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### Course: Class 10 Chemistry (India)>Unit 1

Lesson 2: Balancing a chemical reaction

# Balancing chemical equations

A balanced chemical equation shows the same number of each type of atom on both sides of the arrow.

## Want to join the conversation?

• I'm working on Chemical Reactions: Double and Single Replacement on FLVS. Now my first question for this video is, how do you have 4 aluminum atoms when it says 2Al subscript 2? Do you just add the 2 and the subscript 2? Same thing goes with the O3. I am so lost.

My next question is...how would I write the charges out in this equation?
Would it be like this?
Al + O + O ----> Al + Al + O + O + O

I don't know if I'm doing it right or not. I am completely lost. If someone can help me, I would really appreciate it. Thanks :)
• Al2 is written with a subscript because it is bonded to O3. The leading 2 shows that there are two Al2O3's- giving four aluminum and six oxygen. This is found by multiplying the leading number by the subscript.
• How does Al and O get 2 and 3 atoms?
• Aluminium has a charge of +3 and Oxygen has a charge of -2. So by using the criss-cross method we get Al2O3. Hope you understand. Thank you
• So no matter how many times I watch this video and how many times my Chemistry teacher explains it I cat seem to understand how to balance equations. If I have
N2+ H2---> NH3 what would I get and how would would I get that answer? I have a work sheet due on this this Wednesday so I need help fast.
• Write it out on a piece of paper and underneath each side of the equation write out how many of each atom there are.

N2 + H2 -> NH3

On the left there is 2 N and 2 H
On the right there is 1 N and 3 H

If we tried to balance starting with H you'd need to use a fraction or decimal and would get messy, so let's start with N.
There's 2 on the left and 1 on the right, so we need to change the coefficient of NH3 to 2

Now we have
N2 + H2 -> 2NH3

Total the atoms up again:
On the left there is 2 N and 2 H still
On the right there is 2 N and 6 H now

So now all we need to do is make the left side have 6 H in total. So all we need to do is make the coefficient of H2 3

N2 + 3H2 -> 2NH3
• is not 03 a molecule in the atmosphere?
• Yes, ozone is found in the atmosphere at an overall concentration of 0.6 ppm.
But most of it is found in the upper atmosphere.
Ozone is highly unstable. It rapidly decomposes to oxygen in the lower atmosphere.
In the upper atmosphere (in the ozone layer), ultraviolet rays from the sun are constantly converting oxygen molecules into ozone, and the concentration of ozone there ranges from 2 to 8 ppm.
• At , since the equation reads 2Alsub2 Osub3, does the coefficient from 2Alsub2 apply to Osub3, or is it just assumed that we know that oxygen is doubled or something?
• The coefficient 2 in 2Al₂O₃means 2×(Al₂O₃).
Everything inside the parentheses gets doubled, so there are 4 Al atoms and 6 O atoms.
• do you have to multiply each side by the same number?
Sorry I'm just a bit confused.
And also do you have to multiply each molecule?
• We will have to multiply each sides by the same number if one of the molecules has a fractional value after balancing(1.5 for oxygen becomes 3 by multiplying every molecule on both sides by 2). However, when the reaction is not balanced(when both sides do not have equal number of atoms of every element) we multiply the value of the molecule/atom by a suitable number on the side where it has fewer atoms. We do this until the reaction is perfectly balanced and we are sure that the Law of Conservation of Mass is followed. I hope this answers your question.
• I have a question, for the Al oxide how did he get one?
• I’ll assume you’re referring to . The top equation is balanced. If an equation is balanced and if a chemical doesn’t have a coefficient, then we assume it has a coefficient of 1. It’s implicit as Sal mentioned. Similar to how variables are assumed to have coefficients of 1 if they do not have explicit coefficients (e.i. x = 1x).

Hope that helps.
• I was trying to do this, but I failed, is there an easier way to try this?
• well...not really...
Just practice
• is the equation technically balanced already at ? say we don't care about molecules being half molecules. Just wanna make sure i understood
• Yes, the equation is technically balanced at .