Chapter 8 Flashcards
What is the charge on the group 2 elements?
2+
What are a couple of the group 2 elements?
Be, Mg, Ca, Sr, Ba, Ra
What is the most common type of reaction for a group 2 element?
A redox reaction due to their 2 outer shell electrons.
Each metal atom is oxidised forming a 2+ ion.
Are group 2 elements reducing agents or oxidising agents?
reducing agents.
How do group 2 elements react with oxygen?
group 2 element + oxygen —> metal oxide
2Mg + O2 —> 2MgO
How do group 2 elements react with water?
group 2 element + water —> metal hydroxide and hydrogen gas
Mg +2H2O —-> Mg(OH)2 + H2
How do group 2 elements react with dilute acids?
group 2 element + dilute acids —> salt and hydrogen gas
Mg + 2HCL —-> MgCl2 +H2
Does reactivity increase or decrease down group 2?
It increases.
Why does reactivity increase down group 2?
They react by losing electrons, the formation of 2+ ions relies on the input of 2 ionisation energies (first and second).
The attraction between the nucleus and the outer electrons decreases as a result of increasing atomic radius and increasing shielding.
Do the group 2 elements become stronger or weaker reducing agents down the group?
stronger
Does ionisation energy decrease or increase down group 2?
decreases, less energy is needed to remove electrons due to the decrease in attraction to the nucleus and increased shielding.
What happens if an oxide of a group 2 element (CaO) reacts with water?
CaO + H20 —> Ca + 2OH-
It produces hydroxide ions and alkaline solutions of the metal hydroxide.
How soluble are group 2 hydroxides in water?
slightly soluble.
when the solution becomes saturated and further metal/ hydroxide ions form a solid precipitate.
Ca2+ + 2OH- —> Ca(OH)2
Does the solubility of the hydroxides in water increase / decrease down the group?
Increases.
What other trends are there with the hydroxides of group 2 down the group?
Solubility increases.
pH increases
Alkalinity increases
How do you show the trends down the group 2 hydroxides with an experiment?
1- add a spatula of each group 2 oxide to water in a test tube
2- shake the mixture. You get a saturated solution of each metal hydroxide + white undissolved solid at the bottom.
3- Measure the pH of each solution
What’s the role of group 2 compounds in agriculture? (calcium hydroxide)
Ca(OH)2 is added to fields as lime by farmers to increase the pH of acidic soil (white powder on fields).
It neutralises acid in the soil and forms water.
Ca(OH)2 + 2H+ —> Ca2+ + 2H2O
What is the role of group 2 compounds in medicine? (Magnesium hydroxide)
Mg(OH)2 is used as an antacid (milk of magnesia) to treat acid indigestion. Its a suspension of Mg(OH)2 in water. Calcium carbonates are also used.
The acid in our stomach is HCL.
Neutralisation reactions occur with Mg(OH)2 and CaCO3
Mg(OH)2 + 2HCL —> MgCl2 + H2O
CaCO3 + 2HCL —-> CaCl2 + H20 + CO2
What group are the halogens?
Group 7
What is the state + colour of Florine at RTP?
Pale yellow gas
What is the state + colour of Chlorine at RTP?
Pale green gas
What is the state + colour of Bromine at RTP?
red-brown liquid
What is the state + colour of Iodine at RTP?
Shiny grey- black solid
What trends are there down group 7?
There are more electrons
Stronger London forces
More energy is required to break the intermolecular bonds
Boiling point increases
What is the most common type of reaction halogens undergo?
Redox reactions
Are halogens reduced or oxidised?
They are reduced and gain 1 electron to form a 1- halide ion.
Are halogens reducing agents or oxidising agents?
Oxidising agents.
Does the reactivity of the halogens increase or decrease down the group?
The reactivity decreases.
How do halogen- halide displacements occur?
A solution of each halogen is added to an aq solution of the other halides.
If the halogen added is more reactive then the halide present then the halogen displaces the halide from the solution and a colour change occurs.
Since iodine and bromine can look similar in water (orange-brown colour) how can we tell them apart?
Add cyclohexane and shake.
The non-polar halogens dissolve more easily in cyclohexane then in water so the colours are much easier to tell apart.
What colour is Cl2 in water?
Pale green
What colour is I2 in water?
brown (can be mixed up with Br2)
What colour is Br2 in water?
Orange (can be mixed up with I2)
What colour is Cl2 in cyclohexane?
Pale green
What colour is Br2 in cyclohexane?
Orange
What colour is I2 in cyclohexane?
violet
What does Cl2 displace?
Br and I (the solution becomes the colour of the Br2 and I2)
What does Br2 displace?
