Question
The equation for the reaction between magnesium and copper(II) oxide is shown.
\(Mg+CuO\rightarrow MgO+Cu\)
Which substance is oxidised?
A. Cu B. CuO C. Mg D. MgO
▶️Answer/Explanation
Ans:
C
Oxidation state of Mg in reactants is 0.
In MgO,
Mg+(-2)=0, Mg = +2.
Thus, Mg is oxidized.
Question
In which change is the sulfur, $\mathrm{S}$, in sulfur(I) oxide, $\mathrm{S}_2 \mathrm{O}$, reduced?
▶️Answer/Explanation
Ans:D
In S2O,
O is -2. 2S -2 =0, S = +1.
In S, S=0.
Thus, in D, S gets reduced from +1 to 0.
Question
The equation for burning propane in air is shown.
$
\mathrm{C}_3 \mathrm{H}_8(\mathrm{~g})+\mathrm{xO}_2(\mathrm{~g}) \rightarrow 3 \mathrm{CO}_2(\mathrm{~g})+\mathrm{yH} \mathrm{H}_2 \mathrm{O}(\mathrm{g})
$
Which values of $x$ and $y$ balance the equation?
▶️Answer/Explanation
Ans:C
To balance H atoms, place 4 in front of H2O.
Now to balance O atoms, place 5 in front of O2.
Thus, x=5, y=4.
Question
Carbon reacts with silver oxide to form carbon dioxide and silver.
Which substance is reduced?
A carbon
B carbon dioxide
C silver
D silver oxide
▶️Answer/Explanation
Ans:D
In reactants, Ag2O , 2Ag -2 =0. Ag =+1.
In products, Ag, Ag=0.
Thus, Silver oxide is reduced.
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 (aqueous solution of bromine, Br2) is added to a sample of pentane, no reaction occurs. Pentane being an alkane, which consists of only carbon-carbon single bonds, is not reactive towards bromine water. Alkanes are generally unreactive towards bromine water.
However, when bromine water is added to a sample of pentene, an alkene, a reaction takes place. Alkenes contain at least one carbon-carbon double bond, which makes them more reactive compared to alkanes. The bromine molecule (Br2) adds across the double bond in the presence of water, resulting in the decolorization of bromine water. The orange-brown color of bromine water disappears as the bromine molecule adds to the double bond of pentene, forming a dibromoalkane. This reaction is often used as a test for the presence of alkenes, as the decolorization of bromine water is characteristic of an alkene’s unsaturation.