Question

which of the following is a decomposition reaction? *
\\( \mathrm{al}_2\mathrm{o}_3 + 3\mathrm{h}_2\mathrm{o} \
ightarrow 2\mathrm{al}(\mathrm{oh})_3 \\)
\\( 2\mathrm{hgo} \
ightarrow 2\mathrm{hg} + \mathrm{o}_2 \\)
\\( \mathrm{c}_2\mathrm{h}_4 + 3\mathrm{o}_2 \
ightarrow 2\mathrm{co}_2 + 2\mathrm{h}_2\mathrm{o} \\)
\\( \mathrm{h}_2\mathrm{so}_4 + \mathrm{ba}(\mathrm{oh})_2 \
ightarrow \mathrm{baso}_4 + 2\mathrm{h}_2\mathrm{o} \\)
which option balances the equation? *
\\( \underline{\quad} \mathrm{c}_2\mathrm{h}_6 + \underline{\quad} \mathrm{o}_2 \
ightarrow \underline{\quad} \mathrm{co}_2 + \underline{\quad} \mathrm{h}_2\mathrm{o} \\)
\\( \mathrm{c}_2\mathrm{h}_6 + 5\mathrm{o}_2 \
ightarrow 2\mathrm{co}_2 + 3\mathrm{h}_2\mathrm{o} \\)
\\( 2\mathrm{c}_2\mathrm{h}_6 + 7\mathrm{o}_2 \
ightarrow 4\mathrm{co}_2 + 6\mathrm{h}_2\mathrm{o} \\)
\\( \mathrm{c}_3\mathrm{h}_8 + 3\mathrm{o}_2 \
ightarrow 2\mathrm{co}_2 + 3\mathrm{h}_2\mathrm{o} \\)
\\( \mathrm{c}_3\mathrm{h}_8 + 7\mathrm{o}_2 \
ightarrow 4\mathrm{co}_2 + 2\mathrm{h}_2\mathrm{o} \\)

Explanation:

First Question (Decomposition Reaction)

Step1: Recall Decomposition Reaction

A decomposition reaction is when a single compound breaks down into two or more simpler substances (elements or compounds). The general form is \( AB
ightarrow A + B \).

Step2: Analyze Each Option

• Option 1: \( \text{Al}_2\text{O}_3 + 3\text{H}_2\text{O}

ightarrow 2\text{Al(OH)}_3 \)
This is a combination (synthesis) reaction (two substances combine to form one), so not decomposition.

• Option 2: \( 2\text{HgO}

ightarrow 2\text{Hg} + \text{O}_2 \)
A single compound (\( \text{HgO} \)) breaks down into two simpler substances (\( \text{Hg} \) and \( \text{O}_2 \)). This matches the decomposition pattern.

• Option 3: \( \text{C}_2\text{H}_4 + 3\text{O}_2

ightarrow 2\text{CO}_2 + 2\text{H}_2\text{O} \)
This is a combustion reaction (a hydrocarbon reacts with \( \text{O}_2 \) to form \( \text{CO}_2 \) and \( \text{H}_2\text{O} \)), not decomposition.

• Option 4: \( \text{H}_2\text{SO}_4 + \text{Ba(OH)}_2

ightarrow \text{BaSO}_4 + 2\text{H}_2\text{O} \)
This is a double - displacement (neutralization) reaction, not decomposition.

Second Question (Balancing Equation \( \text{C}_2\text{H}_6 + \text{O}_2

ightarrow \text{CO}_2 + \text{H}_2\text{O} \))

Step1: Balance Carbon Atoms

On the left, \( \text{C}_2\text{H}_6 \) has 2 C atoms. So we need 2 \( \text{CO}_2 \) on the right: \( \text{C}_2\text{H}_6 + \text{O}_2
ightarrow 2\text{CO}_2 + \text{H}_2\text{O} \)

Step2: Balance Hydrogen Atoms

\( \text{C}_2\text{H}_6 \) has 6 H atoms. So we need 3 \( \text{H}_2\text{O} \) (since \( 3\times2 = 6 \)): \( \text{C}_2\text{H}_6 + \text{O}_2
ightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \)

Step3: Balance Oxygen Atoms

On the right: \( 2\text{CO}_2 \) has \( 2\times2 = 4 \) O, and \( 3\text{H}_2\text{O} \) has \( 3\times1 = 3 \) O. Total O on right: \( 4 + 3=7 \). Since \( \text{O}_2 \) is diatomic, we need \( \frac{7}{2}\text{O}_2 \), but we use whole numbers. Multiply all coefficients by 2:
\( 2\text{C}_2\text{H}_6 + 7\text{O}_2
ightarrow 4\text{CO}_2 + 6\text{H}_2\text{O} \)

Now check the options:

• Option 1: \( \text{C}_2\text{H}_6 + 5\text{O}_2

ightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \)
O: Left \( 5\times2 = 10 \), Right \( 2\times2+3\times1 = 7 \). Not balanced.

• Option 2: \( 2\text{C}_2\text{H}_6 + 7\text{O}_2

ightarrow 4\text{CO}_2 + 6\text{H}_2\text{O} \)
C: Left \( 2\times2 = 4 \), Right \( 4 \). H: Left \( 2\times6 = 12 \), Right \( 6\times2 = 12 \). O: Left \( 7\times2 = 14 \), Right \( 4\times2+6\times1 = 14 \). Balanced.

• Option 3: \( \text{C}_3\text{H}_8 + 3\text{O}_2

ightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \)
The reactant is \( \text{C}_3\text{H}_8 \) (not \( \text{C}_2\text{H}_6 \)), so incorrect.

• Option 4: \( \text{C}_3\text{H}_8 + 7\text{O}_2

ightarrow 4\text{CO}_2 + 2\text{H}_2\text{O} \)
Reactant is \( \text{C}_3\text{H}_8 \) and H/O not balanced, so incorrect.

Answer:

First Question: \( 2\text{HgO}
ightarrow 2\text{Hg} + \text{O}_2 \) (the second option in the first question)
Second Question: \( 2\text{C}_2\text{H}_6 + 7\text{O}_2
ightarrow 4\text{CO}_2 + 6\text{H}_2\text{O} \) (the second option in the second question)