Question

Acetylene(ethyne) burns in oxygen to give carbon dioxide and steam. The balanced reaction is:
$\mathrm{A}) \mathrm{C}_{2} \mathrm{H}_{2}+5 \mathrm{O}_{2} \rightarrow 4 \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}$
$\mathrm{B}) 2 \mathrm{C}_{2} \mathrm{H}_{2}+3 \mathrm{O}_{2} \rightarrow 4 \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}$
$\mathrm{C}) 2 \mathrm{C}_{2} \mathrm{H}_{2}+5 \mathrm{O}_{2} \rightarrow 3 \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}$
$\mathrm{D}) 2 \mathrm{C}_{2} \mathrm{H}_{2}+5 \mathrm{O}_{2} \rightarrow 4 \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}$

Answer 1

Hint: The balanced chemical reaction is in accordance with the law of conservation of mass. According to which, the mass can neither be created nor destroyed in a chemical reaction. But the total mass of the atoms of the molecules on the reactant side is always equal to the mass of atoms of the molecules on the product side. The balancing is done by introducing the coefficients in front of the molecules.

Complete step by step solution:
-The chemical equation is defined as the representation of the chemical reaction in terms of substance. These substances can be reactant or the product. The chemical reactions obey the law of conservation of mass. According to the law, the mass cannot be created nor be destroyed, but it can be changed from one composition to the other.
-The balanced chemical reactions are the application of the law of conservation of mass. In the balanced chemical equation, the number of atoms or the number of moles of an atom of all the reactants and the product is equal on both sides of the reaction.
-In other words, the number of moles of an atom on the reactant side always equals the number of moles of the same atom on the product side.
-Now we have asked for balance as a chemical reaction.
-Acetylene is burned in oxygen to emit carbon dioxide and steam.
\mathrm{C}_{2} \mathrm{H}_{2}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}
-Now let us balance this particular reaction.
Here are two carbon atoms in $\mathrm{C}_{2} \mathrm{H}_{2}$ and one in the
$\mathrm{CO}_{2}($ product $) .$ So $,$ we put coefficient 2 in front of $\mathrm{CO}_{2} $ Thus the carbons are balanced.
-There are two hydrogen atoms in $\mathrm{C}_{2} \mathrm{H}_{2}$ (reactant). There are 2 hydrogen atoms as well in $\mathrm{H}_{2} \mathrm{O}$.
-Now, on the reactant side we have 2 oxygen atoms. But, on the product side, there are 5 oxygen atoms. Then we put a $5 / 2$ coefficient in front of $\mathrm{O}_{2}$ on the reactant side.
The equation is:
$\mathrm{C}_{2} \mathrm{H}_{2}+5 / 2 \mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}$
Since the number of atoms on either side of the reaction is well balanced, this is the balanced chemical reaction.
To simply we just multiply the whole reaction by 2 ,
The reaction is:
$2 \mathrm{C}_{2} \mathrm{H}_{2}+5 \mathrm{O}_{2} \rightarrow 4 \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}$

Clearly, the answer is (D).

Note: We never try to balance the equation by changing the subscript of the molecule. The subscript is unique and has characteristics of a specific molecule, which decides the number of atoms in molecules, the stability, reactivity, etc. Changing the subscript would result in the new molecules which may lead to the undesired product. Thus, always balance the reaction by considering the coefficient in front of atoms. In this way, the equation is balanced correctly.