Question

Identify each of the following skeleton equations as representing a synthesis reaction, a combustion reaction or decomposition reaction. Balance each equation.
a. C2H4(g)+ O2(g)~ CO2(g)+H2O(g)

Answer 1

The given skeleton equation represents a **combustion reaction** since it involves the burning of a hydrocarbon (C2H4, ethylene) in the presence of oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).

To balance the combustion reaction, we can follow these steps:

1. Write the unbalanced equation:
\[
C_2H_4(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)
\]

2. Count the number of atoms for each element on both sides:
- On the left, we have:
- Carbon (C): 2 (from C2H4)
- Hydrogen (H): 4 (from C2H4)
- Oxygen (O): 2 (from O2)

- On the right, initially:
- Carbon (C): 1 (in CO2)
- Hydrogen (H): 2 (in H2O)
- Oxygen (O): 3 (1 from CO2 + 1 from H2O)

3. Start by balancing the carbon atoms:
\[
C_2H_4(g) + O_2(g) \rightarrow 2CO_2(g) + H_2O(g)
\]

4. Now count again:
- Carbon: 2 (left) and 2 (right, from 2CO2).
- Hydrogen: 4 (left) and 2 (right, from H2O).

Next, balance the hydrogen by adjusting the number of water molecules:
\[
C_2H_4(g) + O_2(g) \rightarrow 2CO_2(g) + 2H_2O(g)
\]

5. Count again:
- On the left:
- C: 2
- H: 4
- O: 2 (from the O2)
- On the right:
- C: 2 (from 2CO2)
- H: 4 (from 2H2O)
- O: 6 (2 from 2CO2 + 2 from 2H2O)

6. Now, balance the oxygen:
- We have a total of 6 oxygen atoms on the right.
- Since each O2 molecule has 2 oxygen atoms, we need 3 O2 molecules on the left:
\[
C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(g)
\]

7. Final count:
- Left:
- C: 2
- H: 4
- O: 6 (from 3O2)
- Right:
- C: 2 (from 2CO2)
- H: 4 (from 2H2O)
- O: 6 (2 from 2CO2 + 2 from 2H2O)

The balanced equation for the combustion reaction is:
\[
C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(g)
\]