8- Reactivity Trends Flashcards
What is the outer shell electron configuration for Beryllium ions?
(Be2+) - [He]
What is the outer shell electron shell of Calcium ions?
(Ca2+) - [Ar]
How many electrons are in the outer shell of group 2 elements?
2
What happens when a Group 2 electron undergoes a redox reaction?
-metal atom is oxidised
-losses two electrons
-forms a 2+ ions (with electron configuration of a noble gas)
-group 2 metals are reducing agents (reduced another species)
Ca → Ca2+ 2e-
How do group 2 elements react with oxygen?
-form at metal oxide
2Mg + O2 → 2MgO
What are the changes in oxidation numbers when magnesium reacts with oxygen?
2Mg + O2 → 2MgO
Mg - starts with oxidation state of 0 and ends in +2
O- starts with oxidation state of 0 and ends in -2
Mg is oxidised
O2 is reduced
What is the reaction of the group 2 elements with water?
-form an alkaline hydroxide (general formula M(OH2) )
-form hydrogen gas
Sr + 2H2O ➝ Sr(OH)2 + H2
How does reactivity change down Group 2?
least reactive at the top (Beryllium)
most reactive at the bottom (Radium)
What is the reaction of group 2 elements with dilute acids?
-form a salt + hydrogen gas
metal+acid ➝ salt + hydrogen
What is the reaction between magnesium and dilute hydrochloric acid?
Mg + 2HCl ➝ MgCl2 + H2
Mg - goes through oxidation (goes from oxidation state of 0 to +2)
H- goes through reduction (oxidation state of +1 to 0)
Why does the first ionisation energies decrease down group 2?
The first ionisation energies decrease down the group because the attraction between the nucleus and the outer electrons decrease due to increased atomic radius and increased shielding
Why does reactivity increase down group 2?
-↑atomic radius ∴ increase in shielding
-decreased attraction between the outer electrons and the positive nucleus
-less energy is required to remove outer electrons
What happens when you react a carbonate with an acid?
carbonate + acid = salt + water + C02
What is the reaction like between Group 2 oxides and water?
-release hydroxide ions (OH-)
-form alkaline solutions of the metal hydroxides
Write an equation for the reaction between calcium oxide and water
CaO + H2O ➝ Ca2+ + 2OH-
Are group 2 hydroxides soluble in water?
-slightly
-when the solution becomes saturated, any further metal and hydroxide ions will for a solid precipitate
eg.
Ca2+ + 2OH- ➝ Ca(OH)2
How does the solubility of hydroxides change down group 2? And why?
-increases as you go down the group
-results in solutions containing more OH- ions are more alkaline
-solubility increases along with pH and alkalinity
How are Group 2 compounds used in agriculture?
-Calcium Hydroxide (Ca(OH)2)
-Added to fields as lime
-Increase the pH of acidic soils
-Neutralises the soil forming neutral water
Ca(OH)2 + 2H+ ➝ Ca2+ + 2H2O
How are Group 2 compounds used in medicine?
-used as antacids for treating acid indigestion
-magnesium and calcium carbonates as main ingredients
-milk of magnesia is a suspension of magnesium hydroxide (Mg(OH)2) in water
-because magnesium hydroxide is only slightly soluble in water
-acid in stomach is mainly HCl and a neutralisation reaction takes place
Mg(OH)2 + 2HCl ➝ MgCl2 + 2H20
CaCO3 + 2HCl ➝ CaCl2 + H20 + CO2
How do halogens exist?
-diatomic molecules at room temperature and pressure (Cl2, Br2)
What is the trend of boiling points of halogens?
-boiling point increases as you go down the group
-more electrons
-stronger london forces
-more energy required to break the intermolecular forces
How many electrons do halogens have in there outer shell?
7
What is the charge of a halide ion?
1-
How do halogens react in a redox reaction?
-halogen atom is reduced
-gain one electron to form a 1- halide ion (electron configuration of nearest noble gas)
-oxidising agent (oxidised another species)
Cl2 + 2e- ➝ 2Cl-
What happens in a reaction if a halogen is more reactive then the halide?
-halogen displaces halide
-colour change occurs
What do displacement reactions between halogens and halides show?
