Reactivity Trends (8) Flashcards
How many electrons in a Group 2 outer shell
2
What part of the redox reaction do Group 2 elements undergo?
Oxidation. 2 of their electrons are removed in order to have the same structure as the nearest noble gas.
Are Group 2 elements oxidising or reducing agents
They are reducing agents as they are oxidised and the 2 electrons removed from them will be accepted by another element which will become reduced.
Group 2 elements and oxygen
They will react with oxygen to form metal oxides with the molecular formula MO as the +2 and -2 cancel out.
Magnesium + Oxygen
Reacts to make white magnesium oxide and burns with a bright white light. Mg is oxidised and O2 is reduced.
Group 2 elements and water
React to form a metal hydroxide M(OH)2 which is alkali because it releases OH- ions. Water reacts very slowly with the top elements, but as you go down the group it becomes more vigorous.
Magnesium + Water
React really slowly because magnesium isn’t too reactive.
Group 2 elements and dilute acids
metal + acid -> salt + hydrogen
What happens to reactivity down Group 2
As you go down Group 2, the reactivity increases because of the first and second ionisation energies. It is a lot easier to remove the outer electrons on the lower elements due to electron shielding etc.. so the lower you go, the more reactive they are.
Group 2 oxides and water
Makes the group 2 ion and 2 hydroxide ions which means it becomes an alkali solution. If the solution becomes saturated, the M2+ ion and 2OH- ion will combine to make M(OH)2 precipitate.
What happens to solubility of metal hydroxides down Group 2
As you go down the group, the solubility of hydroxides in water will increase because the metals are more reactive so will be more attracted to the water and will result in more OH- ions being released.
What is the trend in pH down Group 2 hydroxides?
The pH will increase as you go down group 2 as it is becoming more alkali as you go down. Magnesium is only slightly soluble in water so it will only release a few OH- ions, whereas Barium is more soluble so, will release a lot more OH- ions and have a bigger pH.
Group 2 compound uses
Used in agriculture or medicine
Group 2 uses in agriculture
The use of Ca(OH2) which is white lime and neutralises acid in soil
Group 2 uses in medicine
Treats acid indigestion by neutralising it, and is called antacid. Neutralised the HCl in your stomach.
How to test for pH of Group 2 oxides
Add a spatula of metal oxide to a test tube and add water. Shake the tube and you won’t have enough water to dissolve all the metal oxide so you will have precipitate at the bottom and a solution which is your metal hydroxide. Test the pH of this to see what you get.
Halogens
Group 7 elements that are the most reactive non-metal elements
How do halogens exist?
As diatomic molecules with simple molecular structures. They exist in all states.
What is the trend of boiling points?
The boiling point will increase down the group and this is because they exist as diatomic molecules, so as you go down the group, the number of electrons increases, so the number of London forces increases. This means that the energy required to break the bond between them will get higher.
Fluorine state and colour
pale yellow gas
Chlorine state and colour
pale green gas
Bromine state and colour
red-brown liquid
Iodine state and colour
black solid
Astatine state and colour
never been seen
What part of the redox reaction do halogens undergo?
They undergo a reduction as they gain an electron and they are oxidising agents as they will oxidise other elements.
Halogen - Halide Displacement Reaction
This is where the halogens will displace halide ions and is used to show that reactivity decreases down the group. You add a halogen to different halide solutions and see if the halogen is more reactive than the halide If it is then a reaction will occur and a colour change will take place
colour of chlorine in water
pale green
colour of bromine in water
orange
colour of iodine in water
brown
What can you use as a solvent for halogens instead of water?
Cyclohexane because the bromine and iodine in water solutions are similar colours in water. The halogens will dissolve more readily in cyclohexane so will have more distinguishable colours
colour of chlorine in cyclohexane
pale green
colour of bromine in cyclohexane
orange
colour of iodine in cyclohexane
purple
Results of the displacement reaction
Chlorine has reacted with both bromine and iodine so has displaced both reactions
Bromine reacted with iodine only and displaced it, but not with chlorine
Iodine didn’t react with any.
What do the halogen halide reactions prove?
That reactivity decreases down the group
What is the reactivity trend and why?
The reactivity decreases down the group. When halogens react, they gain an electron so the more shells they have, the greater the atomic radii, and the smaller the nuclear charge, the less attracted an electron on another atom will be, so it will be harder for it to react.
Disproportionate Reactions
This is when the same element is both reduced and oxidised in the same reaction. Two examples are chlorine + water and chlorine + dilute sodium hydroxide
Chlorine + water
The chlorine will be reduced into hydrochloric acid and chloric acid which can kill bacteria
Chlorine + dilute sodium hydroxide
The chlorine is reduced into NaCl and the chlorine is oxidised into NaClO which is sodium chlorate which is used as bleach. Water is also formed.
Benefits of chlorine
Can kill harmful bacteria, so have been made into tablets and added to drinking and pool water in order to make it purified.
Risks of chlorine
As a gas, it’s extremely harmful as it can irritate the lungs. It can also combine with methane to form chlorinated hydrocarbons which can cause cancer
Quantitative Analysis
Doing an experiment and taking measurements that are with numbers to calculate the result of the experiment
Qualitative Analysis
Relying on observations like colour, gas bubbles etc to determine the result of your experiment
Carbonate Test
Carbonates are CO3 2- anions that react with dilute acid to form carbon dioxide gas.
To test for them put dilute nitric acid on the compound to be tested.
If you see bubbles, the unknown compound could be a carbonate
You can prove the gas is carbon dioxide by doing a lime water test.
Lime Water Test
You run carbon dioxide through lime water which is calcium carbonate. This will form a cloudy white precipitate.
Sulphate Test
Most sulphates are soluble in water, but barium sulphate is very insoluble. So if you want to test a compound to see if it’s a sulphate, add barium nitrate and see if any barium sulphate precipitate it formed. It should be a white precipitate.
Halide Test
Most halides are soluble in water, but silver halides are not. So when you want to test for halide ions:
you add in silver nitrate
you then form a silver halide precipitated that can be different colours depending on the halide
Add dilute then concentrated aqueous ammonia to test the solubility as they can be different colours
Silver chloride precipitate
white
Silver bromide precipitate
cream
Silver iodide precipitate
yellow
Silver chloride solubility
soluble in dilute ammonia
Silver bromide precipitate
soluble in concentrated ammonia
Silver iodide precipitate
insoluble in concentrated ammonia
The sequence of tests when trying to recognise a compound.
Carbonate Test, Sulphate Test, Halide Test
Carbonate Test goes first as it doesn’t have the possibility of an incorrect experiment, sulphates and halides don’t react with dilute acid to form bubbles
Sulphate Test goes after because barium also reacts with carbonate to form a white precipitate, so you have to determine there’s no carbonate first.
Halide Test last because silver sulphate and carbonate both form precipitates.
What if you want to analyse a mixture of ions?
You first do the carbonate test and keep adding nitric acid until the bubbling stop so you can get rid of all the carbonate ions. Then do the sulphate test and add barium nitrate to the solution until all of the barium sulphate has precipitated out, then just filter it out.
Do the halide test last and then determine which silver halide precipitate you get.
Ammonium Ion Test
You add sodium hydroxide to the solution you want to test. It will form ammonia gas and water. The ammonia gas will then evaporate when warmed and you can smell it, or you can put a paper pH indicator and it will turn blue.
