Topic 4: Inorganic Chemistry and the Periodic Table Flashcards
Why does ionisation energy of group 2 elements decrease down the group?
- each element down Group 2 has an extra electron shell compared to one above
- extra inner shells shield the outer electrons from the attraction of the nucleus
- outer electrons are also further away from the nucleus, reducing electrostatic attraction between the nucleus and outer electrons
- the positive charge does increase down the group, but the top two factors override
Does reactivity increase or decrease down group 2?
- reactivity increases down the group
- as atomic radii increase there is more shielding
- the nuclear attraction decreases and it is easier to remove outer electrons and so cations form more easily
What do group 2 metals react with water to make? Write a general equation
- M(s) + 2H2O(l) → M(OH)2(aq) + H2(g)
Which group 2 metals react with cold water? And which doesn’t?
- Ca, Sr, Ba react with cold water to form hydroxides
- Mg needs to react with warm water to form its hydroxide
What would you observe with the formation of a group two hydroxide?
- fizzing (more vigorous down the group)
- metal dissolving (faster down group)
- solution heating up (more down group)
- with calcium: a white precipitate appearing
Describe the reaction between magnesium and steam
- forms magnesium oxide and hydrogen (Mg reacts differently in steam)
- Mg will burn with a bright white flame
- Mg(s) + H2O(g) → MgO(s) and H2(g)
What do group 2 metals react with oxygen to make?
- solid white oxides
- Mg with a bright white flame
What do group 2 metals react with chlorine to make?
- solid white chlorides
- M(s) + Cl2(g) → MCl2(s)
- more vigorous down group
Why are group 2 oxides classed as basic oxides?
- the oxide ions accept protons to become hydroxide ions
Describe the reaction of group 2 oxides with water
- MO(s) + H2O(l) → M(OH)2(aq)
- forms colourless solutions
Which metal oxides are the exceptions to being bases?
- beryllium oxide: doesn’t react with water and is insoluble
- magnesium oxide: reacts slowly and OH isn’t very soluble
Describe the reactions of group 2 oxides and hydroxides with acids
- neutralisation reaction
- salt and water
- white solid to colourless solution
- exothermic reaction
What is the trend in solubility of group 2 hydroxides?
- group 2 hydroxides become more soluble down the group
What is calcium hydroxide used for?
- Testing for CO2:
- a saturated solution of calcium hydroxide is limewater
- CO2 + Ca(OH)2 → CaCO3 + H2O
- white precipitate forms
- Neutralising soils in agriculture
How is magnesium hydroxide used?
- used in medicine to neutralise excess stomach acid (HCl)
- it is safe to use because the very low solubility of magnesium hydroxide means it is only weakly alkaline
What is the trend of solubility of group 2 sulfates?
- group 2 sulfates become less soluble down the group
- magnesium sulfate is classed as soluble
- calcium sulfate slightly soluble
- strontium and barium sulfate insoluble
How do you test for sulfate ions?
- add barium ions (usually barium chloride or nitrate)
- forms a white precipitate of barium sulfate
- carbonate ions can also form white precipitate with barium ions, so there must be H+ ions to prevent it from forming
- add dilute nitric acid or dilute HCl
What is thermal decomposition?
- the use of heat to break down a reactant into more than one product
What is the trend of thermal stability down group 1 and group 2 compounds?
- thermal stability increases down a group
- carbonate and nitrate ions are large negative ions (anions) and can be made unstable by the presence of a cation
- the cation polarises the anion, distorting it
- the greater the distortion the less stable the compound
- large cations cause less distortion than small cations as they have a lower charge density
- so further down the group, the larger the cations, the lower the charge density so the less distortion caused and the more stable the carbonate/nitrate compound
Are group 1 or group 2 compounds less thermally stable?
- group 2 compounds are less thermally stable than group 1
- the greater the charge on the cation, the greater the distortion and the less stable the carbonate/nitrate compound becomes
What do group 1 carbonates decompose to?
- they are thermally stable, so they do not decompose with a Bunsen flame
- except for lithium carbonate because it is small enough to have a polarising effect
- Li2CO3(s) → Li2O(s) + CO2(g)
What do group 1 nitrates decompose to?
