Giant ionic structures
3 marks · NormalA student is comparing the melting points and electrical conductivity of two compounds: sodium chloride (NaCl) and magnesium oxide (MgO).
Sodium chloride melts at around 801°C, while magnesium oxide melts at about 2852°C. Calculate the difference in melting points between these two ionic compounds.
Explain, using the structure and bonding in giant ionic lattice structures, why magnesium oxide has a much higher melting point than sodium chloride.
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(a) 2051 °C
(b) — · must show Ionic lattice structure described for both compounds, MgO has higher charge ions (Mg^{2+}, O^{2-}) compared to Na^+, Cl^-, Stronger electrostatic attraction between Mg^{2+} and O^{2-} than between Na^+ and Cl^-, More energy needed to break the stronger bonds in MgO
Common mistakes: Mixing up which compound has the higher melting point.; Not referring to charge or size of ions.; Talking about molecules instead of ions.
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Marking points
A1 (a) · 1 mark · Correct calculation of melting point difference: 2852 - 801 = 2051°C (accept 2050–2052°C for rounding error)
M1 (b) · 1 mark · States or describes that MgO has ions with higher charges than NaCl, leading to stronger electrostatic attractions in the lattice
A2 (b) · 1 mark · Relates higher strength of attraction to greater energy (higher temperature) needed to break bonds or melt MgO
Worked solution
a) The difference in melting points is calculated by subtracting the melting point of NaCl from that of MgO:
b) Magnesium oxide (MgO) has Mg^{2+} and O^{2-} ions, while sodium chloride (NaCl) has Na^+ and Cl^- ions. The greater the charges on the ions, the stronger the electrostatic attraction between them. MgO's ions both have 2+ or 2- charge, so the force holding its lattice together is stronger than the 1+ and 1- ions in NaCl. Therefore, more energy is needed to break the ionic bonds in MgO, resulting in a much higher melting point.