations. Chlorine can react with organic hydrocarbons forming chlorinated hydrocarbons which are suspected of causing cancer.
How do you test for carbonates?
add dilute nitric acid, if bubbles occur it may be carbon dioxide. To confirm it is carbon dioxide, bubble the gas through lime water. The positive result is a white precipitate which turns the lime water cloudy.
most sulphates are insoluble in water except…
barium sulphate which forms a white precipitate
how do you do halide tests?
add aqueous silver nitrate to an aqueous solution of halide. The silver halide precipitates are different colours- silver chloride is white, silver bromide is cream-coloured and silver iodide is yellow. You can add aqueous ammonia to test the solubility of the precipitate which is useful as the colours can be hard to tell apart.
if you are asked to analyse an unknown inorganic compound, what is the order of tests you need to carry out?
1) carbonate test
2) sulphate test
3) halide test
how do you carry out a sulphate test?
add a solution containing Ba2+ (aq) ions and the positive result is a white precipitate of BaSO4.
why is it important to carry out the carbonate test before the sulphate test?
Barium carbonate, BaCo3 also makes a white precipitate in a sulphate test
why is it important to carry out the carbonate test and sulphate test before the halide test?
although we are looking AgNO3 as a precipitate, silver carbonate and silver sulphate also form precipitates.
how do you analyse a mixture of ions?
1) carbonate test- add dilute nitric acid until bubbling stops to remove all carbonate ions. Use dilute nitric acid, HNO3 as sulphuric acid contains sulphate ions and hydrochloric acid contains chloride ions which would show on the halide tests
2) sulphate test- add excess Ba(NO3) then filter off solution. Don’t use BaCl2 as chloride ions would show on halide test.
3) halide test
Describe chloride, bromide and iodide ions solubility in ammonia, NH3
- chloride is soluble in dilute ammonia
- bromide is soluble in concentrated ammonia
- iodide is insoluble in concentrated ammonia
How do you test for ammonium ions?
add sodium hydroxide and warm the mixture to release bubbles as ammonia is soluble in water . Test gas using moist pH indicator paper, blue is the positive result.