-reactivity of halogens decrease down the group
Due to I2 and Br2 being hard to tell apart in water what can happen to be able to tell them apart?
-organic non-polar solvent (cyclohexane) is added
-mixture shaken
-non-polar halogens dissolve more readily in cyclohexane
-colours much easier to tell apart
What are the colours of Cl2, Br2 and I2 in water?
Chlorine - pale green
Bromine - orange
Iodine - brown
What are the colours of Cl2, Br2 and I2 in water?
Chlorine - pale green
Bromine - orange
Iodine- violet
What is the reaction between chlorine and Br-?
Cl2 + 2Br- ➝ 2Cl- + Br2
What is the reaction between bromine and I- ions?
Br2 + 2I- ➝ 2Br- + I2
What are the trends in reactivity of the halogens?
-reactivity decreases down the group
-atomic radius increases
-more inner shells so shielding increases
-less nucleus attraction to capture an electron from another species
Which halogen is the strongest oxidising agent?
-fluorine
-gaining electrons from other species more readily than the other halogens
What is a disproportionation reaction?
-a redox reaction in which the same element is both oxidised and reduced
What is the reaction of chlorine with water like?
-Cl2 + H20 ➝ HClO + HCl
-disproportionation reaction
How is the reaction of chlorine with water used?
-drinking water treatment
What is the reaction of chlorine with cold, dilute aqueous sodium hydroxide? What is it used for?
-limited by the low solubility of chlorine in water
-water contains dissolved NaOH, much more chlorine dissolves
-disproportionation reaction
Cl2 + 2NaOH ➝ NaClO + NaCl + H2O
-results in a large concentration of ClO- ions from sodium chlorate
-use as household bleach
What are the benefits and risks of chlorine use?
-ensures our water is fit to drink and bacteria are killed, no risk of diseases like typhoid,cholera
-extremely toxic gas
-respiratory irritant in small concentrations, large concentrations are fatal
-chlorine can react with organic hydrocarbons which are suspected of causing cancer
What is the reaction between aqueous halide ions and aqueous silver ions?
Ag+ + X- ➝ AgX (aq)
X = aqueous soloution of any halide
How do you preform the carbonate test?
- In a test tube, add dilute nitric acid to the solid or solution
- If you see bubbles the unknown gas could be a carbonate
- To prove gas is carbon dioxide, bubble the gas through limewater (aqueous calcium hydroxide) . If CO2 if present will make a white precipitate
What is the reaction between sodium carbonate and nitric acid?
Na2CO3 + 2HNO3 ➝ 2NaNO3 + CO2 + H2O
How do you perform the sulfate test?
-add a few drops of dilute HCL
-add a few drops of Barium Chloride
-positive result = white precipitate formed
Write an equation for the sulphate ion test
Ba2+ + SO4 2- ➝ BaSO4
How do you test for halide ions?
- Add aqueous silver nitrate , AgNO3
- Record the colour change
- If difficult to tell colours apart, add aqueous ammonia
- Note the solubility of the precipitate
What is the equation and result for the Cl- test?
-white precipitate, soluble in dilute aqueous ammonia
Ag+(aq) + Cl-(aq) → AgCl(s)
What is the equation and result for the Br- test?
-cream precipitate
-soluble in concentrated ammonia only
Ag+(aq) + Br-(aq) → AgBr(s)
What is the equation and result for the I- test?
-yellow precipitate
-insoluble in concentrated and dilute aqueous ammonia
Ag+(aq) + I-(aq) → AgI(s)
When testing for carbonate, sulfate and halide ions, in which order should you carry out the tests and why?
- Carbonate
- Sulfate
- Halide
Because barium ions forms insoluble precipitate of BaCO3 and silver ions form insoluble precipitate Ag2SO4
How do you test fir ammonium ions, NH4+?
- add sodium hydroxide (NaOH) to the sample
- Warm the mixture
- Test the gas produced with red litmus paper
What are the observations for positive ammonia test?
-litmus paper turns blue
-ammonia has a pungent smell
Write an equation for the ammonia test?
NH4+ (aq) + OH- (aq) ➝ NH3 (aq) + H2O (aq)
