Chemistry 5070/21 — May/June 2018

Walkthrough

Margarine is made by adding hydrogen across the carbon-carbon double bonds of unsaturated vegetable oils. This process is called hydrogenation. From the list, the gas that is used for this is hydrogen.

Key Takeaways

Hydrogen is an important industrial gas; one of its uses is hydrogenating unsaturated oils to make margarine. This is an addition reaction across a C=C double bond.

Common Mistakes

Choosing another gas such as methane or carbon monoxide because they are also flammable gases. The question asks for a specific industrial use, not for a fuel.

Things to Be Careful About

Remember that the mark scheme accepts the name 'hydrogen' only; no formula is needed, but writing $\text{H}_2$ would also be accepted if the question allowed formulae.

Walkthrough

Sulfur dioxide is added to some foods and drinks as a preservative because it kills microorganisms and prevents oxidation. From the list, the gas used as a food preservative is sulfur dioxide.

Key Takeaways

Sulfur dioxide has several uses, including food preservation. It is different from sulfur trioxide, which is mainly an intermediate in making sulfuric acid.

Common Mistakes

Choosing sulfur trioxide instead of sulfur dioxide. Sulfur trioxide is not used as a food preservative.

Things to Be Careful About

Write the full name 'sulfur dioxide' and do not confuse it with 'sulfur trioxide'.

Walkthrough

Ethane is an alkane with the molecular formula $\text{C}_2\text{H}_6$. It contains 2 carbon atoms and 6 hydrogen atoms, so its molecule contains 8 atoms in total. No other gas in the list has exactly 8 atoms in its molecule.

Key Takeaways

The molecular formula tells you how many atoms of each element are present. For ethane, $\text{C}_2\text{H}_6$ means $2 + 6 = 8$ atoms.

Common Mistakes

Choosing methane, which has the formula $\text{CH}_4$ and contains only 5 atoms. Counting the subscript numbers as one number rather than adding them is another common error.

Things to Be Careful About

Count all atoms, not just the number of different elements. Also note that argon and neon are monatomic gases with only 1 atom per particle.

Walkthrough

When vegetable matter decays without oxygen, bacteria produce a gas called methane. This is why methane is found in marshes, landfill sites and biogas. From the list, the gas formed from bacterial decay of vegetable matter is methane.

Key Takeaways

Methane is a greenhouse gas and is produced naturally by anaerobic decay of organic matter. It is the main component of natural gas.

Common Mistakes

Choosing carbon dioxide because decay also releases carbon dioxide. The question specifically asks for the gas formed by bacterial decay of vegetable matter, and the classic product is methane.

Things to Be Careful About

Remember the condition 'bacterial decay' often implies anaerobic conditions, which favour methane production.

Walkthrough

Photochemical smog forms when sunlight acts on nitrogen oxides and hydrocarbons from vehicle exhausts. One of the harmful products is ozone. From the list, the gas formed in photochemical smog is ozone.

Key Takeaways

Ozone at ground level is a pollutant and a component of photochemical smog. This is different from the protective ozone layer high in the atmosphere.

Common Mistakes

Choosing nitrogen oxides, but nitrogen oxides are not in the list. Ozone is the listed gas associated with photochemical smog.

Things to Be Careful About

Do not confuse ground-level ozone (a pollutant) with stratospheric ozone (which protects us from UV radiation).

Walkthrough

Carbon monoxide burns completely in oxygen to form carbon dioxide:

Carbon dioxide is the gas that turns limewater milky because it forms a white precipitate of calcium carbonate. Methane and ethane also produce carbon dioxide on complete combustion, but they also produce water, so they do not form only the gas that turns limewater milky. Therefore the gas is carbon monoxide.

Key Takeaways

Complete combustion of a carbon-containing fuel produces carbon dioxide. The limewater test is the standard test for carbon dioxide: it turns milky/cloudy.

Common Mistakes

Choosing methane or ethane because they contain carbon. Their complete combustion also produces water, so they do not match only a gas that turns limewater milky.

Things to Be Careful About

Check the phrase only a gas. Carbon monoxide produces only carbon dioxide on complete combustion, whereas hydrocarbons produce carbon dioxide and water.