Inorganic Chemistry (Topic 4) Flashcards
What ions do group 2 elements form when they react
+2 ions
How do group 2 elements form ions
Group 2 metals lose 2 electrons to form +2 ions.
What is the general similarity in group 2 electron configuration
Group 2 metals all have 2 electrons in their outer shell, configurations that end in s²
Be - ends in 2s²
Mg - ends in 3s²
Ca - ends in 4s²
What happens to ATOMIC RADIUS as we go DOWN group 2
Atomic radius INCREASES as we go down group 2.
Why does atomic radius increase as we go down group 2
Extra electron shells are added to the atom as we go down group 2.
What happens to 1st ionisation energy as we go down group 2
1st ionisation energy DECREASES as we go DOWN group 2
Why does 1st ionisation energy decrease as you go down group 2
Extra shells are added as we go down group 2.
The outer electrons are further from the nucleus which weakens attraction.
There is more shielding, hence weaker attraction between nucleus and outer electrons.
Both of these factors make it easier to remove the outer electron, so less energy is needed to remove the outer electron.
Protons number increases as we go down, which should increase ionisation energy, but shielding effect overrides this increase in positive charge.
What is 1st ionisation energy
The energy required to remove the 1st electron from the outer shell of an atom, ionising it.
What is shielding
When there is an increase in the electron shells of an atom, the outer electrons are shielded from the nucleus by these shells, so there is less attraction between the nucleus and the outer electrons.
What should increase 1st ionisation energy as we go down group 2, and why does it not increase it
We have an increase in the number of protons as we go down the group, so there should be more electrostatic attraction between these protons and the electrons as we go down, increasing 1st ionisation energy.
However, the shielding effect overrides an increase in positive charge.
What happens when group 2 elements react with water
They form bases. Specifically, they react with water to form water hydroxides + hydrogen
Sr(s) + 2H2O(l) -> Sr(OH)_2(aq) + H_2(g)
So Calcium + Water –> Calcium Hydroxide + Hydrogen
What happens to reactivity with water of group 2 elements as we go down
As we go down group 2, reactivity with water increases. So Be does not react with water, but Sr and Ba react quite strongly with water.
What reacts with water more vigorously, group 1 or group 2 elements
The most reactive group 1 elements, the ones at the top of group 1, react more violently with water than the most reactive group 2 elements, the ones at the bottom of group 2.
Why does reactivity with water increase as we go down group 2
Atoms get larger, which means more electron shells, and electron is further away from nucleus. There is more shielding. Outer electrons are easier to remove, hence the metal is more reactive.
How does magnesium react with cold water
Mg reacts slowly with cold water
How does magnesium react with steam, and what does the reaction produce
Mg reacts vigorously with steam. This produces Magnesium Oxide, MgO, instead of a hydroxide.
How can you tell if an element is reacting with steam rather than water
It will be H2O(g) with a gas state symbol
What do group 2 elements form when they react with oxygen
Group 2 elements react with oxygen to form metal oxides (which are bases)
2Mg(s) + O_2(g) -> 2MgO(s)
What happens to magnesium and oxygen in terms of oxidation number in the reaction between them
2Mg(s) + O_2(g) -> 2MgO(s)
Oxidation number of Mg increases from 0 to +2. Magnesium is being oxidised.
Oxidation number of O decreases from 0 to -2. Oxygen is being reduced.
2Mg(s) + O_2(g) -> 2MgO(s)
This is a redox reaction
What happens when you put a magnesium ribbon in a flame
You get a brilliant white flame, and a white powder is produced.
What do group 2 oxides look like
All group 2 elements are white solids.
What do group 2 elements form when they react with chlorine
They form metal chlorides.
Mg(s) + Cl_2(g) -> MgCl_2(s)
What happens to magnesium and chlorine in terms of oxidation number in the reaction between them
Oxidation number of Mg increases from 0 to +2. Mg is oxidised. Oxidation number of Cl decreases from 0 to -1. Cl is being reduced. This is a redox reaction.
Mg(s) + Cl_2(g) -> MgCl_2(s)
What do group 2 OXIDES, like SrO, BaO,
form when they react with water
Alkaline solutions are formed.
The metal hydroxide formed dissociates into ions which make the solution alkaline.