I (the solution becomes the colour of the I2 )
What does I2 displace?
nothing
Is the more reactive halogen left as the halide or halogen?
A halide, it displaces the halide that was there and makes it become a halogen.
Eg. Cl2 + 2I- –> 2Cl- + I2
Is the more reactive halogen left as the halide or halogen?
A halide, it displaces the halide that was there and makes it become a halogen.
Eg. Cl2 + 2I- –> 2Cl- + I2
Why does reactivity decrease down group 7?
Atomic radius increases
There are more inner shells so shielding increases
So there is less nuclear attraction to capture an electron from another species
So reactivity decreases
(They become weaker oxidising agents down the group)
Why does boiling point increase down group 7?
The halogens are simple molecular structures with weak London forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces.
The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces.
Therefore, the larger the molecule the stronger the London forces between molecules. This is why as you go down the group, it gets more difficult to separate the molecules and the boiling point increases
What is disproportionation?
A redox reaction where the same element is both oxidised and reduced
What are 2 disproportionation reactions with chlorine?
Chlorine + water
chlorine + cold, dilute aqueous alkali
What is the reaction of chlorine with water?
Chlorine can be used to clean water and make it drinkable.
Cl2 + H20 —> HCL +HClO
Chloric acid (HClO) sterilises water by killing bacteria and can also further dissociate in water to form ClO-.
ClO-(aq) also acts as a sterilising agent cleaning the water
chloric acid also works as a weak bleach - This is shown when indicator is added to chlorine in water. The solution will first turn red then will turn colourless due to the bleaching action of chloric acid.
What is the reaction of chlorine with cold, dilute aqueous sodium hydroxide?
Cl2 + 2NaOH —> NaCl + NaClO + H2O
ionic - Cl2 + 2OH- —> Cl- + ClO- +H2O
Chlorine gets oxidised as there is an increase in ox. no. from 0 to +1 in ClO-(aq)
Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)
The mixture of NaCl and NaClO (sodium chlorate (I)) is used as Bleach and to disinfect/ kill bacteria.
What is the reaction of chlorine with cold, dilute aqueous sodium hydroxide?
Cl2 + 2NaOH —> NaCl + NaClO + H2O
ionic - Cl2 + 2OH- —> Cl- + ClO- +H2O
Chlorine gets oxidised as there is an increase in ox. no. from 0 to +1 in ClO-(aq)
Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)
The mixture of NaCl and NaClO (sodium chlorate (I)) is used as Bleach and to disinfect/ kill bacteria.
Benefits of using chlorine?
Adding chlorine to a water supply is an effective way to make the water safe to drink.
Chlorine is used in water treatment to kill bacteria (drinking water and swimming pools)
Risks of using chlorine?
Chlorine in drinking water could react with organic hydrocarbons and create chlorinated hydrocarbons which could cause cancer. But if we didnt add chlorine the quality of drinking water would be compromised and could lead to break outs of typhoid/cholera.
What test do you use to identify a halide and how does It work?
Dissolve the unknown solution in nitric acid and then add silver nitrate solution dropwise.
halide ions react with the silver nitrate solution.
X- + Ag+ —> AgX (s)
If the unknown solution contains halide ions, a precipitate of the silver halide will be formed.
Silver chloride (AgCl) is a white precipitate
Silver bromide (AgBr) is a cream precipitate
Silver iodide (AgI) is a yellow precipitate
How do you distinguish between the 3 precipitates made in the halide test?
white, cream and yellow precipitates could look very similar in colour. So ammonia is often used as a follow up test to determine which halide ion is present.
Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion.
If it dissolves in dilute NH3 its Cl-
if it dissolves in concentrated NH3 its Br-
If it doesn’t dissolve in either it is I-
What test do you use to identify a carbonate and how does It work?
Add dilute nitric acid to to the solution.
if there’s bubbles it could be a carbonate but to confirm you bubble the gas through limewater (Ca(OH)2) if the limewater turns cloudy then calcium carbonate has formed and a carbonate was present.
What test do you use to identify a sulfate and how does It work?
BaSO4 is very insoluble in water.
Add barium ions to the unknown compound and a white precipitate will form if a sulfate was present. Ba2+ ions are added as barium chloride or barium nitrate.
Ba2+ + SO42- —-> BaSO4
What test do you use to identify the ammonium ion and how does It work?
We can test for the presence of ammonium ions, NH4+, by reacting the unknown with warm aqueous sodium hydroxide which would form ammonia gas.
NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
If you place damp litmus paper above the test tube it will go from red to blue.
What’s the correct sequence of tests? and why must they be done in this sequence?
