Chapter 8 - Reactivity trends Flashcards
What is common about all elements in group 2?
They are all reactive metals which form 2+ ions
Why are group 2 elements called reducing agents?
In a redox reaction, they lose electrons (oxidation) which is donated to the other reactant, reducing it
What is the most common type of reaction for group 2 elements?
Redox
What happens when a group 2 metal reacts with oxygen?
It forms a metal oxide with the general formula MO
What is produced when a group 2 metal burns in oxygen?
A solid, white oxide
What is produced when group 2 metals react with water?
An alkaline metal hydroxide and hydrogen gas
What is the general formula for group 2 metal hydroxides?
M(OH)2
What is produced when group 2 metals react with dilute acid?
Metal + Acid -> Salt + Hydrogen
Why does reactivity increase down group 2?
Group 2 metals wish to lose 2 electrons from their outer shell, to gain a full outer shell. Ionisation energy decreases down the group, because of an increase in atomic radius and there are more electrons to shield
Why do the first and second ionisation energies decrease down group 2?
Increasing atomic radius and the shielding effect of other electrons means less energy is needed to overcome the force
What happens when an oxide of a group 2 metal reacts with water?
Metal hydroxides are formed which dissolve to form hydroxide ions (OH-) and alkaline solutions of the metal hydroxide (ions)
Why is slaked lime (Ca(OH)2) often spread onto fields?
It is a base and neutralises acidic soils so crops can grow
Why are group 2 bases, like Mg(OH)2 and CaCO3 used as antacids?
The bases neutralise the hydrochloric acid in your stomach which is causing the discomfort
Ionic equation for neutralisation
H+ (aq) + OH- (aq) -> H2) (l)
What happens when a solution of group 2 hydroxides becomes saturated?
Any metal and hydroxide ions will form a solid precipitate
Because the solubility of group 2 hydroxides increases down the group, what happens to the resulting solution?
They contain more OH- ions and are more alkaline
How to show the increasing alkalinity of group 2 hydroxides?
Add a spatula of metal oxide to some water and shake
Measure the pH. This will increase down the group
Why do the group 7 elements not occur in their elemental forms in nature?
They are highly reactive
At RTP, what will all halogens exist as?
Diatomic molecules (two atoms joined by a single covalent bond)
In their solid states, what will the halogens form lattices with?
Simple molecular structures
Colour and state of fluorine
Pale yellow, gas
Colour and state of chlorine
Green, gas
Colour and state of bromine
Red-brown, liquid
Colour and state of iodine
Grey, solid
As you go down group 7, why do melting and boiling points increase?
As the size of the atom increases, there are more electrons, and so the strength of the induced dipole-dipole forces increases, meaning more energy is needed to overcome the intermolecular forces
What is the most common type of reaction for halogens?
Redox
Why are halogens known as oxidising agents?
In a reaction, they gain one electron from the other reactants, meaning the halogen is reduced and the other is oxidised
If the halogen added to the halide in a displacement reaction is more reactive, what happens?
The halogen will displace the halide from the solution and the reaction will change colour
What chemical is added to the solution during a displacement reaction and why?
Cyclohexane - the non-polar halogens dissolve more readily with this than water and so the colour change is clearer to see
What colour can be observed when bromide ions are added to chlorine?
Orange - Br2 is formed
What colour can be observed when iodide ions are added to chlorine?
Violet - I2 is formed
What colour can be observed when chloride ions are added to bromine?
No reaction
What colour can be observed when iodide ions are added to bromine?
Violet- I2 is formed
What colour can be observed when either chloride ions or bromide ions are added to iodine?
No colour change - iodine is the least reactive and won’t be displaced
How reactive is fluorine?
Very - it reacts with almost any substance it comes into contact with
How reactive is astatine?
It is radioactive and decays rapidly so that is unknown, but it is predicted to be the least reactive
Why does reactivity decrease as you go down group 7?
Halogens want to gain another electron to achieve the full outer shell that is desirable
As you down the group, the atomic radius increases and there more inner electrons, so shielding increases too
There is, therefore, less nuclear attraction to attract the final electron needed for a full outer shell
Reactivity decreases
What is a disproportionation reaction?
A chemical reaction in which an element is both oxidised and reduced
Two examples of disproportionation reactions
Chlorine with water
Chlorine with cold, dilute sodium hydroxide
Why is chlorine added to water?