SrO(s) + H2O(l) -> Sr(OH)_2(aq)
SrO(s) + H2O(l) -> Sr^2+(aq) + 2OH^- (aq)
How does magnesium and beryllium react with water
Magnesium oxide reacts very slowly and the hydroxide barely dissolves. Beryllium oxide doesn’t react with water at all and the hydroxide is insoluble. So beryllium oxide doesn’t form a basic solution at all
What happens to alkalinity of group 2 oxides reacting with water
The further down the group 2 oxides you go, the more alkaline the solution that the metal oxide makes with water is.
BaO(s) + H2O(l) -> Ba^2+(aq) + 2OH^- (aq)
is a very alkaline solution compared to
SrO(s) + H2O(l) -> Sr^2+(aq) + 2OH^- (aq)
Why are group 2 oxides that react with water more alkaline as you go down the group
The hydroxide ions formed by the group 2 oxides further down gr2 are more soluble, they dissociate, meaning there are more OH-(aq) ions, making the solution more strongly alkaline.
The more readily the hydroxide ions dissolve, the more it dissociates, the more alkaline the solution is.
What can group 2 oxides and hydroxides neutralise
Group 2 oxides and hydroxides are bases so they can neutralise acids, giving you salt + water.
What does an acid + base give you
Salt + Water
CaO(s) + 2HCl(aq) -> CaCl_2(aq) + H_2O(l)
Ca(OH)_2(s) + 2HCl(aq) -> CaCl_2(aq) + 2H_2O(l)
What is the general rule of group 2 compounds in terms of solubility
If the anion (negative ion) has a double charge, they become less soluble as we go down the group.
So group 2 sulfates (SO_4 ^2-), become less soluble as we go down the group.
If the anion has a single charge, they become more soluble as you go down the group.
What relationship do group 2 sulfates and group 2 hydroxides have in terms of solubility
They have opposite solubilities as you go down group 2. Group 2 Hydroxides are more soluble as you go down the group, group 2 sulfates become less soluble as you go down the group.
What do you call it when a compound is almost insoluble, so barely soluble in water
Sparingly soluble
What happens to group 2 carbonates and nitrates upon heating
They can decompose. This can be done by putting the compound above a flame.
What is a sign of a thermal decomposition reaction that might allow you to recognize it
There is usually one solid reactant breaking down into products.
CaCO_3(s) -> CaO(s) + CO_2(g)
What do group 2 carbonates form when they thermally decompose
Carbonates break down into metal oxides and carbon dioxide. This occurs via thermal decomposition.
CaCO_3(s) -> CaO(s) + CO_2(g)
What do group 2 nitrates form when they thermally decompose
Nitrates break down into metal oxides, nitrogen dioxide, and oxygen via thermal decomposition.
2Ca(NO_3)_2(s) -> 2CaO(s) + 4NO_2(g) + O_2(g)
What happens to the thermal stability of carbonates/nitrates as you go down group 2
Group 2 carbonates/nitrates become more thermally stable as you go down group 2
What does a compound being more thermally stable mean
It is more difficult for it to thermally decompose, as it is already stable in the form it is in
Why do group 2 carbonates/nitrates become more thermally stable as you go down group 2
Carbonate/nitrate ion has a large electron cloud that can be distorted when it is near positive group 2 metal ions. The more distorted the cloud is, the more the electrons are likely to shift, meaning the less thermally stable the compound is.
Group 2 metal ions all have a 2+ charge, but as you go down the group, the ions become larger (ionic radius increases), meaning the charge is spread out over a large area, making them have a lower charge density. So carbonate/nitrate is more thermally stable when there is lower charge density.
Why is MgCO_3(s) less thermally stable than BaCO_3(s)
Mg^2+ has a high charge density due to 2+ charge but small ionic radius. It distorts the cloud in CO_3^2-/NO_3^- ions more than Ba^2+ which has lower charge density. The less distortion, the more stable the carbonate is.
INSERT PICTURE OF IONS 12:24, ALLERY TOPIC 4 VIDEO
What is the carbonate ion
CO_3 ^2-
What is the nitrate ion
NO_3 ^-
How thermally stable are group 1 carbonates when compared to group 2 compounds
Group 1 carbonates can also decompose upon heating, but they are more thermally stable than group 2 compounds.
