4 Flashcards
Trends in Group 2:
Atomic radius
Atomic radius INCREASES as you go down the group
As you go down group, a full energy level of electrons are added between each element
Trends in Group 2:
1st Ionisation energy
1st ionisation energy DECREASES as you go down the group
All outer electrons are in a full s orbital
Shielding increases, so effective charge on nucleus decreases
This causes a decrease in the attraction to the outer electrons, therefore, less energy is needed to remove it
Trends in Group 2:
Melting/boiling points
Melting/boiling points DECREASES as you go down the group
All form 2+ ions in a metallic structure
Increased shielding causes weaker effective nuclear charge
This leads to a decrease in the strength of the electrostatic attractions with delocalised electrons
Therefore, less energy is needed to overcome them, so melting/boiling point is lower
Group 2 elements and water:
Reactivity of group 2 elements
Reactivity of group 2 elements INCREASES, as you go down the group
Group 2 metals are oxidised , as they lose 2 electrons when they react
Ionisation energies decrease as you go down the group, so the outer electrons are lost more easily
This is due to the increased shielding which causes a lower effective charge on the nucleus, therefore, they react more easily
Group 2 elements and water:
Magnesium and water
Mg (s) + H2O (g) —–> MgO (s) + H2 (g)
- Heat/steam needed in order to react
- Insoluble MgO formed.
Group 2 elements and water:
Cs/Sr/Ba and water
X (s) + 2H2O (l) ——-> X(OH)2 + H2 (g)
- React with cold water
- Basic metal hydroxide formed
Nb: Solubility of group 2 oxides increases down group, so state symbol varies
Reaction of Mg with oxygen
2Mg (s) + O2 (g) ——-> 2MgO (s)
Bright white flame
Reaction of Ca with oxygen
2Ca (s) + O2 (g) ——–> 2CaO (s)
Reaction of Sr with oxygen
2Sr (s) + O2 (g) ———> 2SrO (s)
Reaction of Ba with oxygen
2Ba (s) + O2 (g) ———> 2BaO (s)
Reaction of Mg with chlorine
Mg (s) + Cl2 (g) ———-> MgCl2 (s)
Reaction of Ca with chlorine
Ca (s) + Cl2 (g) ———-> CaCl2 (s)
Reaction of Sr with chlorine
Sr (s) + Cl2 (g) ———-> SrCl2 (s)
Reaction of Ba with chlorine
Ba (s) + Cl2 (g) ———-> BaCl2 (s)
Trend in solubility of group 2 hydroxides?
The solubility of group 2 INCREASES as you go down the group
Trend in solubility of group 2 hydroxides?
The solubility of group 2 hydroxides INCREASES as you go down the group
Trend in solubilty of group 2 sulfates
The solubility of group 2 sulfates DECREASES as you go down the group
Trend in solubility of group 2 sulfates
The solubility of group 2 sulfates DECREASES as you go down the group
Trend in solubility of group 2 carbonates
Group 2 carbonates are INSOLUBLE
Tests for group 2 salts
-Add Na2CO3 (aq) ——> White precipitate for ALL
-Add Na2SO4 (aq) ——–> MgSO4 = no ppt
BaSO4 = Thick ppt
-Add Na2OH (aq) ———> Mg(OH)2 = white ppt
Ba(OH)2 = No ppt
Thermal decomposition:
The use of heat to break down a reactant into multiple products
Thermal decomposition of carbonates
The positive ion causes heterolytic fission of the C—O- bond
The greater the charge density of the positive ion, the more readily this occurs
Thermal decomposition of Group 1 carbonates
Li2CO3 (s) ——-> Li2O (s) + CO2 (g)
Only Lithium carbonate (LiCO3) decomposes
Charge density in the other group 1 carbonates too low, and decomposition does not occur
Thermal decomposition of Group 2 carbonates
Thermal stability of group 2 carbonates INCREASES as you go down the group.
ie. they become more difficult to decompose as charge density DECREASES.
Therefore, the carbonate ion is less likely to be POLARISED.
Test for sulfate ions
Ba2+ ions test for sulfate ions and sulfate ions test for Ba2+
Thermal decomposition of Group 1 nitrates
Lithium:
4LiNO3 (s) ———> 2LiO (s) + 4NO2 (g) + O2 (g)
Rest of group 1 nitrates:
2XNO3 (s) ———–> 2XNO3 (s) + O2 (g)
Thermal decomposition of Group 2 nitrates
2X(NO3)2 (s) ——–> 2XO (s) + 4NO2 (g) + O2 (g)
NO2 (nitrate) observation
Brown gas
Trend in thermal decomposition of Group 2 nitrates
Thermal stability of group 2 nitrates INCREASES down the group
ie. they become more difficult to decompose as charge density DECREASES.