- form metal nitrite (nitrate(III) salt) and oxygen
- e.g. 2NaNO3 → 2NaNO2 +O2
- except lithium nitrate which decomposes like group 2 nitrates to form oxide, oxygen and nitrogen dioxide gas
- 4LiNO3 → 2Li2O + 4NO2 + O2
What do group 2 carbonates decompose to?
- forms group 2 oxides and CO2
- e.g. MgCO3(s) → MgO(s) + CO2(g)
What do group 2 nitrates decompose to? State the observations
- form the oxide, nitrogen dioxide and oxygen
- brown fumes of nitrogen dioxide and white nitrate solid is seen to melt to a colourless solution then resolidify
- e.g. 2Mg(NO3)2 → 2MgO + 4NO2 + O2
How can you test the thermal stability of nitrates?
- how long it takes until a certain amount of oxygen is produced
- how long it takes until NO2, a brown gas, is produced
- needs fume cupboard
How to test thermal stability of carbonates?
- how long it takes for an amount of CO2 to be produced
- test using lime water which turns cloudy with CO2
What flame colour is lithium, sodium, potassium, rubidium, caesium, calcium, strontium, barium?
- Li: red
- Na: orange/yellow
- K: lilac
- Rb: red
- Cs: Blue
- Ca: brick-red
- Sr: crimson
- Ba: green
How to do a flame test
- use a nichrome wire as it is unreactive and will not give a flame colour
- dip in concentrated HCl to sterilise and heat in Bunsen flame
- dip wire in solid and put in Bunsen flame and observe flame
Why do flames have colour?
- energy absorbed from the flame causes electrons to move to higher energy levels (from ground state to excited state)
- colours are seen as electrons fall back down to lower energy levels, releasing energy in the form of light
- the difference in energy between higher and lower levels determines wavelength of light released
- this determines colour of the light
What are the colours, states of the first four halogens?
- fluorine: very pale yellow gas
- chlorine: greenish gas
- bromine: red liquid, that gives off brown, orange fumes
- iodine: shiny grey solid sublimes to purple gas
What are the physical states at room temperature of the first 4 halogens?
- F2: gas
- Cl2: gas
- Br2: liquid
- I2: solid
What is the trend in electronegativity in halogen and why?
- electronegativity decreases down the group
- the atomic radii increases due to the increasing number of shells
- nucleus is therefore less able to attract the bonding pair of electrons
What are the trends in melting and boiling points of halogens?
- they increase down the group
- as the molecules become large, they have more electrons and so have larger London forces between the molecules
- as the intermolecular forces increase, more energy has to be put in to break the forces
What is the trend in reactivity down the halogens group?
- reactivity decreases down group 7
- atoms become larger
- outer electrons are further from the nucleus
- shielded more from the attraction of nucleus
- harder for larger atoms to attract the electron to form an ion
Why are halogens oxidising agents?
- they usually react by gaining an electron, so they are reduced
- they oxidise other substances
Describe the solubility in water of halogens
- halogens are non-polar diatomic molecules
- they have low solubility in water
What do halogens dissolve in?
- they dissolve easily in organic compounds (such as cyclohexane)
What is the colour of chlorine, bromine and iodine in hexane?
- chlorine: virtually colourless
- bromine: yellow
- iodine: purple
What is the colour in water of chlorine, bromine and iodine?
- chlorine: virtually colourless (or very pale green)
- bromine: yellow/orange
- iodine: brown
Which halogen will displace both bromide and iodide ions?
- chlorine
What is a displacement reactions between halogens and halides called?
- a redox reaction
- what is displaced is oxidised, what is displacing is reduced
Write down half equations for the displacement reaction between chlorine and potassium bromide
- Cl2 + 2e- → 2Cl-
- 2Br- → Br2 + 2e-
Write down the ionic equation when chlorine water reacts with potassium bromide
- Cl2 + 2Br- → 2Cl- + Br2
What is the colour change when bromide is displaced and Br2 is formed?