How do group 1 carbonates react under a bunsen
All group 1 carbonates are thermally stable under a bunsen flame, meaning there is no reaction. Except for Lithium carbonate, which forms an oxide and carbon dioxide.
Li_2CO_3(s) -> Li_2O(s) + CO_2(g)
What happens to group 1 nitrates under a bunsen
Group 1 Nitrates break down into nitrites and oxygen, except lithium nitrate.
2KNO_3(s) -> 2KNO_2(s) + O_2(g)
What is a nitrite (nitRITE) ion
NO_2 ^-
What is the exception for thermal decomposition of group 1 nitrates
Lithium Nitrate, LiNO_3,
which decomposes to Li_2O, NO_2, and O_2
Why is magnesium carbonate less thermally stable than sodium carbonate
Mg^2+ has a high charge density and distorts the electron cloud in carbonate/nitrate ions more than Na^+ which has lower charge density. They are similar in size (ionic radius) but Mg 2+ has a bigger charge than Na+. The less distortion, the more stable the carbonate is.
INSERT IMAGE ALLERY TOPIC 4, 14:27
How do you test the stability of nitrates experimentally
Measure how long it takes a specific amount of oxygen to be produced. Using a gas syringe, or the amount needed to relight a glowing splint.
OR
The length of time it takes until a specific amount of NO_2 to be produced. NO_2 is a brown gas, so it is easily observed, but it is toxic, so this must be done in a fume cupboard.
Why does the test for thermal stability of a nitrate be done in a fume cupboard
NO_2 is produced which is a brown gas. This is easily observed but it is toxic to breathe in.
How do you test the stability of carbonates experimentally
The length of time it takes until a specific amount of CO_2 is produced. CO_2 turns lime water cloudy so the quicker this turns cloudy, the more carbon dioxide is produced. You could use a gas syringe too.
How do you test for positive ions (cations) in a compound
Using flame tests
Why are different colours produced in flame tests depending on what cation there is
Electrons in the shells move to higher energy levels as they absorb energy from the flame. When they drop back down to lower energy levels, light is released. Different colours are produced as the difference in energy levels determines the wavelength of light released.
How do carry out a flame test
Dip the nichrome wire in concentrated hydrochloric acid.
Dip wire into sample
Place the loop into the BLUE bunsen flame and observe the colour
Why is the nichrome wire dipped in concentrated hydrochloric acid before conducting a flame test
To clean the wire of anything which might affect the flame colour, like previous samples, and to allow the current sample to stick to the wire better
What is the story to remember the colours of the flame tests
Cali was dark and red hot that night. The gym lad criminals were on the run from the boys in blue who were trying to give them a caes. In their car, they had stolen red rubies and pots of lilac amethysts. The driver of the car was sipping on Fanta Soda, and the other was putting on a thick coat of crimson lipstick. They made their way to their hideout, the green apple bar.
Crimson LIPSTICK
Fanta is Yellow-Orange Soda
Put Lilacs in POTS
Red Ruby
Blue Caes for the cops
California Too Hot
Criminal strong
Green Apple Bar
What does flourine look like
Pale yellow gas
What does chlorine look like
Pale green gas
What does bromine look like
brown-orange liquid
What does iodine look like
grey solid
What happens to the boiling point as you go down group 7 and why
Boiling point increases as you go down the group.
What happens to the physical states of the elements as you go down group 2
Physical states go from a gas at the top of group 7 to a solid at the bottom
Why does boiling point increase as you go down group 7
Because London forces increase due to the increasing size and relative mass of the atoms.
What happens to electronegativity as you go down group 7
Electronegativity decreases as we go down group 7.
What is electronegativity
The ability for an atom to attract electrons towards itself in a covalent bond.
Why does electronegativity increase as you go down group 7
The atoms get larger and the distance between the positive nucleus and bonding electrons increases. There is also more shielding.
How can we observe halogen displacement reactions
You add hexane to the mixture, the halogen present will dissolve into the organic layer of hexane, forming a coloured band layer above the aqueous layer. The colour tells you which halogen is present.
INSERT IMAGE TOPIC 4, ALLERY 20:23
What happens to the reactivity of gr7, halogens, elements as you go down the group
Reactivity in halogens decreases as we go down group 7. So Flourine is the most reactive, iodine is the least.
