CHEMICAL REACTIONS7th Grade Science

Bowling Green Junior High

WHAT ARE CHEMICAL REACTIONS?

Chemical Reaction – a change that takes place when two or more substances (reactants) interact to form new substances (products) with new properties.

COMPOUNDS Matter made of two or more different elements chemically bonded. Cannot be separated by physical means

Has properties that are different from the elements that make it up.

MORE COMMON THAN ELEMENTS DUE TO MANY ELEMENTS BEING REACTIVE WITH EACH OTHER

THE ELEMENTS THAT COMBINE MAKE A NEW SUBSTANCE WITH NEW PHYSICAL PROPERTIES

FOR A COMPOUND TO FORM OR BE BROKEN DOWN, A CHEMICAL REACTION MUST TAKE PLACE

+ =

NaCl TABLE SALT

EVERYDAY EXAMPLES OF CHEMICAL REACTIONS

Respiration (breathing) Photosynthesis Grilling food Starting a vehicle Digestion Rusting metal

HOW DO YOU KNOW WHEN A CHEMICAL REACTION HAS TAKEN PLACE?

A new substance with new properties is formed

SIGNS OF A CHEMICAL REACTION Temperature Change (heat given off or

required) FIZZES OR BUBBLES COLOR CHANGE ODOR LIGHT GIVEN OFF NEW SUBSTANCE FORMED

Precipitate (solid) Precipitate (gas bubbles)

TWO PARTS OF A CHEMICAL REACTIONReactants – Substances that start a chemical reaction (EX: chemicals on match head)Products – Substances produced in the reaction (EX: black material on match)

CHEMICAL EQUATIONS• Chemical equations are symbols used to describe

the details of a chemical reaction. • Shows how the reactants changed into the

product. • This involves indicating all the atoms involved in

the reaction.Fe + O2 FeO2

Reactants:Iron and oxygen

Product:Ferrous oxide(rust)Plus Sign:

Shows substances combine

Arrow:Means “yields”takes the place

of an = sign

Reactants are ALWAYS to the left of the arrow

Products are ALWAYS to the right of the arrow

TYPES OF CHEMICAL REACTIONS

Combustion Synthesis Decomposition Single replacement Double replacement Neutralization Oxidation/Reduction Hydrolysis Endothermic/Exothermic

WHAT DO YOU HAVE TO HAVE TO BURN SOMETHING?

COMBUSTION REACTIONS When oxygen (O2) combines with another

compound to form water and carbon dioxide. Needs a fuel source Takes place at high temperatures Fast process that results in an increase of

temperature and production of fire.

Chemical reactions can be classified

Combustion Reaction – always involves oxygen (O2) as a reactant.

OC

CH4

O

O O

+

+

2O2 CO2+ 2H2OMethane Oxygen Carbon Dioxide Water

+H

H H

H

4 TYPES OF REACTIONS

SYNTHESIS REACTIONS

Two or more substances react to form a new substance(s)

A + B AB

S + O2 SO2

CHEMICAL REACTIONS CAN BE CLASSIFIEDSynthesis Reaction – combines two or more

simpler reactants to form new, more complex products.

N NO O

O O

N2 + 2O2 2NO2

+

Nitrogen Oxygen Nitrogen Dioxide

Simple to complex

DECOMPOSITION REACTION

One substance breaks down into two or more simpler substances

AB A + B

CaCO3 CaO + CO2

Chemical reactions can be classifiedDecomposition reaction – breaks a reactant

into two or more simpler products

2H2OWater

2H2 + O2Hydrogen Oxygen

OHH

OHH

+

Complex to simple

SINGLE REPLACEMENT

One element replaces another element in a compound

AB + C AC + B

Zn + 2HCl H2 + ZnCl2

Chemical reactions can be classified

Replacement Reaction – elements switch places to form new compounds.