It forms chloric acid which kills bacteria so the water is safe to drink
What is produced when chlorine is added to water?
Hydrochloric acid and chloric acid
What happens when chloric acid is ionised?
It forms chlorate (I) ions, which are what kills bacteria
What is produced when chlorine is added to sodium hydroxide?
Bleach
Why is chlorine important in water treatment? (3)
It kills bacteria
Some chlorine remains in the water and prevents reinfection
Prevents the growth of algae, which can cause discolouration and bad smells/tastes
Ethical issues with chlorinating water
We don’t get a choice about having chlorine in our water
Problems with using chlorine in water (3)
It irritates the respiratory system if inhaled
Liquid chlorine causes severe chemical burns
Chlorine can react with organic hydrocarbons to form chlorinated hydrocarbons
What is bad about chlorinated hydrocarbons?
They are carcinogens
Alternatives to chlorinating the water
Use ozone, which is good at killing bacteria, but too expensive to produce
Use UV light, which kills bacteria by damaging their DNA, but it isn’t effective in cloudy water
What is formed when halogens are added to silver nitrate?
A coloured precipitate
How do you test for a carbonate?
Add dilute nitric acid to the solution
If you see bubbles, these could be carbon dioxide
To test for carbon dioxide, bubble the gas through limewater. It should turn cloudy.
If carbon dioxide is produced, a carbonate is present
How to test for halides
Add aqueous silver nitrate to the solution
Record the colour of the precipitate
Add ammonia solution to the mixture
Record the solubility
Colour of silver chloride
White
Colour of silver bromide
Cream
Colour of silver iodide
Yellow
When will silver chloride be soluble?
Dilute ammonia
When will silver bromide be soluble?
Concentrated ammonia
When will silver iodide be soluble?
It won’t - it is insoluble in ammonia
How to test for ammonia compounds
Add sodium hydroxide to ammonia solution
Warm the mixture and ammonia gas will be released
Ammonia is alkaline so will turn red litmus paper blue
What is the correct order of testing for anions?
Carbonate test
Sulfate test
Halide test
Why should the carbonate test go first?
Neither sulfides nor halides produce bubbles with acid. Therefore, it can be carried out with the possibility of an incorrect answer
Why should the sulfate test go second?
Barium carbonate also produces the characteristic white precipitate, which could give you a false positive. Therefore, you have to carry out the carbonate test first to ensure it isn’t a carbonate
Why should the halide test go last?
Silver carbonate and silver sulfate will also produce precipitates, so it is important to have carried out the other tests first to ensure there is no confusion
What should you when conducting the carbonate test on a mixture of ions?
Use nitric acid (sulfuric acid will interfere with the sulfate test and hydrochloric with the halide test) and continue adding until bubbling stops, so all the carbonate has been removed
What should you do when conducting the sulfate test on a mixture of ions?
Add an excess of barium nitrate
Any sulfate ions present will form a precipitate, which can then be removed
What should you do when conducting the halide test on a mixture of ions?
Add silver nitrate to the remaining solution
Any sulfate or carbonate ions will have already been removed, so you know that if it forms a precipitate it will be a halide
Add ammonia to discover which
How to test for a sulfate
Add barium nitrate to your solution (barium chloride introduces halide ions which can affect later tests)
Barium sulfate forms a white precipitate - if this forms, sulfate ions are present
Equation for the formation of bleach
Cl2 + 2NaOH -> NaCl + NaClO + H2O
What conditions must be present for bleach to form?
The sodium hydroxide must be cold and dilute
What is periodicity?
A repeating pattern of properties shown across different periods
What colour does Cl2 go in cyclohexane?
Pale green
What colour does Br2 go in cyclohexane?
Orange
What colour does I2 go in cyclohexane?
Violet - it changes from brown
What happens to the pH of the solution formed when group 2 metals are added to water as you go down the group?
As you go down group 2, the solubility of the metal increases which increases the pH and alkalinity
Why is chlorine more reactive than bromine?
Gains an electron more easily
Chlorine smaller
Decreased shielding
Greater nuclear attraction
How do you test for ammonia?
Add ammonia hydroxide
Warm the mixture to produce gas
Turns indicator blue
