1. Chapter 21 Chemical Reactions
Section 1: Chemical Changes
Section 2: Chemical Equations

2. Warm Up What are the substances called before a chemical reaction?
What are new the substances called after a chemical reaction?

3. Chemical Reactions
A chemical reaction is a change in which one or more substances are converted into new substances
Review: a substance is an element or a compound

4. Chemical Reactions
The reactants are the substances that exist before the reaction
The products are the substances that exist after the reaction

5. Conservation of Mass Conservation of Mass:
The total mass of the products must always equal the mass of the reactants

6. Conservation of Mass Lavoisier’s experiment: 10.0g = 0.7g + 9.3g
Mercury (II) oxide = oxygen + mercury
10.0g = 0.7g g

7. Chemical Reactions
Ex: Ca O2 → CaO
A chemical equation describes the chemical reaction using formulas and symbols (and usually coefficients)

8. Dissolved in water: (aq)
Chemical Equations
Reactants on the Left
→ stands for “yields”
Products on the Right
Solids: (s)
Liquids: (l)
Dissolved in water: (aq)
Gases: (g)
2 Mg (s) + O2 (g) → 2 MgO (s)

9. Chemical Reactions Al + O2 ® Al2O3
Coefficients represent the number of units of each substance that take part in the reaction
      Al +       O2 ®       Al2O3

10. Chemical Equations 4 Al + 3 O2 ® 2 Al2O3
Because mass must be conserved, coefficients are used to balance an equation
The same number of atoms of each element must be on both sides of the equation
4 Al + 3 O2 ® 2 Al2O3

11. Chemical Equations
To write a balanced chemical equation for a reaction:
Write the equation using formulas and symbols
Count the atoms in the reactants and products
Choose coefficients that balance the equation (don’t change subscripts!)
Recheck the numbers. If they aren’t the same, do it again!

12. Examples
Cu + O2 → Cu2O
Al + O2 → Al2O3
N2 + H2 → NH3
Mg + P4 → Mg3P2