3.2 - Group 2 and the halogens Flashcards
What is the electron configuration of the outer shell of group 2 elements ?
- Each group 2 element contains 2 outer shell electrons
- The two electrons are in the outer s sub-shell ( s^2 )
What do group 2 elements do during redox reactions ?
They lose two outer shell electrons to form 2+ ions
Explain how group 2 elements are reducing agents ?
- Each metal atom is oxidised, losing two electros to form a +2 ion
- Another species will gain these two electrons and be reduced
- Group 2 elements are therefore reducing agents as they reduce another species
Write the half equation showing a group 2 element being oxidised ?
X –> X^2+ + 2e-
eg. Ca –> Ca^2+ + 2e-
What happens during redox reactions of group 2 elements with oxygen ?
Group 2 elements all react with oxygen to form a metal oxide with the general formula XO
Write the equation for a redox reaction of group 2 elements with oxygen ?
2X + Ov2 –> 2XO
eg. 2Mg(s) + Ov2(g) –> 2MgO(s)
What happens during redox reactions of group 2 elements with water ?
Group 2 elements react with water to form an alkaline hydroxide with general formula X(OH)v2, and hydrogen gas
Write the equation for a redox reaction of group 2 elements with water ?
X + Hv2O –> X(OH)v2 + Hv2
eg. Sr(s) + Hv2O(l) –> Sr(OH)v2(aq) + Hv2(g)
What happens during redox reactions of metals with dilute acids ?
Metals react with dilute acids to form a salt and hydrogen
Write the equation for a redox reaction between metals and dilute acids ?
Metal + acid –> salt + hydrogen
eg. Mg(s) + 2HCl(aq) –> MgClv2(aq) + Hv2(g)
What is the trend in reactivity of group 2 elements ?
As you go down the group, the reactivity of group 2 elements increases
Explain the trend in relativity of group 2 elements in terms of first and second ionisation energy ?
- As you go down the group, the number of electron shells increases meaning the atomic radius and shielding increases
- This means there is a greater number of repealing electrons and distance between the outer shell electrons and the positive nucleus
- This means that the outer shell electrons are less attracted to the positive nucleus causing the first and second ionisation energy to decrease
- This means less energy is required to remove the first and second outer electrons to form a 2+ ion, causing reactivity to increase
What happens during reactions between group 2 compounds and water ?
The oxides of group 2 elements react with water, releasing OH- ions forming an alkaline solution of metal hydroxide
Write an equation for the reaction between group 2 compounds and water ?
XO + Hv2O –> X(OH)v2
eg. CaO(s) + Hv2O(l) –> Ca(OH)v2(s)
What is the trend in solubility and alkalinity of group 2 hydroxides ?
- As you go down the group , the solubility of group 2 hydroxides in water increases
- This means the group 2 oxides release more OH- ions resulting in solution containing more OH- increasing it’d pH/ alkalinity
What are the uses of group 2 compounds as bases ?
- Ca(OH)v2 in agriculture to neutralise acid soils
- Mg(OH)v2 and CaCOv3 as ‘antacids’ in treating indigestion.
Explain how group 2 compounds are used as bases in agriculture ?
Ca(OH)v2 is added to fields as lime by farmers to increase the pH and neutralise acidic soils, forming neutral water
Write the equation showing group 2 compounds being used as bases in agriculture ?
Ca(OH)v2(s) + 2H+(aq) –> Ca2+(aq) + 2Hv2O(l)
Explain ho group 2 compounds are used as bases in medicine ?
Group 2 bases are often used as antacids for treating acid indigestion ( ie. Mg(OH)v2 and CaCOv3 )
Write the equations showing group 2 compounds being used as bases in medicine ?
Mg(OH)v2(s) + 2HCl(aq) –> MgClv2(aq) + 2Hv2O(l)
CaCOv3(s) + 2HCl(aq) –> CaClv2(aq) + Hv2O(l) + COv2(g)
What is the electron configuration of the outer shell of group 7 elements ?
- Each halogen has 7 outer-shell electrons, two are in the outer s sub shell and five are in the outer p sub-shell ( s^2p^5 )
What do group 7 elements do during redox reactions ?
- Each halogen atom is reduced, gaining one electron to form a -1 halide ion
Explain how group 7 elements are oxidising agents ?
- Each halogen atom is reduced, gaining an electron to form a -1 halide ion
- Another species loses electrons to halogen atoms and is oxidised
- Group 7 elements are therefore oxidising agents as they oxidise another species
Write the half equation showing a group 7 element being reduced ?
Xv2 + 2e- –> 2X-
eg. Fv2 + 2e- –> 2F-
What is the trend in boiling points for group 7 elements ?
As you go down the group, the boiling point increases
Explain the trend in boiling points of group 7 elements ?
- Halogens exist as diatomic molecules
- As you go down the group, there is a greater number of electron shells meaning the size of the molecule increases
- This means there is a greater number of electrons present resting in stronger induced dipoles
- This results in stronger London forces between molecules which require more energy to overcome and separate molecules
- This causes the boiling point to increase
Name the various halogens and their appearance/state at room temperature ?
What is the trend in reactivity of group 7 elements ?
As you go down the group, the reactivity of group 7 elements decreases
Explain the trend in reactivity of group 7 elements ?
