Chemical Reactions and EquationsCHEMISTRY PART 3

Chemical Reaction

What is a chemical reaction?A chemical reaction: is a process in

which one set of substances called reactants is converted to a new set of substances called products.

reactants products

Some physical evidence to look for that shows a reaction has occurred:

a color change

formation of a solid (precipitate) within a clear solution

evidence of a gas

evolution or absorption of heat

How do we know a chemical reaction has occurred ?

Chemical Formula

Substitute chemical formulas for names, to obtain the following expression.

nitrogen react oxygen to nitrogen monoxide with gas form dioxidegas gas

NO + O2 NO2

(c) McGraw Hill Ryerson 2007

• When a chemical reaction occurs, new compounds are created, BUT…– No new matter is created or destroyed; atoms are

just rearranged as the atoms change partners to form new compounds.

– If there are 3 atoms of oxygen in the reactants, there MUST be 3 atoms of oxygen in the products.

– Number of each atom in reactants = number of each atom in products.

• The law of conservation of mass:– Mass of reactants = mass of products

If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O2) = mass of products (exhaust).

Law of Conservation of Mass

Types of Reactions

Synthesis reaction: A + B AB

Decomposition reactions: AB A + B

Single -Displacement reactions: AB + C AC + B

Double-Displacement reactions:

AB + CD AD + CB

Balancing Equations (Stoichiometry)

• Chemical reactions result in chemical changes.– Chemical changes occur when new substances

are created.The original substance(s), called reactants, change

into new substance(s) called products.

Reactants Products

Writing & Balancing Equations• A balanced chemical equation shows all

atoms and the coefficients tells us how many molecules (and atoms) there are.– Balancing ensures that the number of each

atom is the same on both sides of the reaction arrow.

4K + O2 2K2O

K

K

K

K

K

K

K

K

O O O

O

(c) McGraw Hill Ryerson 2007

• Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations.– Word equation: methane + oxygen water +

carbon dioxide

– Skeleton equation: CH4 + O2 H2O + CO2

• To balance the compounds, take note of how many

atoms of each element occur on each side of the

reaction arrow.

See Page 207

Counting Atoms to Balance Equations

Video over how to balance equations

http://www.schooltube.com/video/7f9c94c1a297431d9dfd/Balancing%20Chemical%20Equations%20(Stoichiometry)%20-%20Lesson

Endothermic Reaction VS Exothermic reaction

Energy Diagrams pg 637