Balanced Equations and Reaction Energy
A chemical equation is a shorthand way to write what happens in a reaction, and a balanced equation makes sure it obeys the law of conservation of mass. Balancing equations looks intimidating at first, but it follows a clear rule you can apply every time. This lesson also touches on whether reactions release or absorb energy.
Why Equations Must Balance
In a reaction, atoms are only rearranged — never created or destroyed. So a chemical equation must have the same number of each kind of atom on both sides of the arrow. Consider the formation of water: \[ 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}. \] On the left there are \(4\) hydrogen atoms and \(2\) oxygen atoms; on the right there are also \(4\) hydrogen atoms and \(2\) oxygen atoms. The equation balances because nothing is lost.
The big numbers in front (called coefficients) tell you how many of each molecule you have. You balance an equation by adjusting these coefficients — never by changing the small subscripts inside a formula, which would change the substance itself.
How to Balance, Step by Step
Work through it in order. First, count the atoms of each element on both sides. Then adjust the coefficients so the counts match, one element at a time, saving hydrogen and oxygen for last when possible. Finally, recount to confirm every element balances. For example, to balance \[ \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}, \] you place a \(2\) in front of the water and a \(2\) in front of the hydrogen, giving \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\), which balances.
Reactions and Energy
Reactions also involve energy. An exothermic reaction releases energy, usually as heat — burning fuel warms its surroundings. An endothermic reaction absorbs energy, so it feels cold — some instant cold packs work this way. A quick tell: if a reaction gives off heat or light, it is exothermic; if it takes in heat and cools its surroundings, it is endothermic.
Watch: A Short Video Lesson
Wayne Breslyn (Dr. B.) walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A Routine for Balancing Questions
- Count each kind of atom on both sides of the arrow.
- Adjust coefficients (the big front numbers), not subscripts, to make counts match.
- Recount to confirm every element is balanced.
- Exothermic reactions release energy; endothermic reactions absorb it.
Practice
- Why must a chemical equation be balanced?
- In \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\), how many hydrogen atoms are on each side?
- When balancing, do you change coefficients or subscripts?
- What does an exothermic reaction do with energy?
- A reaction makes its surroundings feel cold. Is it exothermic or endothermic?
- What is the big number in front of a formula called?
Answers
- To obey conservation of mass — the same atoms must appear on both sides.
- Four on each side.
- Coefficients.
- Releases it (as heat or light).
- Endothermic.
- A coefficient.
Where This Fits in Your Science Prep
Balancing equations is conservation of mass in action, building on chemical reactions and the counting skills from units and formulas. See all topics on the Science Topics Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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