Why does reactivity decrease as you go down group 7
For a reaction to occur, an electron is gained. Atoms with a smaller radius attract electrons better than larger atoms, due to electrostatic attraction between the nucleus and the electron it wants to attract. Larger radius means more shielding, so less attraction.
How oxidising are halogens as you go down group 7
Halogens are less oxidising as we go down the group. Iodine is less oxidising than flourine.
When can a halogen displace a halide from its solution
Only if the halide in the solution is further down the group 7 on the periodic table than the halogen. So Potassium Bromide will be displaced by chlorine to make Potassium Chloride.
What does Potassium chloride, potassium bromide and potassium iodide look like
They are all colourless solutions.
What happens when you add chlorine water to potassium chloride
No reaction.
Aqueous layer (KCl) - Colourless
Organic layer (Hexane) - Colourless
What happens when you add chlorine water to potassium bromide solution
The chlorine displaces the bromide in potassium bromide.
Aqueous layer - Yellow
Organic layer - Orange
Cl_2 + 2Br^- –> 2Cl^- + Br_2
What happens when you add chlorine water (almost colourless) to potassium iodide solution
The chlorine displaces the iodide in potassium iodide.
Aqueous layer - Brown
Organic layer - Purple
Cl_2 + 2I^- –> 2Cl^- + I_2
What happens when you add bromine water (orange) to potassium chloride solution
No reaction
Aqueous layer - Yellow
Organic layer - Orange
What happens when you add bromine water (orange) to potassium bromide solution
No reaction
Aqueous layer - Yellow
Organic layer - Orange
What happens when you add bromine water (orange) to potassium iodide solution
Bromine displaces the iodide in potassium iodide
Aqueous layer - Brown
Organic layer - Purple
Br_2 + 2I^- –> 2Br^- + I_2
What happens when you add iodine solution (brown) to potassium chloride, potassium bromide, and potassium iodide solution
All of them have no reaction, as iodine is the least reactive halogen.
For all of them
Aqueous layer - brown
Organic layer - purple
What do halogens form when they react with group 1 elements
Metal halides
Mg(s) + Cl_2(g) –> MgCl_2(s)
What happens when magnesium reacts with chlorine
Forms Magnesium chloride
Mg(s) + Cl_2(g) –> MgCl_2(s)
What kind of reaction is the formation of magnesium chloride and why
Redox, as magnesium is being oxidised, and chloride is being reduced
Mg(s) + Cl_2(g) –> MgCl_2(s)
Half equations
Mg –> Mg 2+ + 2e-
Cl_2 + 2e- –> 2Cl-
What is the reaction for Lithium and chlorine, and what type of reaction is it and why
2Li(s) + Cl_2(g) –> 2LiCl(s)
This is a redox reaction as lithium is being oxidised
Li -> Li+ + e-
And chlorine is being reduced
Cl_2 + 2e- –> 2Cl-
What oxidation states can fluorine form
Fluorine only forms oxidation states of 0 or -1. It cannot form positive oxidation states. So it can only be fluorine or fluoride.
What are the Oxidation states of halogens at -1 oxidation number
Oxidation state: -1
Ion: Cl-/Br-
Name: Chloride/Bromide
What are the oxidation states of halogens at 0 oxidation number
Oxidation state: 0
Ion: Cl/Br
Name: Chlorine/Bromine
What are the oxidation states of halogens at +1 oxidation number
Oxidation state: +1
Ion: ClO^- and BrO^-
Name: Chlorate (I) and Bromate (I)
What are the oxidation states of halogens at +3 oxidation number
Oxidation state: +3
Ion: BrO_2 ^-
Name: Bromate (III)
What are the oxidation states of halogens at +5 oxidation number
Oxidation state: +5
Ion: IO_3 ^-
Name: Iodate (V)
What are the oxidation states of halogens at +7 oxidation number
Oxidation state: +7
Ion: IO_4 ^-
Name: Iodate (VII)
What oxidation numbers can fluorine, chlorine, bromine, and iodine form
Fluorine: -1 and 0 ONLY
Chlorine: -1, 0, +1
Bromine: -1, 0, +1, +3
Iodine: +5, +7
What happens when a halogen reacts with COLD alkalis
A disproportionation reaction
What is a disproportionation reaction
When the same species is both oxidised and reduced in 2 different products.
