chemical reactions and equations chemistry part 3
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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
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