1. Writing Chemical Equations
Building Science Champions
Writing Chemical Equations

2. Objectives Describe the information conveyed in a chemical equation.
Apply the principle of conservation of mass to chemical reactions.
Identify and describe three classes of chemical reactions.

3. Key Terms Chemical equation Subscript Reactant Product
Conservation of mass
Coefficient
Synthesis
Decomposition
Replacement

4. Chemical Equations
Chemical Equations are short, easy ways to show a chemical reaction by using elements’ symbols instead of words.
Symbols are easy to understand.

5. Writing Chemical Equations
The formula will show the ratio of elements in a compound.
C12H22O11 is the chemical formula for sugar.
The letter corresponds to a particular element on the periodic table.
The subscript corresponds to the element that precedes the subscript.
If there is no subscript after a letter then there is one (1) of that particular element.
CO2

6. Structure of an Equation
A chemical equation summarizes a chemical reaction.
The material you begin with is the REACTANT.
The completed reaction is the PRODUCT.
Reactant + Reactant Product + Product
= YIELDS

7. Structure of an Equation
When there are two or more reactants they are separated by a plus sign.
When there are two or more products they are separated by a plus sign.
The ratio of products to reactant can change.
Reactant + Reactant Product
Reactant Product + Product
Reactant + Reactant + Reactant Product

8. Conservation of Mass
Regardless of the total number of reactants and products involved, all the atoms at the start of the reaction have to be present at the end of the reaction.
The amount of matter involved in a chemical reaction does not change. The total mass of the reactants must equal the total mass of the product.

9. The LAW of CONSERVATION
The law of conservation states:
“Matter cannot be created nor destroyed during a chemical reaction.”

10. Review Chemical equations are short hand for chemical reactions.
Subscripts go to the element before them.
To the left of the yield signs are reactants and to the right are products.
There can be multiple products and reactants.
Matter cannot be created nor destroyed.

11. Checkpoint
How do masses of the atoms in the reactants of a chemical reaction compare with the atoms in the products?
The number of atoms remain equal and the total of mass is unchanged.

12. Balancing Chemical Equations
A chemical reaction must show the same number of each type of atom on both sides of the equation.
When this happens the equation is BALANCED.

13. Balancing Equation H2 + O2 H2O
Does the left side equal the right side?
2 Hydrogen and 2 Oxygen on the left
2 Hydrogen and 1 Oxygen on the right
Not equal. So what can be done to balance the equation?

14. H2 + O2 H2O Add a coefficient of 2 in front of the H2O
Coefficient is the number placed in front of chemical formulas in an equation.
A coefficient tells how many atoms or molecules there are.
The coefficient is factored out until the next sign.
H2 + O H2O
Left side – 2 Hydrogen and 2 Oxygen
Right side – 4 hydrogen and 2 Oxygen
How can we balance them out?

15. H2 + O2 2 H2O Place a 2 in front of the H2 in the reactant.
Left side – 4 Hydrogen and 2 Oxygen
Right side – 4 Hydrogen and 2 Oxygen
We are balanced!!!!!!!!!!!!!!!!!

16. Mg + O2 MgO Left side – 1 Magnesium and 2 Oxygen
Right Side – 1 Magnesium and 1 Oxygen
Is the equation balanced?
Now what?
Add 2 in front of Magnesium in the reactant and Magnesium in the product.
2Mg + O MgO
Are we balanced?

17. You Try C + Cl2 CCl4 C + 2 Cl2 CCl4 Al2O3 Al + O2 2 Al2O3 4 Al + 3 O2
***You can only change coefficients you CANNOT change subscripts. ***

18. Classifying Chemical Reactions
There are 3 major classifications of chemical reactions:
Synthesis
Decomposition
Replacement

19. Synthesis
When two or more substances combine to make a more complex substance the process is called synthesis.
Put things together.
Simple + Simple Complex
More reactants than products
2 SO2 + O2 + 2 H2O 2 H2SO4
Sulfur dioxide + Oxygen + water yields sulfuric acid

20. Decomposition
Decomposition breaks down reactants into smaller products.
Complex Simple + Simple
More products than reactants
2 H2O2 2 H2O + O2
Hydrogen peroxide yields water and oxygen gas

21. Replacement
When one element replaces another in a compound, or when 2 elements in different compound trade places, this is replacement.
Reactant + Reactant Product + Product
CaCO3 + 2 HCl CaCl2 + H2O3
2 CuO + C 2 Cu + CO2

22. Review Chemical equations show chemical reactions
Matter cannot be created nor destroyed
Three types of chemical reactions
Synthesis
Decomposition
Replacement

23. References
Anderson, M. et all (2012) Physical Science. McGraw-Hill: Columbus
Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey