1. Chemical Equations
In this lesson, you will go from chemical formulas to chemical equations, a required step in stoichiometry.
If you have trouble writing chemical formulas, please review that concept before continuing here.

2. Chemical Equations
What happens to matter when it undergoes chemical changes?
The law of conservation of mass says that atoms are neither created nor destroyed, during any chemical reaction.
Thus, the same collection of atoms is present after a reaction as before the reaction. The changes that occur during a reaction just involve the rearrangement of atoms.
A chemical equation shows how the atoms are rearranged.

3. Chemical Equations 2H2 + O2  2H2O
Chemical reactions are represented on paper by chemical equations. For example, hydrogen gas (H2) can react (burn) with oxygen gas (O2) to form water (H20). The chemical equation for this reaction is written as:
2H2 + O2  2H2O
The '+' is read as 'reacts with' and the arrow '' means 'produces'. The chemical formulas on the left represent the starting substances, called reactants. The substances produced by the reaction are shown on the right, and are called products. The numbers in front of the formulas are called coefficients (the number '1' is usually omitted).

4. Chemical Equations Below are the parts of a chemical equation.
The reactants are listed on the left side and show what went into the reaction.
The products are on the right side and show what was produced by the reaction.
The arrow is like an = sign, but shows the direction of the reaction.
The coefficients show the ratio in which the substances reacted/were produced. When no coefficient is present, a 1 is implied.

5. Chemical Equations
Because atoms are neither created nor destroyed in a reaction, a chemical equation must have an equal number of atoms of each element on each side of the arrow (i.e. the equation is said to be 'balanced').

6. Chemical Equations
Steps involved in writing a 'balanced' equation for a chemical reaction:
1. Write the chemical formula for each of the named substances. Don’t forget that the diatomic elements always have a subscript of two if not combined with another element.
2. Write the skeleton equation using formulas of reactants and products. Don’t forget the arrow between reactants and products.
3. Balance the equation by determining coefficients that provide equal numbers of each type of atom on each side of the equation (generally, whole number values).
Note! Subscripts should never be changed when trying to balance a chemical equation. Changing a subscript changes the actual identity of a product or reactant. Balancing a chemical equation only involves changing the relative amounts of each product or reactant using coefficients.

7. Chemical Equations CH4 + O2  H2O + CO2
Consider the reaction of burning the gas methane (CH4) in air. We know experimentally that this reaction consumes oxygen (O2) and produces water (H2O) and carbon dioxide (CO2). Thus, we have accomplished step #1 above. We now write the unbalanced chemical equation (step #2):
CH4 + O2  H2O + CO2
Let’s see if the two sides of the equation are balanced:
Carbon – 1  Carbon – 1
Hydrogen – 4  hydrogen – 2
Oxygen – 2  oxygen – 3
The equation is not balanced! What can we do? We can add coefficients to balance it.

8. Chemical Equations CH4 + O2  2H2O + CO2
By adding the coefficient 2 to the water, we double the number of hydrogens and the number of oxygens in that substance. We now have 4 hydrogens on each side of the equation. That’s one problem solved.
The oxygens, however, are still unbalanced. Of I look at everything on the products side, I have 4 oxygens. The reactant side only has two oxygens. How can I fix this imbalance?
CH4 + 2O2  2H2O + CO2
A coefficient of 2 on the oxygen gas fixed it!

9. Chemical Equations CH4 + 2O2  2H2O + CO2
We now have fulfilled step #3, we have a balanced chemical equation for the reaction of methane with oxygen. Thus, one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.
CH4 + 2O2  2H2O + CO2
The physical state of each chemical can be indicated by using the symbols (g), (l), and (s) (for gas, liquid and solid, respectively):
CH4(g) + 2O2(g)  2H2O(l) + CO2 (g)

10. Chemical Equations Try this:
Ammonium chloride reacts with sodium carbonate to produce sodium chloride and ammonium carbonate.
1. Start by figuring out the formula for each substance.
Ammonium chloride is
Sodium carbonate is
Sodium chloride is
Ammonium carbonate is

11. Chemical Equations NH4Cl + Na2CO3  NaCl + (NH4)2CO3
Because the carbonate had a charge of -2, the only way to combine sodium and carbonate into a neutral compound is to use two sodiums for each carbonate.
Note how the ammonium carbonate required parentheses, as two ammoniums were needed to balance the charge of the carbonate. Subscripts are used within a chemical formula to show the number of atoms in a single unit.
Now see of you can balance the equation with coefficients.

12. Chemical Equations NH4Cl + Na2CO3  NaCl + (NH4)2CO3
The ammonium’s are not balanced! What shall we do?
2NH4Cl + Na2CO3  NaCl + (NH4)2CO3
The ammoniums are fixed, but that changed the chlorines.
We now have 2 chlorines on the product side and one on the reactant side.
What coefficient do we add next?
2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
That fixed the sodiums AND the chlorines.
Anything else?
Check the carbonates, as that’s all we have left.

13. Chemical Equations 2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
The carbonates are balanced. The equation is all balanced by just adding those two coefficients, hence:
2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
This equation tells us that two formula units of ammonium chloride react with one formula unit of sodium carbonate to produce two formula units of sodium chloride and one unit of ammonium carbonate.
Those same ratios apply to moles of the substances.
Two moles of ammonium chloride react with one mole of sodium carbonate to produce two moles of sodium chloride and one mole of ammonium carbonate.

14. Chemical Equations 2NH4Cl + Na2CO3  2NaCl + (NH4)2CO3
Those coefficients give us the ratios of all substances involved:
There are two ammonium chlorides for each sodium carbonate. The ratio of ammonium chloride to sodium carbonate is 2:1.
The ratio of ammonium chlorides to sodium chlorides is 1:1. Why?
The ratios are always reduced to their lowest form.
2:2 reduces to 1:1.