About this experiment
Atoms are not created or destroyed in a chemical reaction, they are only rearranged. Pick a reaction, change the coefficients in front of each formula and watch the molecule models and the atom tally update. The equation is balanced when every element has the same number of atoms on both sides. Ten reactions run from making water to burning propane, so the class can start easy and build up.
Learning goals
- Explain why a chemical equation must have the same number of each kind of atom on both sides
- Tell the difference between a coefficient and a subscript in a chemical formula
- Balance equations by adjusting coefficients and reduce them to the smallest whole numbers
- Link a balanced equation to the conservation of mass
How to use it
- 1Start with reaction 1. Count the hydrogen and oxygen atoms on each side in the atom tally.
- 2Change one coefficient at a time with the plus and minus buttons and watch which rows turn green.
- 3When every row is green, check that the coefficients cannot all be divided by the same number.
- 4Compare the reactant and product masses, then move on to the next reaction.
The model
Assumptions
- Each coefficient is a whole number from 1 to 9. A coefficient of 1 is not written in the equation.
- The molecule models show which atoms each formula contains. Ionic compounds and metals are drawn as single formula units, not as their real lattices.
- Masses are calculated as if each coefficient were that many moles, using standard relative atomic masses.
Try this in class
- Q1Why can you not balance H₂ + O₂ → H₂O by changing it to H₂ + O₂ → H₂O₂?
- Q2In burning methane, which element is easiest to balance last, and why?
- Q3Iron reacting with oxygen needs the coefficients 4, 3 and 2. Why does trying 2, 1.5 and 1 first help?
- Q4Set the water reaction to 4, 2 and 4. Is it balanced? Why do chemists still prefer 2, 1 and 2?
Frequently asked questions
Can I change the small numbers in a formula?
No. Subscripts tell you what the substance is. H₂O is water but H₂O₂ is hydrogen peroxide. Only the coefficients in front can change.
Is there a quick strategy for balancing?
Balance elements that appear in only one substance on each side first, leave hydrogen and oxygen until near the end, and balance any element that appears on its own, such as O₂, last.
Why do the masses match only when the equation is balanced?
Mass comes from the atoms. If every atom on the left appears on the right, the total mass must be the same. This is the law of conservation of mass.