WJEC Chemistry for AS: Study and Rev Guide

Atomic hydrogen spectrum Rhiannon’s answer David’s answer (a) The spectrum is a pattern of separate lines. 3 ✗  Because the lines are separate it shows that the energy levels in the atom are quantised. 3 ✗✗  (b) The energy lines converge to a limit as they go from n = 1 to n = ∞ and using ΔE = hf this represents the ionisation energy. 3  (c) ΔE = hf therefore f = ΔE = 1310 = 1.98 × 10 36 Hz 3 ✗✗  h 6.63 × 10 –34 (a) The visible emission spectrum of atomic hydrogen is a series of lines 3 which become closer as the energy increases. 3 The lines are caused by excited electrons dropping back to a lower energy level ✗  therefore electrons exist in discrete energy levels. 3 As energy increases, the energy levels become closer so the lines become closer. 3 (b) For the Lyman series, n = 1, the convergence limit represents the ionisation of the hydrogen atom. 3 The convergent frequency (difference from n = 1 to n = ∞) can be measured 3 and the ionisation energy can be calculated from ΔE = hf. 3 (c) I.E. = L ΔE and ΔE = hf therefore I.E. = Lhf f = I.E. = 1 310 000 = 3.28 × 10 15 s –1 333  Lh (6.02 × 10 23 )(6.63 × 10 –34 ) Examiner commentary  Rhiannon gains one mark for the description. To gain the second mark she needs to describe the pattern that the lines form.  Rhiannon gets one mark for correctly using ‘quantised’; however, she has not explained how the lines form or why they get closer.  Rhiannon’s answer is very vague. Although she has stated the relationship between ionisation energy and the Lyman series, she has not explained how the ionisation energy can be derived.  Rhiannon has not realised that molar ionisation energy = ionisation of an atom × Avogadro’s constant and has not changed I.E. into J mol –1 . Rhiannon achieves 4 out of 11 marks. Examiner commentary  However, in the visible emission spectrum (Balmer series) the lines result from electronic transitions from higher energy levels to energy level n = 2.  David has correctly converted I.E. into J mol –1 since the unit for Planck’s constant is Js. David achieves 10 out of 11 marks. Q & A 117 Questions and answers 3 (a) Describe the visible emission spectrum of atomic hydrogen and explain how its features relate to electronic levels within the hydrogen atom. [5] (b) Explain how the ionisation energy of hydrogen can be derived from the Lyman series in the atomic hydrogen spectrum. [3] (c) Given that the molar ionisation energy of hydrogen is 1310 kJ mol –1 , calculate the value of the frequency at the start of the continuum in the hydrogen emission spectrum. [3]