C8- Reactivity Trends Flashcards
What are the group 2 elements?
Metals also known as the alkaline earth metals due to the alkaline properties of the metal hydroxides.
- are reactive metals and do mot occur in their elemental form naturally.
- on earth they are found in stable compounds.
Explain group 2 elements as reducing agents.
Each group 2 element has two outer shell electrons in the s sub-shell
= two more than electron configuration of a noble gas.
- redox reactions are the most common type of reaction in group 2.
- each metal atom is oxidised, losing two electrons to form a 2+ ion with the electron configuration of a noble gas.
- another species will gain them and become reduced so therefore the group 2 element = reducing agent.
Redox reactions of group 2 elements with oxygen.
They all react with oxygen to form a metal oxide.
This is a redox reaction.
- oxygen us reduced.
- group 2 element oxidised (reducing agent)
Redox reactions of group 2 elements with water.
React with water to form an alkaline hydroxide and hydrogen gas.
- reactivity increases down the group.
- not all the hydrogen atoms are reduced.
Redox reactions of group 2 elements with dilute acids.
Metal + acid = salt + hydrogen
- reactivity increases down the group.
- total changes in oxidation number balance.
Explain the link between the trend in reactivity down group 2 and ionisation energy.
Down the group:
- atomic radius and shielding increases.
- therefore nuclear attraction decreases.
=ionisation energies decrease.
The first and second ionisation energies make up most of the energy input during reactions. However as the energy input from ionisation energies decreases as you go down, the elements become more reactive and stronger reducing agents.
Reactions of group 2 oxides with water.
The oxides react with water, releasing hydroxide ions and forming alkaline solutions of the metal hydroxide.
- the hydroxides are only slightly soluble in water. When the solution becomes saturated, any further metal and hydroxide ions form a solid precipitate.
Solubility of hydroxides, group2.
Solubility of the hydroxides in water increases down the group, meaning the resulting solutions contain more OH- ions and are more alkaline
As you go down:
- solubility increases
- pH increases
- alkalinity increases
(You can add the oxides to water and test each pH with universal indicator to see alkalinity increase down the group)
What are the uses of group 2 compounds as bases?
Due to their basic properties and ability to neutralise acids.
In agriculture:
- calcium hydroxide is added to fields as lime by farmers to increase the pH of acidic soils. It neutralises acid in the soil, forming neutral water.
In medicine:
- used as antacids for treating indigestion. Many indigestion tablets include Mg and calcium carbonates. They neutralise the acid in the stomach and due to being mostly insoluble in water they are safe.
What are the halogens and how do they exist at RTP?
Group 7- are the most reactive non-metallic group.
- on earth, halogens occur as stable halide ions dissolved in sea water or combined with sodium/potassium as solid deposits.
At room temp and pressure:
- all exist as diatomic molecules.
- group contains elements in all 3 physical states. (Gas-liquid-solid as you go down).
- When solid, they form lattices with simple molecular structures.
What is the appearance and state of the halogens at RTP?
F2- pale yellow gas. Cl2- pale green gas. Br2- red-brown liquid. I2- shiny grey-black solid. At2- never been seen
Explain the trend in boiling points of the halogens.
As you go down:
- more electrons.
- stronger london forces.
- more energy needed to break the IMF.
- boiling point increases.
Redox reactions of halogens.
- most common reaction of group 7.
- each halogen atom is reduced. Gain one electron to form 1- halide ion with electron configuration of nearest noble gas.
- another species loses electrons= oxidised.
- halogen = oxidising agent.
Halogen-halide displacement reactions. (How is it carried out)
Displacement reactions of halogens with halide ions can be carried out on test tube scale.
- results of displacement reactions show reactivity decreases down group 7.
1. Solution of each halogen is added to aqueous solution of the other halides.
2. If halogen added is more reactive than halide present, reaction takes place, solution changes colour.
After a halogen-halide displacement reaction takes place, how is the displacement identified?
Colour.
- solutions of iodine and bromine in water appear a similar orange-brown colour, depending on conc.
- organic non polar solvent like cyclohexane can be added and shaken. Non polar halogens dissolve more readily in cyclohexane.
Cl2: pale green = pale green (reacts with Br- and I-)
Br2: orange= orange. (reacts with I- only)
I2: brown= violet. (doesn’t react)
What about fluorine and astatine?
Fluorine- reacts with almost any substance it contacts.
Astatine- radioactive. Decays rapidly. Never been seen. Predicted to be least reactive.
Explain the trend in reactivity of the halogens.
Down group 7:
- atomic radius increases.
- more inner shells (shielding increases).
- less nuclear attraction (harder to capture electron from another species).
- reactivity decreases.
(Therefore become weaker oxidising agents as you go down)
Explain disproportionation.
a redox reaction in which the same element is both oxidised and reduced.
Eg (on spec so learn):
- reaction of chlorine with water.
- reaction of chlorine with cold, dilute sodium hydroxide.
Benefits/risks of chlorine use?
- extremely toxic gas.
- respiratory irritant in small concentrations. Large concentrations can be fatal.
- chlorine in drinking water can react with organic hydrocarbons (eg methane), forming chlorinated hydrocarbons= associated with causing cancer.
- in low concentrations can purify water and prevent diseases (eg. Typhoid/ cholera)
Why is chlorine used in water purification?
- when small amounts of chlorine are added to water = disproportionation reaction. (Learn equation)
- two acids are formed: chloric(I) acid (HClO) and HCL.
- bacteria is killed by chloric (I) acid and chlorate (I) ions. The acid also acts as a weak bleach.
- can be shown by adding indicator to solution of chlorine in water. First turns red, colour then disappears as bleaching action of chloric (I) acid takes effect.
How is bleach formed?
- chlorine has low solubility in water so only reacts to an extent.
- if the water contains dissolved sodium hydroxide, much more chlorine dissolves and another disproportionation reaction takes place.
- resulting solution contains a large concentration of chlorate (I) ions from the sodium chlorate that is formed.
- this solution is used as a household bleach and is formed by the reaction of chlorine with cold dilute aqueous sodium hydroxide.
Carbonate test.
- carbonates react with acids to form carbon dioxide gas.
1. In a test tube, add dilute nitric acid to the solid or solution to be tested.
2. If you see bubbles, unknown compound could be carbonate.
3. To prove gas is carbon dioxide: - bubble gas through lime water (saturated solution of calcium hydroxide)
- co2 reacts to form a fine white precipitate of calcium carbonate, which turns limewater cloudy (learn equation).
Sulfate test?
- unlike most sulfates, barium sulfate (BaSO4) is insoluble in water.
- aqueous barium ions are added to a solution of the unknown compound, formation of a white precipitate of barium sulfate shows the presence of a sulfate.
- the Ba2+ ions are added as aqueous barium chloride or barium nitrate.
- if a halide test will be carried out after, use barium nitrate. With barium chloride, chloride ions are introduced to the solution.
Halide test?
- unlike most halides, silver halides are insoluble in water.
- aqueous silver ions react with aqueous halide ions to form precipitates of silver halide.
1. Add aq silver nitrate to an aq solution of a halide.
2. The silver halide precipitates are different colours.
3. Add aq ammonia to test the solubility of the precipitate. Useful because colours of precipitates can be difficult to tell apart.
