WATP (2016)

Question 1

Explain how energy changes in bond breaking & formation relate to the overall change in enthalpy in an exothermic reaction. (2)

Answer 1

• The energy input required in the bond breaking, and formation is much less than the energy output in the bond formation.
• => overall release of energy, therefore an exothermic reaction.

Question 2

Why do alkenes undergo addition reactions but alkane and benzene do not? (4)

Answer 2

• Addition reactions require the presence of a double bond which is present in alkenes.
• Double bond is broken to a single bond and another substance is incorporated, added to each side of the carbon of the double bond.
• Alkanes don’t contain double bond => can only undergo substitution reaction.
• Benzene doesn’t contain double bonds, ring structure is stable and can only undergo substitution reactions.

Question 3

Describe how the absorption and emission spectrum of an element is related to electron energy levels, and how this is utilised in AAS. (6)

Answer 3

• The electrons of an atom are confined to specific energy levels or shells.
• Electrons are able to move between these energy levels by absorbing (to jump outwards) or emitting (to jump inwards) particular amounts of energy.
• The amount of energy they absorbed of emitted by these electrons as they move between shells correspond to a particular wavelength of light which create the corresponding absorption or emission spectra.
• These spectra are unique for each element, since the energy levels of the electron shells are slightly different for every element.
• In AAS the known absorption spectrum of a particular element is used and light of a corresponding wavelength is shone through the sample being tested.
• The higher the concentration of the particular element in the sample, the more light it absorbs.