Periodicity, Group 2 & 7 (+redox3.17) Flashcards
What happens to atomic radius across a period?
It decreases
- as the number of protons increases and nuclear charge increases
- this means electrons are pulled closer to the nucleus making the atomic radius smaller
- shielding stays roughly the same
What happens to atomic radius down group 2?
It increases because extra electron shells are added
What is the general trend in first ionisation energy down a group?
It decreases:
- electrons have more shielding
- outer electrons further away from the nucleus so weaker attraction
- so easier to remove outer electrons
What happens to the reactivity and melting points down group 2?
Reactivity - increases
Melting points - decrease
What are the uses of group 2 elements?
CaOH - agriculture (neutralises acid soils)
MgOH - indigestion tablets (neutralises stomach acid)
BaSO4 - barium meals
What is the trend in boiling point down group 7 and why
Increase down the group
- Size of atom increases as more occupied electron shells
- stronger van der waals forces of attraction between molecules take more energy to break
What’s the trend in electronegativity down group 7 and why
Decreases down group
- more occupied electron shells
- greater atomic radius and outer electrons are further from the positive charge of the nucleus
- lower force of attraction between the nucleus and electron pair in the covalent bond
What do you use to test for halide ions?
Acidified AgNO3 (silver nitrate)
Why do you add HNO3 and not HCl
- to remove CO3(2-)
- adding HCl would add Cl- ions giving a false positive result
What is the trend in oxidising ability down group 7 (4m)
Decreases down group
- Cl has fewest occupied electron shells
- greatest force of attraction between outer electrons and nucleus
- easiest to gain electrons and be reduced —> best oxidising agent
What’s the equation for Cl2 oxidising 2I-
Cl2 + 2I- —> 2Cl- + I2
Why does chlorine make drinking water safe
- forms CIO- ions which oxidise all microorganisms in water
What’s the trend in hydroxide solubility down group 2
Increases down the group
- Mg(OH)2 is almost insoluble
- Ba(OH)2 creates a strong alkaline solution
What’s the trend in sulfate solubility down group 2
Decreases down the group
- MgSO4 is soluble
- BaSO4 is insoluble
What’s the trend in melting point down group 2
Decreases down the group because atom size gets bigger
- because sea of delocalised electron is further from the positive charge of the nucleus—> weaker metallic bonds which take less energy to weaken
What’s the equation for the extraction of titanium using magnesium and what’s the role of Mg
TiCl4 + 2Mg —> 2MgCl2 + Ti
role of Mg is reducing agent
What are electron acceptors?
Oxidising agents (loss of electrons)
What are electron donors
Reducing agents (gain of electrons)
Reduction is the —— of hydrogen?
Gain
Oxidation is the —— of hydrogen?
Loss
Reducing agents get ———?
Oxidised
Oxidising agents get ———?
Reduced
What’s the oxidation state of elements on their own
0
What’s the oxidation state of group 1 metals
+1
What’s the oxidation state of group 2 metals
+2
What’s the oxidation state of aluminium
+3
What’s the oxidation state of Flourine
-1
What’s the oxidation state of group 7
-1
What’s the oxidation state of Hydrogen
+1 (except when in a metal hydride eg NaH)
What’s the oxidation state of oxygen
-2 (except H2O2)
What happens (+equations) to each of the silver halide precipitates when dilute/conc NH3 are added
AgCl (s) + 2NH3(aq) -> Ag(NH3)2+(aq) + Cl- (dissolves in both conc+dilute)
AgBr(s) + 2NH3(aq) -> Ag(NH3)2+(aq) + Br- (only dissolves in conc)
Iodine doesn’t dissolve in either
What’s the risk of adding chlorine to drinking water
Chlorine is toxic and damages the respiratory system or can cause carcinogens with hydrocarbons
What’s the equation for the reaction of Cl2 with cold water
Cl2 + H20 -> HClO + HCl (reversible reaction arrow)
What type of reaction is chlorine + water
Disproportionation: chlorine is both oxidised and reduced
What’s the equation for making bleach?
Cl2 + 2NaOH -> NaCl + NaCIO + H20
What’s the equation for the reaction of chlorine and water in the presence of sunlight
2Cl2 + 2H20 -> 4HCl + O2
If an elements oxidation state increases it has been —-?
Oxidised (lost electrons)
If an elements oxidation state decreases it has been —-?
Reduced (gained electrons)
Why do melting points increase across period 3 metals
- cation charge increases
- so more delocalised electrons
- so more electrostatic attraction
Describe all the halogens Flourine, chlorine, bromine and iodine at room temp
F2 - pale yellow gas
Cl2 - pale green gas
Br2 - brown liquid
I2 - grey solid
Describe why ionisation energy drops across period 3 between Mg and Al (the 1st dip)
- Al’s first I.E is lower than Mg
- because unlike Mg whose electron is removed from the 3s sublevel, Al electron is removed from the 3p sublevel
- which is further away from the nucleus and has more shielding by 3s electrons
- so Al’s electron requires less energy to remove
Describe why ionisation energy drops across period 3 between phosphorus and sulfur (2nd dip)
- S first I.E is lower than P
- because unlike P whose electron in the 3p sublevel are unpaired, S has a pair of electrons in one of its 3p orbitals
- paired electrons repel each other more than unpaired
- so less energy is required to remove an electron from sulfur
Equations for Ca, Sr and Ba with water
Ca + 2H20 (I) Ca(OH)2(aq) + H2(g)
Sr + 2H20 (I) Sr(OH)2(aq) + H2(g)
Ba + 2H20 (I) Ba(OH)2(aq) + H2(g)
Equation for use of CaO in removing SO2 from flue gases
SO2 + CaO —> CaSO3
Explain why successive ionisation energies increase
Because electrons are removed from an increasingly positive ion which requires more energy
How does the graph of first ionisation energies give evidence for shells
The electron is being removed from an inner shell
What is the general trend in first ionisation energy across a period
It increases:
- nuclear charge increases
- shielding stays roughly the same
- outer electron more strongly attracted to nucleus
- so harder to remove
What happens to first ionisation energy down group 2
Decreases
- outer e- further from nucleus so easier to remove
- more shielding
What is the trend in reducing ability down group 7
- increases down the group
- atomic radius increases
- outer e- further away from nucleus + more shielding
- so the outer e- is lost more readily making it a more powerful reducing agent
Define periodicity
Regular repeating Patterns in properties of elements across periods of the periodic table
