1. Chemical Reactions

2. Properties and Changes in Matter
- Observing Chemical Change
Properties and Changes in Matter
Matter can undergo both physical change and chemical change.

3. Properties and Changes of Matter
- Observing Chemical Change
Asking Questions
Properties and Changes of Matter
Question
Answer
What are physical properties of matter?
Physical properties are characteristics that can be observed without changing one substance into another.
What is the evidence for chemical reactions?
The evidence for chemical reactions is the formation of new substances and changes in energy.

4. Chemical Reactions Rearrange atoms Involve changes in energy
Exothermic: release energy (heat, fire, fizz, etc)
Endothermic: absorb energy (drop in temp.)

5. Evidence of Chemical Changes
Clue
Example
Description
Color change
Bread dough baking
Changes
from
white to
brown
Smell
Eggs rotting
Smells
bad
New matter created
Wood burning
Wood
changes to
ash
Gas created
Baking soda added to vinegar
Creates
carbon
dioxide
gas
Heat or Light created
Fireworks
Burning
chemicals
create light
and heat
Cut along dotted lines and paste into your science notebook.

6. Properties and Changes in Matter
- Observing Chemical Change
Properties and Changes in Matter
Chemical changes occur when bonds break and new bonds form.

7. Describing Chemical Reactions
Cellular phone messages make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

8. What Are Chemical Equations?
- Describing Chemical Reactions
What Are Chemical Equations?
Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction.
The arrow symbol is read YIELDS which means “results in” or “produces”

9. Chemical Equations ___ CH4 + ___ O2 ___CO2 + ___ H2O
methane and oxygen yield carbon dioxide and water
___ CH4 + ___ O ___CO2 + ___ H2O
Reactants
Products
The molecules that combine are called REACTANTS and the molecules that result are called PRODUCTS

10. - Describing Chemical Reactions
Chemical Formulas
The formula of a compound identifies the elements in the compound and the ratios in which their atoms are present.

11. - Describing Chemical Reactions
Conservation of Mass
The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products.

12. Balancing Chemical Equations
- Describing Chemical Reactions
Balancing Chemical Equations
To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

13. Balancing Chemical Equations
- Describing Chemical Reactions
Balancing Chemical Equations
Magnesium metal (Mg) reacts with oxygen gas (O2), forming magnesium oxide (MgO). To write a balanced equation for this reaction, first write the equation using the formulas of the reactants and products, then count the number of atoms of each element.

14. Balancing Chemical Equations
- Describing Chemical Reactions
Balancing Chemical Equations
Balancing Chemical Equations:
Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

15. Balancing Chemical Equations
- Describing Chemical Reactions
Balancing Chemical Equations
Balancing Chemical Equations:
Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

16. Classifying Chemical Reactions
- Describing Chemical Reactions
Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.

17. A + B → AB AB → A + B AB + C → AC + B AB + CD → AC + BD
Chemical Reactions
Type of Reaction
Definition
 Equation
Synthesis
Decomposition
Single Replacement
Double Replacement
Two or more elements or compounds combine to make a more complex substance
A + B → AB
AB → A + B
Compounds break down
into simpler substances
Occurs when one element replaces another one in a compound
AB + C → AC + B
Occurs when different atoms in two different compounds trade places
AB + CD → AC + BD
A = Red B = Blue C = Green D = Yellow

18. Identifying Chemical Reactions
Use the code below to classify each reaction.
S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement
____ P O2 → P4O10 ____ Mg O2 → MgO
____ HgO → Hg O2 ____ Al2O3 → Al O2
____ Cl NaBr → NaCl Br2 ____ CuCl H2S → CuS HCl

19. Building Vocabulary - Describing Chemical Reactions Key Terms:
Examples:
closed system
coefficient
synthesis
decomposition
Key Terms:
Key Terms:
Examples:
replacement
Examples:
chemical equation
A chemical equation is a short, easy way to show a chemical reaction.
In a closed system, matter is not allowed to enter or leave.
Replacement is the process in which one element replaces another in a compound or two elements in different compounds trade places.
reactant
A coefficient is a number in a chemical equation telling you how many atoms or molecules of a reactant or product take part in the reaction.
A substance you have at the beginning of a reaction is a reactant.
product
A new substance produced in a reaction is a product.
conservation of mass
Conservation of mass means that during a chemical reaction, matter is not created or destroyed.
Synthesis means combining two or more elements or compounds to make a more complex substance.
open system
Decomposition means breaking down compounds into simpler products.
In an open system, matter can enter from or escape to the surroundings.

20. End of Section: Describing Chemical Reactions