Chapter 8 - Reactivity trends Flashcards
What is the reaction of group 2 elements with water?
Group 2 elements react with water forming a metal hydroxide and hydrogen.
How does magnesium react with water?
Magnesium reacts slightly with cold water (and more vigorously with steam)
How does calcium react with water?
Calcium reacts to form a white precipitate of calcium hydroxide.
How does strontium and barium react with water?
Strontium reacts with water slowly, it reacts with water quicker than calcium, which is placed directly above strontium in the periodic chart, and slower than barium, placed directly below strontium. Strontium is water insoluble, but it does react with water.
Strontium reacts to form less precipitate (with more dissolved product) and barium reacts forming even less precipitate.
How do group 2 elements react to being heated in air, give an example with magnesium.
They burn vigorously when heated in air to form white oxides.
2Mg + O2 –> 2MgO
How do group 2 elements react with acids?
E.g. Mg and HCl
Group 2 elements react with acids to form a salt and hydrogen.
E.g. Mg + 2HCl –> MgCl2 +H2
Give two observations of magnesium when heated in oxygen.
Bright white light and white ash
What is the trend in solubility with the group 2 hydroxides and explain how this trend occurs
Going down group 2, the hydroxides become more soluble. This is because more OH- ions are released into the solution, which become more alkaline. So going down they also get more alkaline.
Suggest the uses of group 2 compounds as bases - Calcium hydroxide, Calcium carbonate and magnesium hydroxide.
Calcium hydroxide is used to neutralise acidic soils.
Calcium carbonate is used to treat acid indigestion.
Magnesium hydroxide is used to treat acid indigestion.
What observations can be made when Group 2 oxides react with water - MgO, CaO, SrO and BaO
MgO has no observable change.
CaO, SrO and BaO reactions are very exothermic. The white oxide expands as water is added, producing a lot of steam.
Explain how calcium carbonate can be used to alleviate indigestion
Indigestion is caused by excess acid in the stomach. Some antacid tablets contain calcium carbonate which neutralises the excess acid
Suggest why sodium hydroxide tablets are not used to alleviate indigestion.
Sodium hydroxide is corrosive and it would burn the body.
Write an equation for the reaction of barium oxide with water.
BaO(s) + H2O(l) –> Ba(OH)2
Explain whether the pH of calcium hydroxide would be higher or lower than barium hydroxide.
pH is lower because calcium hydroxide is less soluble so there are fewer hydroxide ions present.
How do the halogens (group 7 elements) exist?
As diatomic molecules.
Explain the trend in the halogens boiling point
The boiling point increases going down the group because as the size of the molecules increase, the London forces increase.
How is bleach made?
Bleach is made by reacting chlorine with sodium hydroxide solution to make sodium chlorate (I) solution.
Cl2 + 2NaOH –> NaCl + NaOCl
What is disproportionation and give an example
Where the same element is oxidised and reduced in the same reaction. E.g. Chlorine in the reaction to make bleach.
Cl2 + 2NaOH –> NaCl + NaOCl
Cl2 is oxidised from 0 to +1 in NaOCl
Cl2 is reduced from 0 to -1 in NaCl
Explain the advantages and disadvantages of using chlorine for water treatment.
Chlorine kills bacteria in the water because it’s toxic however it may form chlorinated hydrocarbons, which may be carcinogenic.
Write an equation of equilibrium for the reaction when chlorine is used to treat water - a chlorine molecule reacts with a water molecule to form chloric (I) acid, HClO
Cl2 + H2O ⇌ HClO +HCl
Why is Cl2 + H2O ⇌ HClO +HCl classed as a disproportionation reaction?
Chlorine’s oxidation number changes from 0 in Cl2 to +1 in HClO, which is oxidation, and to -1 in HCl, which is reduction. The oxidation and reduction of chlorine in this reaction is disproportionation.
What ions do the halogens form?
The halogens have an outer shell electronic configuration of S2P5, they form anions with a 1- charge and a full outer shell.
Explain the trend in reactivity down group 7 (the halogens)
Further down the group, less energy is released when the halide ion is formed. Reactivity reduces down the group because:
The added electron is further from the nucleus, it’s shielded by more electrons in inner shells, the electrostatic attraction of the added electron to the nucleus is reduced, less energy is released by forming the ion.
What displacement reactions occur with halogens.
A more reactive halogen will displace a less reactive halogen from its salt. Halogen solutions can be added to solutions of halide ions such as sodium chloride, sodium bromide and sodium iodide which are colourless.
