Chapter 8 - Reactivity Trends Flashcards
What are the elements in group 2 found as?
They are found in stable compounds and are reactive metals
How many outer shell electrons do group 2 elements have?
Two outer shell electrons
Which sub-shell are the outer electrons in group 2 atoms in?
The outer s sub-shell
What is the most common type of reaction of group 2 elements?
Redox reactions
What happens to group 2 elements during a redox reaction?
- the metal is oxidised, loosing 2 electrons
- another species will gain these 2 electrons and be reduced
- this makes the group 2 element a reducing agent
What happens when group 2 elements react with oxygen?
A metal oxide is formed with the general formula of MO (M being the group 2 metal)
What is the equation for the formation of magnesium oxide?
2Mg (s) + O2 (g) -> 2MgO (s)
What is oxidised and reduced when magnesium oxide is formed?
Mg is oxidised: 0 -> +2
O is reduced: 0 -> -2
What happens when group 2 elements react with water?
An alkaline hydroxide with the general formula M(OH)2 and hydrogen gas are formed
What happens to reactivity going down group 2?
Reactivity increases
What is the reaction between Sr and water?
Sr (s) + 2H2O (l) -> Sr(OH)2 (aq) + H2 (g)
What happens when group 2 metals react with dilute acids?
Group 2 metal + dilute acid -> salt + hydrogen
What is the reaction between magnesium and dilute hydrochloric acid?
Mg (s) + 2HCl (aq) -> MgCl2 (aq) + H2 (g)
Why does the reactivity increase going down group 2?
- the atoms of group 2 elements react by loosing electrons to form +2 ions
- this requires the input of 2 ionisation energies (first and second)
- the ionisation energies decrease down the group because the attraction between the nucleus and the outer electrons decreases as a result of increasing atomic radius and increasing shielding
What happens when group 2 oxides react with water?
- hydroxide ions and ions of the metal are released, forming an alkaline solution of the metal hydroxide
- the group 2 hydroxides are only slightly soluble in water
- when the solution becomes saturated, any further metal and hydroxide ions will form a solid precipitate
What happens to the solubility of group 2 hydroxides going down the group?
- The solubility increases down the group
- the resulting solutions contain more hydroxide ions and are more alkaline
How can you experimentally show the trend in solubility of hydroxides?
- add a spatula of each group 2 oxide to water in a test tube
- shake the mixture
- you will have a saturated solution of each metal hydroxide with some white solid undissolved at the bottom of the test tube
- measure the pH of each solution
How are group 2 compounds used in agriculture?
Calcium hydroxide is added to fields as lime by farmers to increase the pH of acidic soils
The calcium hydroxide neutralises acid in the soil, forming neutral water
How are group 2 compounds used in medicine?
Group 2 bases are often used as antacids for treating acid indigestion
Stomach acid is mainly hydrochloric acid, which can be neutralised by group 2 bases
How do the halogens naturally occur?
They naturally occur as stable halide ions dissolved in sea water/combined with sodium or pottasium
They are the most reactive non-metallic group and the elements do not occur in their elemental form naturally
What type of molecules are the halogens at RTP?
They exist as diatomic molecules
How do the physical states of the elements change going down the group?
They change from gas to liquid to solid
What is the structure of halogens in their solid states?
The halogens form lattices with simple molecular structures
What is fluorine’s appearance and state at RTP?
Pale yellow gas
What is chlorine’s appearance and state at RTP?
Pale green gas
What is bromine’s appearance and state at RTP?
Red-brown liquid
What is iodine’s appearance and state at RTP?
Shiny grey-black solid
Why is there a trend in increasing boiling point going down group 7?
- there are more electrons
- which means stronger London forces
- so more energy is required to break the intermolecular forces
- so boiling point increases
How many outer shell electrons do halogens have and which sub-shells are they in?
They have 7 outer-shell electrons
2 are in the outer s sub-shell
5 are in the outer p sub-shell
What are the most common type of reaction of the halogens?
Redox reactions
What happens to halogens in redox reactions?
- each halogen atom is reduced
- another species looses electrons to halogen atoms
- so the halogen is an oxidising agent
What type of reaction can be carried out to show that the reactivity of the halogens decreases down the group?
Displacement reactions of halogens with halide ions
What happens in a halogen-halide displacement reaction if the halogen added is more reactive than the halide present?
- a reaction takes place, the halogen displacing the halide from solution
- the solution changes colour
Why might you add an organic non-polar solvent to the solution of a hydrogen-halide displacement reaction?
The non-polar halogens dissolve more readily in an organic non-polar solvent (e.g. cyclohexane) than in water
So the colours are easier to tell apart
What happens when chlorine is reacted with Br- and I- halide ions?
- Cl2 (aq) + 2Br- (aq) -> 2Cl- (aq) + Br2 (aq)
Turns an orange colour from Br2 formation - Cl2 (aq) + 2I- (aq) -> 2Cl- (aq) + I2 (aq)
Turns an violet colour from I2 formation
What happens when bromine is reacted with Cl- and I- halide ions?
- No reaction between bromine and Cl-
- Br2 (aq) + 2I- (aq) -> 2Br- (aq) + I2 (aq)
Turns an violet colour from I2 formation
What happens when iodine is reacted with Cl- and Br- halide ions?
- No reaction between iodine and Cl-
- No reaction between iodine and Br-
What are the overall results of the halogen-halide displacement reactions?
