1. Which of the following is not a balanced chemical equation?
QUESTION:
Which of the following is not a balanced chemical equation?
A. 2 H2 + O2  2 H2O
B. 4 H2 + 2 O2  4 H2O C. Both A and B D. None of the above
In a chemical equation, number of atoms must be the same on both sides of the arrow, for each element.
To count number of atoms: coefficient x subscript
Choice A. Left side: H: 2 x 2 = 4, O: 1 x 2 = Right side: H: 2 x 2 = 4, O: 2 x 1 = 2
Choice B is also balanced.
It is customary, but not required, to use smallest set of whole numbers.
Which of the following is not a balanced chemical equation?
A. 2 H2 + O2  2 H2O
B. 4 H2 + 2 O2  4 H2O C. Both A and B D. None of the above PAUSE PAUSE
CLICK
In a chemical equation, the number of atoms must be the same on both sides of the arrow, for each element.
CLICK To count number of atoms, we simply multiply the coefficient by the subscript.
Let’s take a look at the choice A.
On the left-hand side. The coefficient of H2 is 2.
CALLOUT coefficient
The subscript of H in H2 is 2.
So the number of hydrogen atoms shown on the left is 2 x 2, or 4.
HIGHLIGHT 2x2 = 4
There is no coefficient written for O2. This means that the coefficient of O2 is 1. A coefficient that is not written is implied to be 1.
CALLOUT “implied 1”
The subscript of oxygen in O2 is 2.
HIGHLIGHT subscript of O2
Therefore, the number of oxygen atoms on the left-hand side is 1 x 2, or 2.
HIGHLIGHT 1x2 = 2
Now let’s take a look at the right-hand side.
The coefficient of H2O is 2.
HIGHLIGHT coefficient in chemical equation and in “H: 2 x 2 = 4”
The subscript of hydrogen in H2O is 2.
HIGHLIGHT subscript of H in H2O
Therefore, the number of hydrogen atoms on the right is 2 x 2, or 4.
The subscript of oxygen in H2O is implied to be 1.
CALLOUT
We multiply this by the coefficient of H2O to get the number of oxygen atoms on the right.
The number of oxygen atoms on the right is 2x1, or 2.
HIGHLIGHT 2x1=2
We can see that the number of hydrogen atoms is the same on both sides of the arrow.
And the number of oxygen atoms is also the same on both sides of the arrow.
Therefore, what we have in choice A is a balanced chemical equation.
You should be able to verify that we also have a balanced chemical equation in choice B.
Therefore, the correct answer is C.
One thing you should realize from this question is that there is an infinite number of ways of
balancing a chemical equation. Coefficients in a balanced equation are just relative numbers.
It is, however, customary to use the smallest set of whole number coefficients.
But it is not required.
CLICK PAUSE END RECORDING

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08