You may ask, “Why should we need these coefficients? Why not just write it out as simply as possible?” Well, if we write out what really happens in the reaction, our equation would look like this:
What we mean by this is that the number and type of atoms from the reactants on the left side of the equation do not match the products on the right side. We know that in the real world this cannot be the case, because the law of conservation of mass states that atoms are neither created nor destroyed in any chemical reaction.
Take a look at our last equation again. On the left there are two hydrogen atoms and on the right there are two oxygen atoms. On the right there are two hydrogen atoms (this is good), but only one oxygen atom. This implies that one of the oxygen atoms just disappeared, and we know this cannot be! This means the equation is unbalanced. To fix this, we need to add our coefficients. Let’s start out by bringing back the missing oxygen atom. We do this by putting the coefficient 2 in front of water. This means now there are two molecules of water, each of which has one oxygen atom, for a total of two oxygen atoms.
Let’s try another synthesis reaction—the reaction between nitrogen and hydrogen to form ammonia:
Now let’s try another, more challenging reaction, such as the displacement reaction between aluminum and hydrochloric acid (remember, a displacement reaction happens when atoms of one element displace the atoms of another element in a molecule). In this reaction, aluminum displaces hydrogen as the bonding partner with chloride:
First, let’s look at the chloride atoms, since this is the largest imbalance between the two sides. To fix this imbalance, we would need to put a coefficient 3 in front of the hydrochloric acid:
Let’s try one more, the combustion of propane in oxygen from the air:
NOTE: The most important step of balancing a chemical equation is simply making sure that your reactants and products are written correctly. If they are not, the equation will never balance and you will spend many fruitless hours struggling. If you are having a lot of trouble getting a reaction to balance, this is usually a good place to start looking for answers!
Fill in the blanks with the proper coefficients.
1) _Al + _O2 = _Al2O3
2) _Na + _H2O = _NaOH + _H2
3) _C2H4 + _O2 = _CO2 +_H2
4) _Si2H6 + _O2 = _SiO2 + _H2O
5) _CH3OH + _O2 = _CO2 + _H2O
ONLY WHEN YOU ARE DONE....go ahead and scroll down!
1) 4, 3, 2
2) 2, 2, 2, 1
3) 1, 2, 2, 2
4) 2, 7, 4, 6
5) 2, 3, 2, 4