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# Balancing chemical equations

This lesson is by Lynda Jones. It was originally on a website, balancingequations.info, but that website no longer exists.

### Example: 3 red, 2 yellow and 5 blue atoms go into the reaction. ## Coefficients and Subscripts:

### small numbers in blue are called “subscripts,” because they are written below the line. (“Script” for “writing” and “sub” for “below.”) ## Color Coding to make it simple

### For now we’ll use colored circles as our “atoms” and the first letters of their color names as our “chemical symbols.” For example: ## Subscripts Tell How Many Atoms of Each Kind

### The subscript tells us how many atoms of each kind exist in any formula. The subscript is ALWAYS written AFTER the symbol of the atom to which it refers. ## Combining Two Different “Color” Atoms

### What happens when we combine two or more different atoms together? How do we write the formulas then? We’ll write the formula for a molecule made of one atom of blue and one atom of white. ### Now we will write the formula for 1 atom of blue and 2 atoms of white. ### Now we will combine 1 atom of blue and 3 atoms of white. ### Here is one more set of examples showing how the subscript gives us information about the number of each kind of atom in a formula. ## Coefficients are Multipliers

### A coefficient is a multiplier.  Here are some examples. ### (Remember, when there is only one atom of a kind in the formula there is no subscript written, so we use “1” as the subscript multiplier.) ## Don’t make this mistake: ## Balancing Equations

Example 1:

### The “reactants side” of the equation is anything written BEFORE the arrow. The “products side” of the equation is anything written AFTER the arrow. …. ### We have to add at least one red atom to the right side, but in order to do that, we have to add one entire group, so let’s do that and see what we get. ### Adding the group balances our red atoms, giving us 2 red atoms on each side. However, now the whites are unbalanced. We have 2 white atoms on the left side of the arrow but 4 on the right side. What should we do? Of course, “add a group.” ### Remember, you cannot represent “2 W2” as “W4.” ## So the balanced equation is: ## Example 2

nnn ### In order to get one more atom of blue on the right side, what do we need to add? Yes, we need to add an entire group. So let’s do that. ### And how do we get 6 atoms of white on the left hand side? Yes, by adding groups. What is the total number of groups we need on the left to balance the 6 whites on the right? So now we have another correctly balanced equation. ## Example 3 ### With this equation, first let’s look at the yellow atoms. There is 1 yellow on the left, but 2 yellows on the right. What do we do? ### OK. This balances our yellows, but our reds are still unbalanced. What do we do next?  ### So what groups should we add? If we add one group of R2, as shown below, this still doesn’t balance the reds. ### What is the least common multiple of 2 and 3? [Answer: 6] How do we get 6 reds on both sides? ### But now the yellows are unbalanced again. What do we do next? Remember, 4 Y does NOT equal Y4. ### We change the coefficient in front of “Y” to 4. ### So the final balanced equation is: ### One last word: When balancing equations, you ALWAYS want the lowest possible numbers. For example, the above equation may also be written as Review:

### 4. P.S. You ALWAYS want the lowest possible numbers.

Go to this game: Balance all 10 equations. Write the balanced equation.

Balancing act: Jefferson Lab

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Chembalancer from FunBasedLearning

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Online alternate texbook Chap 4: Chemical Rxns and Balancing equations

## Learning Standards

Massachusetts Science Tech Curriculum Frameworks

HS-PS1-7. Use mathematical representations and provide experimental evidence to support the claim that atoms, and therefore mass, are conserved during a chemical reaction. Use the mole concept and proportional relationships to evaluate the quantities (masses or moles) of specific reactants needed in order to obtain a specific amount of product.

College Board Standards For Success in Science

C-PE.1.4.5 Given a simple chemical reaction (e.g., synthesis of water, decomposition of hydrogen peroxide, combustion of methane)

C-PE.1.4.5a Write a balanced chemical reaction.

C-PE.2.3.1 Construct a balanced symbolic representation, based on given reactants and products, of a chemical reaction. Construct a molecular-level representation of the chemical reaction, and explain, using the concept of atoms, why matter is conserved during any change.