What is the difference between redox and disproportionation reactions
Redox means any 2 species are reduced and oxidised. As long as something is being oxidised and something is being reduced, the reaction can be classed as redox.
In disproportionation, the SAME species has to be both reduced and oxidised.
General equation of halogen reacting with COLD alkali
X_2 + 2NaOH –> NaOX + NaX + H_2O
X - Halogen
The halogen is being both reduced and oxidised
Na is a spectator, it has the same oxidation state throughout
General IONIC equation of halogen reacting with cold alkali
X_2 + 2OH- –> OX- + X- + H_2O
X - Halogen
The halogen is being both reduced and oxidised
Na is a spectator, it has the same oxidation state throughout
What do you call a species that is not oxidised or reduced in a reaction
A spectator. In the reaction between halogens and cold alkalis, metal is a spectator
What is equation of Bromine reacting with cold alkali NaOH
Br_2 + 2NaOH –> NaOBr + NaBr + H_2O
What is equation of Bromine reacting with cold alkali NaOH
Br_2 + 2NaOH –> NaOBr + NaBr + H_2O
General equation of halogen reacting with HOT alkali
3X_2 + 6NaOH –> NaXO_3 + 5NaX + 3H_2O
What is the ionic equation of halogen reacting with HOT alkali
3X_2 + 6OH- –> XO_3- + 5X- + 3H_2O
What is the equation of chlorine reacting with HOT alkali NaOH
3Cl_2 + 6NaOH –> NaClO_3 + 5NaCl + 3H_2O
What is the difference between the products of halogen reacting with COLD alkali vs HOT alkali
In cold, Sodium Chlorate (I) is formed. In hot, sodium chlorate (V) is formed. The oxidation number of the sodium halide is changed.
How is bleach made
Mixing chlorine and sodium hydroxide will form sodium chlorate (I) solution, Bleach.
What is the equation to form bleach, and what type of reaction is this and why
2NaOH(aq) + Cl_2(g) –> NaClO(aq) + NaCl(aq) + H_2O(l)
This is a disproportionation reaction as Cl_2 has oxidation state of 0, NaClO, the Cl has ox state of +1, in NaCl, Cl has ox state of -1.
Chlorine is simultaneously reduced and oxidised.
What is the oxidation state of an element in its standard state, Cl_2
Elements in their standard states have OX state of 0.
What are the uses of sodium chlorate (I), bleach
Treating water, Bleaching paper and fabrics, Cleaning Agents.
How do you sterilise water and what does it do
You add chlorine to water to kill the bacteria in the water.
What is produced from adding chlorine to water,
Adding water to chlorine will produce chlorate(I) ions (ClO-) which kills bacteria. Useful in drinking water and pools
What is the equation of adding chlorine to water, water sterilisation
H_2O(l) + Cl_2 (g) <=> HCl(aq) + HClO (aq)
THEN
HClO(aq) + H_2O(l) <=> ClO- (aq) + H_3O+(aq)
Chloric (I) acid ionises to make chlorate (I) ions.
What does a reducing agent do
They LOSE electrons. Reduction is the gain of electrons but reducing agents lose electrons so the other species can gain them.
What happens to reducing power (power for something to act as a reducing agent, reducing other species), as we go down group 7
Reducing power increases as we go down group 7
What is reducing power
The power for something to act as a reducing agent, reducing other species
Why does reducing power increase as we go down group 7
As we go down the group, ionic radius increases. Distance between nucleus and outer electrons becomes larger, and there is more shielding, so attractive force between nucleus and outer electrons get weaker. The outer electron is lost more readily, and this is the reason why (I-) is a more powerful reducing agent than (F-)
What the group 7 reducing agents
Halide ions (X-). F- is a reducing agent, I- is a reducing agent.
What are the tests to prove that reducing power increases as we go down group 7
Reaction with sulfuric acid, and reaction with silver nitrate solution.
What are the 4 product points in the reaction between halide ions and sulfuric acid
Point A: NaHSO_4
Point B: SO_2
Point C: S
Point D: H_2S
What happens to Chloride, bromide and iodide ions reacting with sulfuric acid at point A
All of Cl, Br, I react at point A.
