To identify which of the given reactions shows a valid combustion reaction, let's examine each option based on the standard criteria of a combustion reaction. A standard combustion reaction typically involves a hydrocarbon or another fuel reacting with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O).
1. [tex]\(2 \text{Al} + 2 \text{O}_2 \rightarrow 2 \text{AlO} + \text{O}_2 \)[/tex]:
- This reaction involves aluminum (Al) and oxygen (O₂).
- Aluminum is a metal, not a hydrocarbon.
- The products are aluminum monoxide (AlO) and molecular oxygen (O₂).
- This reaction does not produce CO₂ and H₂O.
- Therefore, this is not a valid combustion reaction.
2. [tex]\(\text{C}_2\text{H}_4 + 3 \text{O}_2 \rightarrow 2 \text{CO}_2 + 2 \text{H}_2\text{O} \)[/tex]:
- This reaction involves ethylene (C₂H₄), a hydrocarbon, and oxygen (O₂).
- The products are carbon dioxide (CO₂) and water (H₂O).
- This fits the criteria for a standard combustion reaction.
- Therefore, this is a valid combustion reaction.
3. [tex]\(\text{2 CH}_4 + \text{O}_2 \rightarrow 2 \text{CO} + 4 \text{H}_2 \)[/tex]:
- This reaction involves methane (CH₄), a hydrocarbon, and oxygen (O₂).
- The products are carbon monoxide (CO) and hydrogen gas (H₂).
- Typically, combustion of methane should produce CO₂ and H₂O, not CO and H₂.
- Therefore, this is not a valid combustion reaction.
4. [tex]\(\text{Ca} + \text{O}_2 \rightarrow \text{CaOH} \)[/tex]:
- This reaction involves calcium (Ca) and oxygen (O₂).
- Calcium is a metal, not a hydrocarbon.
- The product is calcium hydroxide (CaOH), which is not a typical product of a combustion reaction.
- This does not fit the standard criteria for a combustion reaction.
- Therefore, this is not a valid combustion reaction.
After evaluating each option, the valid combustion reaction is:
[tex]\[ \text{C}_2\text{H}_4 + 3 \text{O}_2 \rightarrow 2 \text{CO}_2 + 2 \text{H}_2\text{O} \][/tex]
So, the correct answer is:
[tex]\[ \boxed{2} \][/tex]