- As you go down the group, the number of outer electron shells increases meaning the atomic radius and shielding increases
- This means there is a greater number of repealing electrons and greater distance between the outer shell electrons and positive nucleus
- This means there is weaker attraction between the outer shell electrons and positive nucleus meaning is harder to gain an electron since it is less attracted to the positive nucleus
- This causes the reactivity to decrease
What type of reactions show the trend in reactivity of group 7 elements ?
Halogen displacement reactions with halide ions
What happens during halogen displacement reactions with halide ions ?
- During the displacement reaction, the more reactive halogen will displace the less reactive halide from it salt
- This results in the formation of the less reactive halogen
What are the colours halogen solutions in water during halogen displacement reactions?
Chlorine = pale green solution in water
Bromine = Orange solution in water
Iodine = Brown solution in water
Why is cyclohexane used during the halogen displacement reaction with halide ions ?
- Aqueous solutions of bromine and iodine can be difficult to tell apart due to having a similar orange-brown colour
- Cyclohexane is therefore used to tell them apart
What are the colours halogen solutions in cycloalkane during halogen displacement reactions ?
Chlorine = pale green solution in cyclohexane
Bromine = orange solution in cycloalkane
Iodine = violet solution in cycloalkane
What apparatus can be used to separate the layers ?
Separating funnel
What are the results of halogen displacement reactions ?
- Chlorine reacts with both Br- and I- ions
- Bromine only reacts with I- ions
- Iodine react with neither ions
Write the full and ionic equation for the displacement reaction between chlorine solution and sodium bromide ?
Clv2(aq) + 2NaBr(aq) –> 2NaCI(aq) + Brv2(aq)
Clv2(aq) + 2Br- (aq) –> 2Cl- (aq) + Brv2(aq)
Write the full and ionic equation for the displacement reaction between chlorine solution and sodium iodide ?
Clv2(aq) + 2NaI(aq) –> 2NaCI(aq) + Iv2(aq)
Clv2(aq) + 2I- (aq) –> 2Cl- (aq) + Iv2(aq)
Write the full and ionic equation for the displacement reaction between bromine solution and sodium iodide ?
Brv2(aq) + 2NaI(aq) –> 2NaBr(aq) + Iv2(aq)
Brv2(aq) + 2I- (aq) –> 2Br- (aq) + Iv2(aq)
What is a disproportionation reaction ?
It is a redox reaction where the same element is both oxidised and reduced
What are the benefits of using chlorine ?
- Chlorine can be used in water treatment to kill/ destroy bacteria
- Chlorine can be used to produce a household bleach
Explain how chlorine is used in water treatment to kill harmful bacteria ?
- When chlorine reacts with water, a disproportionation reaction occurs producing HClO ( hydrogen chlorate (l) ) and HCl ( hydrochloric acid )
- HClO is responsible for killing harmful bacteria
Explain another role that HClO has ?
- HClO is also a weak bleach
- Therefore, in the presence of damp blue litmus paper it will turn it ed before turning it white
Write the equation for chlorine reacting with water when used in water treatment to kill bacteria ?
Clv2(aq) + Hv2O(l) -> HCIO(aq) + HCI(aq)
Explain how chlorine can be used to produce household bleach ?
- When chlorine reacts with cold AND DILUTE NaOH/ sodium hydroxide solution, a disproportionation reaction occurs
- This results in NaClO ( sodium chlorate (l) ), NaCl and Hv2O being produced
- NaClO has high concentration of ClO- / chlorate ions meaning it can act as a household bleach
What are the conditions for making household bleach using chlorine ?
Cold and dilute aqueous sodium hydroxide ( NaOH )
Write the equation for chlorine being used to produce household bleach ?
CIv2(aq) + 2NaOH(aq) -+ NaCIO(aq) + NaCI(aq) + Hv20(l)
Can you explain how chlorine reacting with water and cold sodium hydroxide solution is a disproportionation reaction ?
Yes
What are the risks of chlorine ?
- Chlorine is an extremely toxic gas
- Chlorine can react with hydrocarbons to produce chlorinated hydrocarbons which are possible carcinogens
Explain the risks of chlorine with regards to it being an extremely toxic gas ?
- Chlorine is an extremely toxic gas
- In small concentrations, it can act as respiratory irritant
- In large concentrations is can be fatal/ lead to death
Explain the risks of chlorine with regards to it producing chlorinated hydrocarbons ?
- In drinking water, chlorine can react with organic matter ( eg. methane from decaying vegetation ) in a free radical substitution reaction
- This can produce chlorinated hydrocarbons which are possible carcinogens which cause cancer
What is chlorine used despite its high risks ?
- If chlorine is not used to treat water, we risk bacteria flourishing which can lead to terrible diseases such as cholera of typhoid
- Providing a clean water supply is vital, if chlorine were not used this would be compromised
What is used to test for the presence of halogens ?
Aqueous silver nitrate solution [ AgNOv3 (aq) ]
Describe how to test for halides ?
- Add aqueous silver nitrate solution [ AgNOv3 ( aq ) ] to an aqueous solution of a halide
- Note the colour of the precipitate that forms
- Add aqueous ammonia to test the solubility of the precipitate
What are the colours of the precipitate when aqueous silver nitrate is added to aqueous solution of a halide ?
Chlorine = White precipitate
Bromine = Cream precipitate
Iodine = Yellow precipitate
Why is aqueous ammonia used to test the solubility of the precipitate ?
- This is a confirmative test used to tell the different precipitate colours apart
- White, cream and yellow can be easily misinterpreted
What is the solubility of the different precipitates when aqueous ammonia is added ?