With NaCl: H_2SO_4 + NaCl –> NaHSO_4 + HCl
With NaBr: H_2SO_4 + NaBr –> NaHSO_4 + HBr
With NaI: H_2SO_4 + NaI –> NaHSO_4 + HI
What is the oxidation state of sulfur in each of the points in the reaction between halide ions and sulfuric acid
Point A: NaHSO_4 (+6)
Point B: SO_2 (+4)
Point C: S (0)
Point D: H_2S (-2)
Which halide ion stops reacting at point A of the reaction between halide ions and sulfuric acid
Cl-. There is no further reduction of sulfuric acid by Cl- past point A.
White misty fumes are produced
How can you tell that NaCl (or any metal chloride) was used and reacted with sulfuric acid
White misty fumes are produced. The reaction stopped at point A, forming NaHSO_4 + HCl(this produces the white misty fumes)
Which halide ion stops reacting at point B of the reaction between halide ions and sulfuric acid
Br-. There is no further reduction of sulfuric acid by Br- past point B. Orange vapour of Br_2 is produced.
How can you tell that NaBr (or any metal bromide) was used and reacted with sulfuric acid
This is in addition to point A. Everything produced at point A is also produced. Orange vapour of Br_2 is produced. The reaction stopped at point B, forming Br_2.
Does NaCl, NaBr, NaI have to be reacted with sulfuric acid for the reaction to work
No, any metal halide can be reacted, for example KCl, KBr, KI.
What are the half equations of bromide and sulfuric acid at point B
2Br- –> Br_2 + 2e- (Br- ions oxidised)
H_2SO_4 + 2H+ + 2e- –> SO_2 + 2H_2O (S being reduced)
2e- from Br- oxidising equation and 2e- from S reduction equation cancel out.
What is the overall ionic equation of point B in the reaction between NaBr and H_2SO_4
H_2SO_4 + 2H+ + 2Br- –> Br_2 + SO_2 + 2H_2O
How can you tell that NaI (or any metal iodide) was used and reacted with sulfuric acid
Yellow solid of S is produced at point C, and Rotten egg smell of H_2S is produced at point D.
How does the point A, B, C, D system work when relating to halide ions reacting with sulfuric acid
For example, Cl- reaches point A. Meaning it only forms NaHSO_4. But I- reaches point D, meaning it forms EVERYTHING from Point A, B, C, and THEN forms the products from point D. It works like addition.
What are the half equations of iodide and sulfuric acid at point C
6I- -> 3I_2 + 6e- (I- ions oxidised) x3 to get 6e-
H_2SO_4 + 6H+ + 6e- –> S + 4H_2O (S being reduced)
I- oxidised equation is multiplied by 3 to match the number of electrons as the S being reduced equation
6e- cancels out from oxidising equation and reduction equation
What is the overall ionic equation of iodide and sulfuric acid at point C
H_2SO_4 + 6H+ + 6I- –> 3I_2 + S + 4H_2O
What are the half equations of iodide and sulfuric acid at point D
8I- –> 4I_2 + 8e- (I- ions oxidised) x4 to get 8e-
H_2SO_4 + 8H+ 8e- –> H_2S + 4H_2O (S being reduced)
8e- cancels out from oxidising equation and reduction equation
What is the overall ionic equation of iodide and sulfuric acid at point D
H_2SO_4 + 8H+ 8I- –> 4I_2 + H_2S + 4H_2O
Are hydrogen halides acidic or alkali
Hydrogen halides are acidic
What do hydrogen halides form in water
Hydrogen halides are gases that dissolve in water to form acidic solutions
What do hydrogen halides form when they react with water in the air
White misty fumes
What state are hydrogen halides in
Gas
What do hydrogen halides do when they react with ammonia gas, what is the reaction equation for example with ammonia gas and HCl
Make white fumes of ammonium halides
NH_3 (g) + HCl(g) –> NH_4Cl(s)
What is the equation of hydrogen halides, for example HCl, reacting with water
HCl(g) –> H+(aq) + Cl- (aq)
HCl(g) + H_2O –> H_3O+ (aq) + Cl-(aq)
^This one is what you normally get, the top one is a simplified form
H_3O+ (aq) is a hydroxonium ion
What acid does each hydrogen halide produce when dissolved in water
Hydrogen Chloride forms Hydrochloric Acid
Hydrogen Bromide forms Hydrobromic Acid
Hydrogen Iodide forms Hydroiodic Acid
How do you test for halide ions using silver nitrate
First, you have to add dilute nitric acid (HNO_3) to your solution. This will mop up any other anions other than halides, like carbonates. Then you add silver silver nitrate (AgNO_3). The colour of the precipitate will help you identify which halide ion is present. Then you can confirm it by seeing if the precipitate dissolves in ammonia (NH_3)
Why do we add dilute nitric acid before adding silver nitrate when we are testing for halide ions
The nitric acid reacts with any anions other than halides which are present, like carbonates. If we didn’t do this, the silver nitrate may react with those, giving us a false positive by forming something like silver carbonate, which is a white precipitate. We would think it is Cl- ions when in reality, it isn’t.