1) Single Replacement

ZnZinc

H2+ZnCl2HydrogenZinc Chloride

2HClHydrochloric Acid

+

HCl+

HCl

HCl

HCl

Zn

+

DOUBLE REPLACEMENT

Elements from two different compounds switch places

AB + CD AC + BD

HCl + NaOH NaCl + H2O

Cl

Chemical reactions can be classified

Replacement Reaction – elements switch places to form new compounds.

Double Replacement

Cl

FeSIron Sulphide

H2S+FeCl2Hydrogen SulfideIron Chloride

2HClHydrochloric Acid

+

Fe

S

+ +H

H

All chemical reactions are going to release (give off) energy or absorb (take in) energy. Some will require energy to start the reaction

(activation energy) EX: before you use a new cell phone, what’s got to

happen?

Activation energy=energy required to start a chemical reaction.

ENDOTHERMIC VS. EXOTHERMIC PROCESSES

EXOTHERMIC REACTIONS OR PROCESSES

Exothermic reactions are exactly the opposite. While they take some energy to get going, called the activation energy of reaction, these reactions give off heat during the reaction Good examples of exothermic reactions are

explosions like fireworks or combustion in engines. Forming a chemical bond releases energy and is

exothermic

Usually feel hot because it is giving heat to you

ENDOTHERMIC REACTIONS OR PROCESSES

Endothermic reactions are those which absorb heat during the reaction. They take in more energy than they give off, which leaves the surroundings cooler than the starting point  Evaporation of water by sunlight is a great example. The sun

and the liquid water combine and the water absorbs energy and eventually becomes as gas.

Breaking a chemical bond requires energy and is endothermic

Usually feel cold because it is taking heat away from you

CATALYST Substance which speeds up a chemical

reaction but is chemically unchanged at the end of the reaction.

•The catalytic converter in a car contains platinum, which serves as a catalyst to change carbon monoxide, which is toxic, into carbon dioxide.•If you light a match in a room with hydrogen gas and oxygen gas, there will be an explosion and most of the hydrogen and oxygen will combine to create water molecules.

A way of writing which type of atoms and how many of each there are in a compound.

CHEMICAL FORMULAS

Written as: C4H10

Butane

Written as: CH4

Methane

Subscripts= how many atoms

= how many total molecules

COUNTING ATOMS

FeO2

H2O

CO2

MgBr2

C6H12O6

3OH 2H2O

COUNTING ATOMS IN CHEMICAL EQUATIONS

2Na + MgF2 2NaF + Mg

COUNTING ATOMS IN CHEMICAL EQUATIONS

2K + Cl2 2KCl

COUNTING ATOMS IN CHEMICAL EQUATIONS

2Na2O 4Na + O2

Law of Conservation of Matter Matter cannot be created or

destroyed, it just changes forms.*The total mass of the reactants

MUST EQUAL the total mass of the product.

Law of Conservation of Mass

http://www.sky-web.net/science/balancing_chemical_equations_examples.htm

Alka-Seltzer and Water

BALANCING EQUATIONS• The number of atoms of the

reactants must equal the number of atoms in the product. (Law of Conservation of Matter)

Ex: 2Na + Cl2 -> 2NaCl

4P + 5O2 -> P4O10

BALANCING EQUATIONS• Rules

– Make sure that all atoms are equal on both sides.

– You can only add coefficients.• Changing the subscripts will change the

identity of the compound. – H2O & H2O2

EX: 2Na + Cl2 -> 2NaCl

H2 + O2 -> 2H2O

(Not balanced… So…)

2H2 + O2 -> 2H2O

Balancing Chemical Equations

Hg + O2 HgO

H2 + Cl HCl

Mg + O2 MgO

O2 + H2 H2O

CH4 + O2 CO2 + H2O

Fe + Cl2 FeCl3

Hg + O2 HgO

H2 + Cl HCl

Mg + O2 MgO

O2 + H2 H2O

Fe + Cl2 FeCl3

CH4 + O2 CO2 + H2O