Chemical equation exercises
Correct answers:
HOW TO BALANCE CHEMICAL EQUATIONS
The law of conservation of mass (or the Lavoisier law), says:
“The mass of a closed system will remain constant, regardless of the processes acting inside the system”.
Sentence above means that number of atoms of a chemical element, both in the reagents and products, must be the same.
This is exactly the balance’s aim. So it is necessary to introduce some numbers, called coefficients, before each formula.
Let’s start to balance.
There aren’t hard and fast rules, however is advisable to start off to balance with metals, non-metals and finally hydrogen and oxygen. If you encounter some difficulties, you can to start again by another element.
Example: H3PO4 + Ca(OH)2 → Ca3(PO4)2 + H2O
Let’s begin with a metal: calcium (Ca). We can see 3 calcium atoms among products; among reagents only 1. So we add a coefficient, in this case 3, before Ca(OH)2.
H3PO4 + 3 Ca(OH)2 → Ca3(PO4)2 + H2O
After that we focus on a non-metal: phosphorus (P). We count 2 phosphorus atoms between the products and 1 between the reagents. Therefore we write 2 before H3PO4.
2 H3PO4 + 3 Ca(OH)2 → Ca3(PO4)2 + H2O
Now pay attention to hydrogen (H): in the first part of the reaction there are 12 hydrogen atoms whereas in the second part only 2. We must introduce 6 before water molecule.
2 H3PO4 + 3 Ca(OH)2 → Ca3(PO4)2 + 6 H2O
At last we control oxygen (O) that, if we have done a good work, it turns out right.
In fact we count 14 oxygen atoms between reagents and products.
The equation is balanced.
Often you cannot balance equations in the way explained above.
This is the instanceof redox equations. In these cases we have to account of oxidation numbers.
Example:
HNO3 + H3AsO3 –> NO + H3AsO4 + H2O
As you see, N and As undergo a change of oxidation numbers.
Is necessary determine the net increase in oxidation number for the element that is oxidized (As) and the net decrease in oxidation number for the element that is reduced (N).
As +3 to +5 Net Change = +2
N +5 to +2 Net Change = -3
We must determine a ratio of oxidized to reduced atoms that would yield a net increase in oxidation number equal to the net decrease in oxidation number.
Briefly, we have to find out a least common multiple:
As +3 to +5 Net Change = +2 x 3 = +6
N +5 to +2 Net Change = -3 x 2 = -6
The ratio of As atoms to N atoms is 3:2.
Let’s introduce these coefficients:
2 HNO3 + 3 H3AsO3 –> NO + H3AsO4 + H2O
From this point you can balance the rest of the equation like a not redox reaction.
2 HNO3 + 3 H3AsO3 –> 2 NO + 3 H3AsO4 + H2O
