Chemical Reactions

• Matter is anything that has MASS and takes up SPACE

What’s the MATTER?

• Changes in the physical properties of a substance: including changes in size, shape, or its state of matter (whether it’s solid, liquid, or gas).

• How are these pictures physical changes??

Physical Changes

• Examples:• Tearing paper

• Freezing water

• Water boiling and becoming water vapor

• Cutting your hair

• Cutting food

• Flipping a light switch

• Mixing Kool-Aid and water

Physical Changes

•Can you name some chemical changes?

Chemical Changes

• Changes that produce new substances with chemical

properties different from the original substances.

Chemical Changes

• Examples:• Process of rusting

• Burning paper

• Silver tarnishing

• Light bulb coming on

• Milk spoiling

• Baking soda and vinegar reacting

Chemical Reactions• A process that produces a chemical

change is a chemical reaction. • Signs that a chemical reaction has

occurred include: Color change Bubbles released Heat released Light given off Temperature change New substance formed Substances CANNOT be changed

back to original substances by physical means

NEXT 4 SLIDES are for Pre-AP

CLASSES

**Bonus for General Ed

Classes

• Exothermic reactions RELEASE heat energy resulting in the raising of temperature.

Energy in Chemical Reactions

• Endothermic reactions ABSORB heat energy resulting in the lowering of temperature

• The extra energy is released in various forms—light, sound, and heat.

• Example: burning is an exothermic chemical reaction in which a substance combines with oxygen to produce heat along with light, carbon dioxide, and water.

Exothermic Reactions

• Example: instant cold pack• The heavy plastic cold pack holds ammonium nitrate and water.• The two substances are separated by a plastic divider.

Endothermic Reactions

• When you squeeze the bag, you break the divider so that the ammonium nitrate dissolves in the water.

• The reaction requires energy which is absorbed, used in the reaction, and thus the temperature DROPS and the pack gets cold!

• The dissolving process absorbs heat energy.

Endothermic OR Exothermic?

• Chemical formulas are combinations of chemical symbols and numbers that indicates how many atoms of each element are in a compound

• Subscripts, located to the right of a symbol, tell you the number of atoms of that element only (O3)

• If an atom has no subscript, it means that only one atom of that element is in the compound.

NaHCO3 (baking soda)

Chemical Formulas

• To describe a chemical reaction, you must know which substances react and which substances are formed in the reaction.

• A chemical equation tells chemists at a glance the reactants, products, and the amounts of each substance present in the reaction.

Chemical Equations

Reactants & Products• Reactants are the

substances that exist before the reaction begins (LEFT side of arrow)

• Products are the substances that form as a result of the reaction (RIGHT side of arrow)

• The arrow in the equations is known as the yield arrow and it means: “produces”

The Law of Conservation of Mass

• States that mass/matter is neither created nor destroyed, but only changes from one form to another.

The Law of Conservation of Mass

• Chemical equations are much like math equations: the left and right sides of the equation are numerically EQUAL

• The number and type of atoms must be the same on each side of the equation (the mass of the reactants must be the same as the mass of the products)

• This means the equation is BALANCED and the law of conservation of mass/matter is observed.

Practice #1

• Reactants:– # of atoms of Na =– # of atoms of Cl =

Does this example follow the law of conservation of mass?

• Products:– # of atoms of Na =– # of atoms of Cl =

Na + Cl NaCl

YES!

11

11

Practice #2

• Reactants:– # of atoms of H =– # of atoms of O =

Does this example follow the law of conservation of mass?

• Products:– # of atoms of H =– # of atoms of O =

H2 + O2 H2O

NO!

22

21

Adjusting Coefficients• When balancing chemical

equations, the numbers placed BEFORE the formulas are coefficients.

• They are the ONLY numbers changed when balancing equations (unlike when building compounds)

• Example: 2H2O • The coefficient numbers are

distributed across entire compound

H = 4

O = 2

1. 2KF

Practice with Coefficients

2. 4CO2 C = 4

O = 8

3. 5NaCl

Na = 5

Cl = 5

4. 3AlCl3

Al = 3

Cl = 9

K = 2

F = 2

5. 6CaCO2

Ca = 6

C = 6

O = 12

Practice #2, Continued…

• Reactants:– # of atoms of H =– # of atoms of O =

• Products:– # of atoms of H =– # of atoms of O =

H2 + O2 H2O

22

21

22

42

4

Is it now BALANCED and follow the law of conservation of mass?YE

S!

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