Question

How do you balance a single replacement reaction?

Answer 1

Balancing any reaction is the same as another. You just have to place coefficients before reactants or products to make the number of atoms equal on both sides. Balancing is little bit different for hydrates which contains some water of hydration as we see in $CuS{O_4}.\,5\,{H_2}O$ , so in case of hydrates you have to eliminate them and then have to balance the reaction.

Complete step-by-step answer:
For balancing a chemical reaction we have to see firstly the number of atoms on both sides, then to make them equal we have to multiply coefficients of both sides by number so that at last we get a balanced number of atoms. Let’s take an example of a displacement reaction of sodium in zinc chloride. Now what a displacement reaction is? It is called so, basically because of the displacement of an atom by another from its compound.

In the given reaction as we see sodium is single as an atom while zinc is present as compound zinc chloride. $Na(s)\, + \,ZnC{l_2}(aq) \to NaCl(aq) + \,Zn(s)$

So here on both sides count the number of each type of an atom. We see that in the left side of the reaction there is one sodium $Na$, one zinc $Zn$and two chlorine atoms. But on the right hand side the count is little different, there is one chlorine, one zinc $Zn$ and one sodium $Na$ . To make this count equal we have to make the right hand side two chlorines by multiplying the coefficient of $NaCl$ by $2$ .

After that let’s see how the equation gets changed.
$Na(s)\, + \,ZnC{l_2}(aq) \to 2NaCl(aq) + \,Zn(s)$

Now there are two chlorines, two sodium $Na$ and one zinc $Zn$ atom on the right hand side of reaction. Still the equation is not balanced because of the sodium atoms $Na$. There are two sodium $Na$ on the right hand side and one sodium $Na$ at the left hand side, to balance them we have to multiply coefficient of $Na(s)$ by a number $2$ again let’s see the changed equation.
$2Na(s)\, + \,ZnC{l_2}(aq) \to 2NaCl(aq) + \,Zn(s)$

Here the number of each atom becomes equal on both sides, so this is our balanced chemical equation.
There are some cases of reactions where the count of atoms of the same are equal like the displacement reaction between zinc and copper sulphate. On both sides the atoms of $Cu$ , $Zn$ , $S$ , $O$ are equal in number.
$Zn(s)\, + \,CuS{O_4}(aq)\, \to \,Cu(s)\, + \,ZnS{O_4}(aq)$

Note: You have to firstly check whether the reaction is unbalanced or not. If it is already balanced then no need to. Within the reaction we have to show the state of all components with the symbol of heat or sunlight if involved and making the number of atoms on both the sides equal. Then the reaction is known to be a balanced chemical equation.