Therefore, the nitrate ion is less likely to be POLARISED.
How do we test the stability of carbonates?
One is to heat a known mass of carbonate in a side arm boiling tube and pass the gas produced through lime water.
Time for the first permanent cloudiness to appear in the limewater.
How do we test the stability of nitrates?
Measure length of time it takes until a specific amount of NO2 is produced.
NO2 is a brown gas that can easily be observed, but is toxic so must be done in a fume cupboard
How to carry out a flame test?
- Use a nichrome wire
- Sterilise the wire by dipping in concentrated hydrochloric acid and then heating in Bunsen flame
- Make sure the sample powdered or grinded
- Dip wire in solid and put in Bunsen flame and observe flame
Group 1 flame test colours:
Li - red Na - Yellow/orange K - Lilac Rb - Deep red Cs - Violet
Group 2 flame test colours:
Mg - No colour
Ca - Orange/red
Sr - Red
Ba - Apple green
Explanation of the flame test colours
The heat causes the electron to get excited and be promoted to higher energy levels in the atoms
As they fall back down their ground state, they emit excess energy in the form of an electromagnetic wave
The further they fall, the higher the frequency emitted
Different frequencies = different colours
Why does magnesium show no colour in a flame test
The frequency emitted by Mg may not fall in the visible light part of the spectrum
Reaction of group 2 oxides with water
They are basic metal oxides, and react readily with water
Same equation for all, but magnesium hydroxide is solid, whereas rest are aqueous
Mg (s) + H20 (l) ——-> Mg(OH)2 (s)
X (s) + H20 (l) ——-> X(OH)2 (AQ)
Reaction of group 2 oxides with dilute acids
- Neutralisation reaction
- Forms basic salt and water
MO (s) + HCl (l) ——-> MCl2 (aq) + H20 (l)
MO (s) + H2SO4 (l) ———> MSO4 + H20 (l)
Why is the Mg(OH)2 formed from the reaction between MgO and H20 only PH9 but the other hydroxides formed are PH12?
Mg(OH)2 is not as alkaline as it is only slightly soluble in water, so fewer OH- (aq) ions in solution
Note on group 2 sulfates
State symbol of MSO4 can be (s) or (aq).
Solubility of group 2 sulfates DECREASES down the group
Reaction of group 2 hydroxides and acid
-They neutralise acids
M(OH)2 + 2HCl (l) ——–> MCl2 (aq) + 2H20 (l)
Trend in melting/boiling points of the group 7 elements
Melting/boiling points INCREASE down the group
Halogens exist as non polar diatomic molecules
Weak induced dipole forces exists between the molecules (these are broken when melted or boiled)
Induced dipole forces become stronger as you go down the group as Mr increases. This is because more electrons are involved in the induced dipoles
Trend in electronegativity of the group 7 elements
Electronegativity DECREASES down the group
Fluorine is the most electronegative as it has the greatest effective charge on the nucleus due to low shielding
This increases its ability to attract a bonding pair of electrons
Testing for halide ions
- Acidified silver nitrate
2. Aqeuous ammonia
Why is the silver nitrate acidified?
Nitric acid is added to test solution first to remove (neutralise) any CO3^2- or OH- impurities present
CO3^2- / OH- interfere with test because:
Ag2CO3 -white ppt
AgOH - brown ppt
Results after adding acidified silver nitrate:
Fluoride ions
Ag+ (aq) + F- (aq) ——> AgF(aq)
No ppt
Results after adding acidified silver nitrate:
Chloride ions
Ag+ (aq) + Cl- (aq) ——> AgCl (s)
White ppt
Results after adding acidified silver nitrate:
Bromide ions
Ag+ (aq) + Br- (aq) ——> AgBr (s)
Cream ppt
Results after adding acidified silver nitrate:
Iodide ions
Ag+ (aq) + I- (aq) ——> AgI (s)
Pale yellow ppt
Results after adding aqueous ammonia:
Chloride ions
AgCl (s) dissolves in DILUTE NH3 (aq)
Results after adding aqueous ammonia:
Bromide ions
AgBr (s) dissolves in CONC NH3 (aq)
Results after adding aqueous ammonia:
Iodide ions
AgI (s) will NOT dissolve
Reaction of chlorine and water
Water reacts with chlorine to form HCl and chloric acid
Cl2 + H20——–> HCl + HClO
It is a disproportionation reaction as chlorine is oxidised from 0 in Cl2 to +1 in HClO and reduced from 0 in Cl2 to -1 in HCl
HClO is antibacterial (oxidising agent), hence addition of chlorine to swimming pools and in water treatment
Reaction of chlorine and water in the presence of UV light
In the presence of UV light, chlorine and water do not produce chloric acid
2Cl2 + 2H20 ——> 2HCl + O2
Occurs in the presence of UV light, so outdoor pools need chlorinating more often
Reaction of chlorine and COLD DILUTE sodium hydroxide solution to form bleach
Chlorine reacts with COLD DILUTE sodium hydroxide solution to form sodium chloride, sodium chlorate (1) and water
Cl2 + 2NaOH ——–> NaCl + NaClO + H20
Chlorine is oxidised from 0 in Cl2 to +1 in NaClO and reduced to -1 in NaCl
ClO- ions are oxidising agents and the active ingredient in bleach
Reaction of chlorine and HOT CONC sodium hydroxide solution to form bleach
Chlorine reacts with HOT CONC sodium hydroxide solution to form sodium chloride, sodium chlorate (5) and water
3Cl2 + 6NaOH ——–> 5NaCl + NaClO3 + 3H20
Chlorine is oxidised from 0 in Cl2 to +5 NaClO3 and reduced to -1 in 5NaCl
What are halogens?
