Question
Which statement about ethane is correct?
A. It decolourises bromine water.
B. It burns in excess oxygen to form water and carbon dioxide.
C. Its molecular formula is C2H4.
D. Its atoms are joined together by ionic bonding.
▶️Answer/Explanation
Ans:
B
Ethane (C2H6) reacts with seven molecules of oxygen (O2) to produce four molecules of water (H2O) and two molecules of carbon dioxide (CO2). The reaction occurs in the presence of excess oxygen, meaning there is an ample supply of oxygen for the complete combustion of ethane.
Question
Which processes produce methane?
1 complete combustion of carbon-containing compounds
2 decomposition of vegetation
3 digestion in animals
4 respiration in animals
A 1 and 4
B 1 and 3
C 2 and 3
D 2 and 4
▶️Answer/Explanation
Ans:C
When organic matter, such as vegetation, decays or decomposes in anaerobic conditions (low or absence of oxygen), methane-producing microorganisms called methanogens break down the organic material. During this process, they produce methane as a metabolic byproduct. Wetlands, rice paddies, and landfills are examples of environments where anaerobic decomposition of vegetation can lead to significant methane emissions.
Certain animals, such as cows, sheep, and other ruminants, have specialized digestive systems that allow them to digest cellulose and other complex carbohydrates found in plant material. During the digestive process in the rumen (the first stomach compartment of these animals), anaerobic microbes help break down the plant material, resulting in the production of methane as a byproduct. This methane is then released from the animals when they burp or fart, and it contributes to methane emissions from livestock agriculture.
Both natural processes, decomposition of vegetation, and digestion in animals play a role in the global methane cycle and can have significant impacts on atmospheric methane concentrations and, consequently, the Earth’s climate. Efforts to reduce methane emissions from these sources are essential in mitigating the effects of climate change.
Question
Some information about compound L is listed.
1 L is an organic compound which contains four hydrogen atoms.
2 L is soluble in water.
3 An aqueous solution of L reacts with copper(II) carbonate to produce a gas.
What is L?
A methane
B ethene
C ethanoic acid
D ethanol
▶️Answer/Explanation
Ans: C
L is an organic compound containing four hydrogen atoms: Ethanoic acid (CH3COOH) has four hydrogen atoms.
L is soluble in water: Ethanoic acid is indeed soluble in water. It forms a polar covalent bond with water molecules, making it miscible in water.
An aqueous solution of L reacts with copper(II) carbonate to produce a gas: When ethanoic acid (CH3COOH) reacts with copper(II) carbonate (CuCO3), it produces carbon dioxide (CO2) gas as one of the products.
Question
Which statement about alkanes is correct?
A They burn in oxygen.
B They contain carbon, hydrogen and oxygen atoms.
C They contain double bonds.
D They contain ionic bonds.
▶️Answer/Explanation
Ans: A
Alkanes have only C and H connected by covalent single bonds.
Question
Pentane is an alkane and pentene is an alkene.
What is observed when bromine water is added to a sample of each compound?
▶️Answer/Explanation
Ans: C
When bromine water is added to pentane, no visible reaction occurs. Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. They are relatively unreactive towards bromine water or other electrophilic reagents. Therefore, the bromine water remains unchanged, and the pentane sample does not show any color change.
On the other hand, when bromine water is added to pentene, a chemical reaction occurs rapidly. Alkenes are unsaturated hydrocarbons, containing at least one carbon-carbon double bond. The carbon-carbon double bond is reactive, and it can undergo addition reactions with bromine.
The reaction with bromine water is called bromine addition. During this reaction, the brownish-orange color of the bromine water disappears as the bromine is consumed in the reaction. The alkene’s double bond breaks, and two bromine atoms add across the double bond to form a dihalide product. The result is a colorless solution or precipitate, depending on the specific alkene used.