Ocr Chemistry F321, Groups 2&7

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Topic 11a – Group 7, the Halogens
Revision Notes

1)Trends in physical properties of the elements

• At room temperature, Cl2 is a pale green gas, Br2 is a brown liquid, I2 is a blue-black solid. • In Group 7, boiling point increases down the group because the molecules have more electrons and, therefore, stronger van der Waal’s forces which require more energy to overcome them • Electronegativity is the ability of an atom to attract the electrons in a covalent bond • Down Group 7, the shared electrons are further from the nucleus and more shielded. There is a weaker attraction for the bonding electrons down the Group

2)Trends in chemical properties of the elements

• The Group 7 elements are oxidising agents which gain an electron when they react • The ability to oxidise decreases down Group 7
• This can be shown by halogen displacement reactions where elements higher up the group will displace elements further down the group • This can be done with chlorine dissolved in water or by bubbling chlorine gas through NaBr(aq) or NaI(aq)

Cl2 + 2NaBr ( 2NaCl + Br2
Pale green solutionYellow solution/orange in hexane

Cl2 + 2NaI ( 2NaCl + I2
Pale green solutionBrown solution/purple in hexane

Br2 + 2NaI ( 2NaBr + I2
Orange solutionBrown solution/purple in hexane

• These equations can also be written in ionic form

Cl2 + 2Br- ( 2Cl- + Br2

Cl2 + 2I- ( 2Cl- + I2

Br2 + 2I- ( 2Br- + I2

• Down the group the outer electron is further from nucleus and more shielded so gained less easily

3)Trends in properties of the halides

• The halides (chlorides, bromides and iodides) are reducing agents which lose an electron when they react • The ability to act as a reducing agent increases down the Group • Down the group the outer electron is further from nucleus and more shielded so lost more easily • This can be shown by the reactions of the solid halides with concentrated sulphuric acid

| | | | | |Sodium halide |Observations |Products |Type of reaction | | | | | | |NaF |Steamy fumes |HF(g) |Acid-base (F- acting as base) | | | | | | |NaCl |Steamy fumes |HCl(g) |Acid-base (Cl- acting as base) | | | | | | |NaBr |Steamy fumes |HBr(g) |Acid-base (Br- acting as base) | | |Colourless gas |SO2 |Redox (reduction product of H2SO4) | | |Brown fumes |Br2 |Redox (oxidation product of Br-) | | | | | | |NaI |Steamy fumes |HI(g) |Acid-base (I- acting as base) | | |Colourless gas |SO2 |Redox (reduction product of H2SO4) | | |Yellow solid |S |Redox (reduction product of H2SO4) | | |Smell of bad eggs |H2S...
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