What colour and precipitate does silver form when reacting with Cl- ions
WHITE PRECIPITATE FORMS (Silver Chloride)
Ag+(aq) + Cl-(aq) –> AgCl(s)
What colour and precipitate does silver form when reacting with Br- ions
CREAM PRECIPITATE FORMS (Silver Bromide)
Ag+(aq) + Br-(aq) –> AgBr(s)
What colour and precipitate does silver form when reacting with I- ions
YELLOW PRECIPITATE FORMS (Silver Iodide)
Ag+(aq) + I-(aq) –> AgI(s)
Once you have added silver nitrate to the halide solution, how do you confirm which halide is present
You add ammonia (NH_3) solution to the precipitates
Cl- White precipitate dissolves in dilute NH_3
Br- Cream precipitate dissolves in concentrated NH_3
I- Yellow precipitate INSOLUBLE in concentrated NH_3. This will not dissolve in NH_3 solution at all
Why would you need to further confirm which halide is present with ammonia solution after you have already formed precipitates with silver nitrate
The precipitates are very similarly coloured. It is easy to mistake the cream precipitate of AgBr with the white precipitate of AgCl or the yellow precipitate of AgI. The ammonia solution allows us to know which precipitate is present for sure.
INSERT IMAGE 43:32 ON ALLERY TOPIC 4 OF DIFFERENT PRECIPITATE COLOURS.
What does precipitate mean
It is a solid compound, which means it is insoluble in the solvent.
How do fluoride ions, F-, react with silver nitrate
They do not form a precipitate at all. AgF is soluble in the solution, so you will not see any precipitate. Even though it is not visible, a reaction still has happened and AgF is produced. It is just soluble without the addition of any ammonia, so you cannot see it.
How do you test for carbonates (carbonate ions)
You add an acid, HCl, to a carbonate or hydrogencarbonate (both ions have the same test are the same), and you form CO_2 gas. When bubbled through limewater, it turns cloudy.
CO_3 ^2- + 2H+ –> CO_2 + H_2O
HCO_3^- + H+ –> CO_2 + H_2O
INSERT IMAGE ALLERY TOPIC 4, 45:00 OF LIMEWATER
How do you test for sulfates (sulfate ions)
First, you add HCl to remove any carbonates in it, otherwise you might get barium carbonate precipitate forming, giving you a false positive that there are sulfates present. THEN you add barium chloride.
You will get a WHITE PRECIPITATE if there are sulfates present. This is insoluble. This is barium sulfate.
Ba^2+ (aq) + SO_4 ^2- (aq) –> BaSO_4(s)
Why is barium chloride used as the test for sulfate ions
Because barium sulfate is insoluble, so there is a visible white precipitate
What is the ionic equation of the formation of barium sulfate
Ba^2+ (aq) + SO_4 ^2- (aq) –> BaSO_4(s)
How do you test for ammonium compounds
Add sodium hydroxide (NaOH) to the unknown compound, then gently heat. If ammonium compound is present, ammonia gas will be produced, which you will be able to smell.
Then use damp red litmus paper, damped with water. The ammonia will dissolve in the water on the litmus and turn the paper blue.
NH_4^+ (aq) + OH- (aq) –> NH_3(aq) + H_2O(l)
How do you test for hydroxides
Hydroxides are alkaline and they will turn red litmus paper blue.
Why is the test for hydroxides not very effective
This test doesn’t mean there are definitely hydroxides, as many things, like any alkali, will turn red litmus blue. You will need to do further tests like spectroscopy to help identify an alkali, identify for hydroxides.
