1. Chemical Reactions

2. Physical vs. Chemical
What is the difference between a chemical and physical property?

3. Physical Change
Physical change: a change that occurs that does not change the identity of the substance
Melting ice
Freezing Kool-aid
Tearing paper
Boiling water

4. Chemical Changes
Chemical change: a change that occurs causing the identity of the substance to change
Burning paper
Digesting food
Electrolysis of water
A chemical change is called a chemical reaction

5. Is it Physical or Chemical?
Change
Physical
Chemical
Melting cheese
Burning wood
Milk souring
Wadding up paper
Bicycle rusting

6. Chemical Reactions
A chemical reaction produces new substances by changing the way in which atoms are arranged.
Bonds between atoms are broken and new bonds form between different atoms
The new arrangements of atoms form different substances

7. Reactants and Products
Reactants are the substances present at the beginning of a chemical reaction.
Products are the substances formed by a chemical reaction
Reactant + Reactant Product
The mass of the reactants is always equal to the mass of the products

8. Detecting a Chemical Reaction
Color Change
Formation of a Precipitate
Formation of a Gas
Temperature Change
Evolution of Light

9. Color Change Substance often change color during a chemical reaction.
Example: when gray iron rusts, the product that forms is brown.

10. Formation of a Precipitate
Precipitate: A solid product formed when chemicals in two liquids react.
Example: Seashells are formed when sea creatures release a liquid that reacts with seawater.

11. Formation of a Gas
Chemical reactions may produce a gas, like that often formed when antacid pills are mixed with excess stomach acid.

12. Temperature Change Most chemical reactions involve temperature change
Example: Concrete gets warmer as it hardens

13. Evolution of Light
Energy can be released in the form of light during a chemical reaction
Example—Glow Sticks

14. Endothermic and Exothermic Reactions
Exothermic Reactions: If more energy is released when the products form than is needed to break the bonds in the reactants then energy is released during the reaction
Energy is released
Endothermic Reactions: If more energy is required to break the bonds in the reactants than is released when the product is formed then energy must be added to the reaction
Energy is absorbed

15. Chemical Equations

16. Chemical Equation
Shorthand form for writing what reactants are used and what products are formed in a chemical reaction
Sometimes shows whether energy is produced or absorbed
Example:
C6H12O6 + 6O2  6CO2 + 6H2O + energy

17. Components of a Chemical Equation
Formulas
Chemical
Formula 
2H2 + O2
2H2O
Subscript
Coefficient
Coefficient
Subscripts
(Yield)
(Products)
(Reactants)
Sometimes you will see a “yields” sign that looks like this.
What do you think it means?

18. Law of Conservation of Mass
Proposed by Antoine Lavoisier
In a chemical reaction, atoms are neither created nor destroyed
All atoms present in the reactants are also present in the products
Chemical equations must account for/show the conservation of mass  balancing equations
In its present form, does this chemical equation show a
conservation of mass? 2
H2 + O2  2
H2O
Reactants
Products
How would you balance this equation to
show the conservation of mass?
H  2
O  2 4 2
H  2
O  1 4 2

19. Hints For Balancing Equations
Count the atoms
List the number of atoms of each element to see which elements must be balanced
Use a coefficient to add atoms to one side of the equation
Start with the reactant or product that has the greatest number of different elements
Add a coefficient to another reactant or product
Make sure that the coefficients in your balanced equation are the smallest whole numbers possible (they should have no common factor other than one)

20. Tutorial on Balancing Equations
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