- chlorine reacts with both Br- and I-
- bromine only reacts with I-
- iodine doesn’t react at all
What is the full equation of the reaction of chlorine with sodium bromide?
Cl2 (aq) + 2NaBr (aq) -> 2NaCl (aq) + Br2 (aq)
What is oxidised and reduced during the reaction of chlorine with sodium bromide?
Chlorine is reduced: 0 -> -1
Bromine is oxidised: -1 -> 0
How reactive is fluorine?
Fluorine is a pale yellow gas, reacting with almost any substance that it comes into contact with
How reactive is astatine?
Astatine is extremely rare because its radioactive and decays rapidly
The element has never actually been seen but is predicted to be the least reactive halogen
What is the trend in reactivity going down group 7 and why?
- atomic radius increases
- more inner shells so shielding increases
- less nuclear attraction to capture an electron from another species
- reactivity decreases
What is the trend in strength of oxidising agents going down group 7?
The halogens become weaker oxidising agents down the group
What is disproportionation?
A redox reaction where the same element is both oxidised and reduced
What are two examples of disproportionation reactions?
- Chlorine and water
- Chlorine and cold, dilute sodium hydroxide
What happens when chlorine is added to water?
A disproportionation reaction takes place:
Cl2 (aq) + H2O (l) -> HClO (aq) + HCl (aq)
For each chlorine molecule, one chlorine atom is oxidised and the other chlorine atom is reduced
Why does chlorine purify water?
The two products in the reaction between chlorine and water are acids, chloric (I) acid and hydrochloric acid
The bacteria are killed by the chloric (I) acid
What happens when chlorine is added to cold, dilute aqueous sodium hydroxide?
- the reaction of chlorine with water is limited by the low solubility of chlorine in water
- if the water contains dissolved sodium hydroxide, much more chlorine dissolves and another disproportionation reaction takes place:
Cl2 (aq) + 2NaOH (aq) -> NaClO (aq) + NaCl (aq) + H2O (l)
What are the risks of adding chlorine to a water supply?
- chlorine is a respiratory irritant in small concentrations, and in large concentrations can be fatal
- chlorine in drinking water can react with organic hydrocarbons such as methane to form chlorinated hydrocarbons which are suspected of causing cancer.
What are the benefits of adding chlorine to drinking water?
the quality of drinking water would be compromised and diseases such as typhoid and cholera might break out
What are the tests for halide ions?
Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides:
Ag+ (aq) + X- (aq) -> AgX (s)
(X represents a halide)
What does qualitative analysis rely on?
Simple observations which can often be carried out quickly on a test tube scale
How do you test for carbonates?
- In a test tube, add dilute nitric acid to the soiled/solution being tested
- If you see bubbles, the unknown compound could be a carbonate
- To prove that the gas is carbon dioxide, use the limewater test:
- bubble the gas through lime water
- carbon dioxide reacts to form a fine white precipitate of calcium carbonate, which turns the lime water milky
How do you test for sulfates?
- Add aqueous barium ions to the solution being tested
- If it is a sulfate a white precipitate will be formed
How do you test for halides?
- Add aqueous silver nitrate to an aqueous solution of a halide
- The silver halide precipitates are different colours:
- silver chloride is white
- silver bromide is cream
- silver iodide is yellow - Add aqueous ammonia to test the solubility of the precipitate:
- silver chloride is soluble in dilute ammonia
- silver bromide is soluble in concentrated ammonia
- silver iodide is in insoluble in ammonia
What is the correct order for test for anions?
- Carbonate
- Sulfate
- Halides
Why is the carbonate test done first?
Neither sulfate nor halide ions produced bubbles with dilute acid, so the carbonate test can be carried out without the possibility of an incorrect conclusion
Why is the sulfate test done second?
Barium carbonate is also insoluble in water
So if you carry out a sulfate test on a carbonate, you will get a white precipitate too
Why is the halide test done last?
Silver carbonate and silver sulfate are both insoluble in water and will form precipitates in this test
So they will be false positives
What do you do if you are asked analyse a mixture of chemicals to see what anions are present?
You carry out the tests in the same sequence (carbonate, sulfate, halide) and on the same solution
How do you do the carbonate test on a mixture of chemicals?
- if you see bubbles, continue adding dilute nitric acid until the bubbling stops
- all carbonate ions will then have been removed and there will be none left to react in the next tests
- if you intent to test for sulfate or halide ions, it is important to use dilute nitric acid
- sulfuric acid contains sulfate ions and hydrochloric acid contains chloride ions which will show up in the sulfate and halide tests
How do you do the sulfate test on a mixture of chemicals?
- add an excess of barium nitrate
- filter the solution to remove the barium sulfate
- do not use barium chloride because the chloride ions will show up in the halide test
How do you do the halide test on a mixture of chemicals?
- add silver nitrate
- any carbonate or sulfate ions initially present have already been removed, so any precipitate formed must involve halide ions
- add ammonia to confirm which halide you have
How do you test for ammonium ions?
- Aqueous sodium hydroxide is added to a solution of an ammonium ion
- Ammonia gas is produced (you are unlikely to see gas bubbles as ammonia is very soluble in water)
- The mixture is warmed and ammonia gas is released
- Test with pH indicator paper. Ammonia is alkaline and its presence will turn the paper blue.