Halogens are oxidising agents
The oxidising power DECREASEs down the group, with fluorine being the strongest oxidising agent and iodine the weakest
This is because as you go down, the effective nuclear charge decreases due to the increase in shielding
What are halides?
Halides are reducing agents
The reducing power INCREASES down a group with fluorine being the weakest reducing agent and iodine the strongest
This is because as you go down, the effective nuclear charge decreases due to the increase in shielding
Displacement
redox reaction between a halogen and a halide
A stronger oxidising agent displaces a weaker oxidising agent
Observation when chlorine is displaced
Pale green
Observation when bromine is displaced
Brown
Observation when iodine is displaced
Orange
Test for sulfate ions
Acidified barium solution. Produces barium sulfate which is a thick white ppt
Testing for group 1 ions
ALL are soluble
Testing for group 2 nitrate ions
All are soluble
Testing for group 2 hydroxide ions
Insoluble at top, soluble at bottom
Testing for group 2 sulfate ions
Soluble at top, insoluble at bottom
Testing for group 2 halide ions
All are soluble
Testing for group 2 carbonate ions
All insoluble white ppts
Reaction between Ag+ and OH- ions
Brown ppt
AgOH(s)
Reaction between Ag+ and S04^2- ions
White ppt
Ag2SO4
Reaction between Ag+ and CO3^2- ions
White ppt
Ag2CO3
Reaction between Ag+ and NO3- ions
Soluble
Reaction of halide ions and concentrated sulfuric acid:
Fluoride ions
NaF + H2SO4 ——–> No reaction
No redox
Reaction of halide ions and concentrated sulfuric acid:
Chloride ions
NaCl + H2SO4 ——–>HCl + NaHSO4
Acid/ base reaction
No redox
HCl = steamy fumes
Reaction of halide ions and concentrated sulfuric acid:
Bromide ions
NaBr + H2SO4 ——–> HBr + NaHSO4
2H+ + 2Br- + H2SO4 ——–> Br2 + SO2 + 2H2O
Redox - sulfur reduced from +6 to +4
Reaction of halide ions and concentrated sulfuric acid:
Iodide ions
NaI + H2SO4 ——–> HI + NaHSO4
2H+ + I- + H2SO4 ——–> I2 + SO2 + 2H20
6H+ + 6I- + H2SO4 ——–> 3I2- + S + 4H20
8H+ + 8I- +H2SO4 ——–> 4I2- + H2S + 4H20
Observations during reactions of sodium halides with conc sulfuric acid:
Hydrogen halides
HCl, HBr, HI
Steamy fumes
Observations during reactions of sodium halides with conc sulfuric acid:
Bromine
Brown fumes
Observations during reactions of sodium halides with conc sulfuric acid:
Iodine
Purple fumes
Observations during reactions of sodium halides with conc sulfuric acid:
sulfur dioxide
colourless gas with choking smell
Observations during reactions of sodium halides with conc sulfuric acid:
Sulfur
Yellow solid
Observations during reactions of sodium halides with conc sulfuric acid:
Hydrogen sulphide
H2S
pungent gas with rotten egg smell
Reactions of hydrogen halides with water
All readily dissolve in water
Hx (g) + H2O (l) ——–> H3O+ (aq) + X- (aq)
Classed as strong acids as so they fully disassociate to form H+ (aq). This increases the concentration of hydrogen ions, so creates an acidic solution
Reactions of hydrogen halides with ammonia
All readily react with ammonia
Hx (g) + NH3 (g) ——–> NH4x (s)
solid salt produced but presents as white smoke