- colourless to orange mixture formed
What is the colour change when iodide is displaced and iodine is formed?
- colourless to brown
What would shaking a halogen/halide displacement reaction mixture with an organic solvent help to do?
- the halogen present will dissolve in the organic solvent
- easier to see the colour changes
Describe how halogens react with group 1 and group 2 metals
- forms halide salts
- most vigorous reactions are elements at the bottom of group 1 and 2 with elements at the top of group 7
- redox reaction
What is a disproportionation reaction?
- an element is simultaneously oxidised and reduced
Describe the reaction between chlorine and water and its uses
- Cl2(g) + H2O(l) → HClO(aq) + HCl(aq)
- disproportionation reaction
- product is called chloric(I) acid
- chlorine is used in water treatment to kill bacteria
What is the reaction between chlorine (and other halogens) with cold, dilute aqueous sodium hydroxide?
- 2NaOH(aq) + Cl2(g) → NaClO(aq) + NaCl(aq) + H2O(l)
- disproportionation reaction
- sodium chlorate is bleach
- colour of halogen solution fades to colourless
What is sodium chlorate used for?
- water treatment
- bleach paper
- textiles
What is the disproportionation reactions of chlorine (and other halogens) with hot alkalis?
- 3Cl2 + 6NaOH → 5NaCl(aq) + NaClO3(aq) + 3H2O(l)
Describe the trend in reducing power of halide ions
- reducing power increases down the group
Describe the reaction between NaF and NaCl with H2SO4
- sulfuric acid is not strong enough an oxidising agent to oxidise the chloride and fluoride ins
- no redox reactions occur, only acid-base reactions
- white steamy fumes observed
Describe the reaction between NaBr and H2SO4
- Br- ions are stronger reducing agents than Cl- and F- and after the initial acid-base reaction reduce the sulfur in H2SO4 from +6 to +4 in SO2
- white steamy fumes of HBr
- red fumes of bromine and a colourless acidic gas of SO2
Describe the reaction between NaI and H2SO4
- I- ions are the strongest halide reducing agents
- they reduce sulfur from +6 in H2SO4 to +4 in SO2 to 0 in S to -2 in H2S
Observations:
- white steamy fumes of HI
- black solid and purple fumes of iodine
- colourless, acidic gas of SO2
- yellow solid of sulphur
- H2S gas, like rotten eggs
What colour are hydrogen halides?
- colourless gases
Do hydrogen halides dissolve in water?
- yes
- they are polar diatomic molecules
Describe the reactions of hydrogen halides with water
- they readily react with water to form acidic solutions, which are all colourless
What do hydrogen halides react with ammonia gas to give?
- forms salts, all of which are white ionic solids
- e.g. ammonia and chlorine gas give ammonium chloride
- white fumes
Describe silver nitrate to test for halides
- add dilute nitric acid to remove any carbonates present that may react with silver ions
- add silver nitrate
- a precipitate of silver halide is formed
What colour precipitate is formed when reacting silver nitrate with chloride?
- white precipitate
What colour precipitate is formed when reacting silver nitrate with bromide?
- cream precipitate
What colour precipitate is formed when reacting silver nitrate with iodide
- yellow precipitate
Why would you add ammonia solution to silver halide?
- AgCl: precipitate dissolves in dilute ammonia solution to give a colourless solution
- AgBr: Precipitate remains unchanged if dilute ammonia solution is added, but will dissolve in concentrated ammonia solution to give colourless solution
- AgI: precipitate does not dissolve even in conc ammonia
What does dilute HCl test for?
- carbonates and hydrogencarbonates
- they give off CO2 (and H2O)
- test CO2 with limewater
How do you test for sulfate?
- add dilute HCl to get rid of traces of carbonate, which may produce precipitate
- add barium chloride solution
- if a white precipitate of barium sulfate forms, there are sulfates
How do you test for ammonium compounds?
- to test whether substance has NH4+, add sodium hydroxide and gently heat
- if ammonia is given off, then there are ammonium ions
- test for ammonia using damp piece of red litmus paper, which will turn blue
