This fuel-cell demonstration is used by a college group to show that hydrogen can react with oxygen to form water. Fig. 1 is an incomplete dot-and-cross dia...
Question 1 Report
This fuel-cell demonstration is used by a college group to show that hydrogen can react with oxygen to form water. Fig. 1 is an incomplete dot-and-cross diagram for one water molecule. Small dots represent electrons supplied by oxygen. Crosses represent electrons supplied by hydrogen. The student must complete the electron arrangement before writing the word equation for the reaction.
The relative atomic masses used by the group are H = 1 and O = 16. In a second trial, 3.6 g of water is collected after the reaction. The group discusses why this small covalent molecule has properties very different from sodium chloride, an ionic compound.
(a) Draw the missing electrons on Fig. 1 to complete the dot-and-cross diagram. [4]
(b) Give the name of the type of bond between oxygen and each hydrogen atom. [1]
(c) Describe how a covalent bond forms in this molecule. [2]
(d) Use the relative atomic masses to calculate the relative formula mass of water. [2]
(e) Use your answer to part (d) to calculate the amount, in moles, of water in 3.6 g. [3]
(f) How does the electrical conductivity of pure liquid water compare with molten sodium chloride? Give a reason for each. [3]
Answer Details
(a) The completed dot-and-cross diagram has one dot from oxygen in each O-H shared pair, and two lone pairs remaining on oxygen.
One oxygen dot in each bonding pair earns [2], and four oxygen electrons as two lone pairs earn [2].
(b) Each O-H bond is a covalent bond [1].
(c) A covalent bond forms when a pair of electrons is shared [1] between oxygen and hydrogen atoms [1].
(d) \[M_r(\mathrm{H_2O})=(2\times1)+16=18\] [2]
(e) \[\text{amount}=\frac{\text{mass}}{M_r}=\frac{3.6\text{ g}}{18}=0.20\text{ mol}\] [3]
(f) Pure liquid water conducts very poorly, or does not conduct [1], because it has no mobile ions or delocalised electrons [1]. Molten sodium chloride conducts because its ions are free to move [1].
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