What reaction does a chloride ion have with bromine and iodine.
They have no reaction
Write an equation for chlorine and iodide ions, state the colour of the solution formed.
Cl2 + 2I- –> 2Cl- + I2
Brown solution
Chlorine can oxidise the bromide ions (in, for example, potassium bromide solution) to bromine. Write an equation for this and state the colour of the solution.
Cl2 + 2Br- –> 2Cl- + Br2
The bromine appears as an orange solution.
Write an equation for Bromine solution reacting with iodide solution.
What colour is the mixture
Br2 + 2I- –> 2Br- + I2
A brown mixture is formed
What would happen if cyclohexane is added to a brown mixture of bromide ions and iodine.
The iodine dissolves into the cyclohexane forming a purple solution with another aqueous layer.
Describe an experiment that can be carried out in which bromine solution is added to sodium chloride solution and sodium iodide solution. Give the observations, and use these to deduce the relative reactivity of the halogens, chlorine, bromine and iodine, in their redox reactions with halide ions. (6 marks)
lool
What is the test for carbonate ions ? And what would a positive result look like?
Add dilute nitric acid to sample,
Pass any gas formed through limewater.
A positive result is white precipitate in limewater. The ‘milkiness’ in limewater is a suspension of calcium carbonate.
Write two equations for the carbonate ion test.
First the carbonate with acid
Then the limewater and carbon dioxide
CO3^2-(s) + 2H+(aq) —> CO2(g) + H2O(l)
Ca(OH)2(aq) + CO2(g) —> CaCO3(s) + H2O(l)
What is the test for sulfate ions and what would be the positive result?
Make a solution of the sample, Add drops of barium nitrate solution. A positive result would be a white precipitate. Ba^2+(aq) + SO4^2-(aq) —> BaSO4(s) The white precipitate is barium sulfate
What is the test for halide ions and what would be a positive test?
Make a solution of the sample,
Add drops of silver nitrate solution.
A positive result is white precipitate if chloride ions are present, cream precipitate is bromide ions are present, and yellow if iodide ions.
The result is confirmed using ammonia
How is the halide test confirmed using ammonia?
The precipitate in the chloride ion solution dissolves I’m dilute or concentrated ammonia.
The precipitate in the bromide ion solution dissolves in concentrated ammonia.
The precipitate in the iodide ion solution is insoluble in dilute and concentrated ammonia.
What’s the test for cation: ammonium? And what’s a positive result?
To test if there are ammonium ions, add soloing hydroxide solution to sample, warm, test any gas evolved with damp red litmus paper.
If the litmus paper turns blue it’s a positive result.
What’s the equation for the ammonium cations test?
NH4+(s) + OH-(aq) —> NH3(g) + H2O(l)
What does the litmus paper in the test for ammonium cations detect?
The litmus paper detects alkaline ammonia gas
What three tests for anions should you know? And what tests for ammonium should you know?
Tests for anions: carbonate, sulfate, halides.
Test for cation: ammonium
What’s the order to test an unknown substance for its anion?
First test for carbonate ions, then sulfate ions and then halide ions.
Given three unlabelled solutions - dilute hydrochloric acid, silver nitrate and sodium iodide - how would you identify which was which using no other substances?
Mix all pairs. The pair with no precipitate is sodium iodide and hydrochloric acid.
The other solution is silver nitrate.
Add the silver nitrate to the other solutions.
The one that gives a white precipitate is hydrochloric acid and a yellow precipitate is sodium iodide.
Write an equation for the reaction of silver nitrate with sodium carbonate and describe what you would see
2AgNO3(aq) + Na2CO3(aq) —> 2NaNO3(aq) + Ag2CO3(s)
You would see a white precipitate
A solution containing sodium iodide and sodium carbonate has some silver nitrate solution added, followed by some dilute hydrochloric acid.
Explain that you would see.
Pale yellow precipitate die to yellow AgI(s) mixed with white Ag2CO3; after acid is added, precipitate becomes yellower due to silver carbonate reacting.
Describe a test to show whether some ammonium iodide was contaminating a sample of ammonium carbonate.
Dissolve a sample in distilled water and add nitric acid until the effervescence stops.
Add silver nitrate solution. If a yellow precipitate forms, add some concentrated ammonia solution. If a yellow precipitate doesn’t dissolve, iodide ions are present
Remember a carbonate also gives a precipitate with silver nitrate solution. It reacts with the carbonate ions giving CO2
Remember a carbonate also gives a precipitate with silver nitrate solution. It reacts with the carbonate ions giving CO2
