A technician prepares a reference card for a chemical store. The card compares four covalent molecules that may be supplied as gases or liquids. Table 1 giv...
Question 1 Report
A technician prepares a reference card for a chemical store. The card compares four covalent molecules that may be supplied as gases or liquids. Table 1 gives the atoms in one molecule and the temperature at which each substance boils. The formula column has been left blank for the student to complete.
| substance | atoms in one molecule | formula | boiling point / degrees C |
|---|---|---|---|
| hydrogen | 2 hydrogen atoms | blank | -253 |
| hydrogen chloride | 1 hydrogen atom and 1 chlorine atom | blank | -85 |
| ammonia | 1 nitrogen atom and 3 hydrogen atoms | blank | -33 |
| water | 2 hydrogen atoms and 1 oxygen atom | blank | 100 |
(a) Complete the formula column for all four substances. [2]
(b) State which substance is a liquid at 25 degrees C. [1]
(c) Give the state of hydrogen chloride at -100 degrees C. [1]
(d) Use Table 1 to calculate the temperature range over which hydrogen chloride is a gas but hydrogen is a liquid. [3]
(e) Explain why hydrogen and hydrogen chloride have low boiling points compared with silicon dioxide. [3]
(f) Which molecule contains the greatest number of atoms? Give its name. [1]
(g) State the type of bonding within a water molecule. [1]
Answer Details
(a) The formulae are hydrogen: \(\mathrm{H_2}\); hydrogen chloride: \(\mathrm{HCl}\); ammonia: \(\mathrm{NH_3}\); water: \(\mathrm{H_2O}\). [2]
(b) Water is liquid at 25 degrees C. [1]
(c) Hydrogen chloride is a liquid at \(-100\) degrees C because this is below its boiling point of \(-85\) degrees C. [1]
(d) Hydrogen chloride is a gas above \(-85\) degrees C. The supplied boiling-point data show hydrogen has a lower boiling point, \(-253\) degrees C, so hydrogen is already gaseous whenever hydrogen chloride is gaseous. Therefore there is no temperature range in which hydrogen chloride is a gas but hydrogen is a liquid. [3]
(e) Hydrogen and hydrogen chloride are simple molecular substances. Weak forces act between their molecules, requiring much less energy to overcome than the strong covalent bonds in giant silicon dioxide. Therefore they have low boiling points compared with silicon dioxide. [3]
(f) Water contains the greatest number of atoms per molecule: three atoms. [1]
(g) The bonding within a water molecule is covalent. [1]
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