Describing
Chemical Reactions
By : Amr Khodary
G : 8/B
chemical equation
•
A chemical equation is a way to show a chemical reaction, using symbols
instead of words.
•
A chemical formula is a combination of symbols that represents the
elements in a compound. For example, CO₂ is the formula for carbon dioxide.
The formula tells you that the ratio of carbon atoms to oxygen atoms in this
compound is 1 to 2. Carbon dioxide is a molecular compound.
•
A chemical equation tells you the substances you start with in a reaction and
the substances that are formed at the end.
chemical equation
The substances you have at the beginning are the reactants, and the new
substances are called the products.
Reactant + Reactant → Product + Product
Reactant → Product + Product
Reactant + Reactant → Product
The number of reactants and products can vary. Some reactions have only one
reactant or product. Others have two, three, or more reactants or products. For
example, the reaction that occurs when limestone, or calcium carbonate
(CaCO₃), is heated has one reactant and two products (CaO and CO₂).
CaCO₃ → CaO + CO₂
Law of Conservation of Mass
•
In a chemical reaction, all of the atoms present at the start of the reaction
are present at the end of the reaction.
•
This principle was first demonstrated by Antoine Lavoisier in 1774.
•
Atoms are not created or destroyed; however, they may be rearranged to
form new substances.
•
The amount of matter does not change.
•
Total mass stays the same before and after the reaction.
Balancing Chemical Equations
•
The principle of the conservation of mass means that the total number of
atoms of each element in the reactants must equal the total number of
atoms of each element in the products.
•
To balance the equation, use coefficients.
•
A coefficient is a number placed in front of a chemical formula in an
equation. It tells you the amount of a reactant or a product that takes part in
a reaction. The coefficient applies to every atom of the formula it is in front
of.
Open and Closed Systems
•
Open and Closed Systems is not always easy to measure all the matter
involved in a reaction. Products can escape into the air, but how much?
•
In an open system, matter can enter from or escape to the
surroundings. If you want to measure all the matter before and
after a reaction, you have to be able to contain it.
•
In a closed system, matter does not enter or leave. A chemical
reaction that occurs inside a sealed, airtight container is a closed
system
•
A fish bowl is an example of an open system. Ecosphere is closed.
What are Three Types of Chemical Reactions?
•
The three types of chemical reactions are synthesis, decomposition, and
replacement.
•
Synthesis – when two or more elements or compounds combine to make a
more complex substance
Example: The reaction of phosphorus with oxygen P₄ + 3 O₂ → P₄O6
•
Decomposition – when compounds break down into simpler products
Example: If you keep a bottle of hydrogen peroxide for a very long time,
(several years) you’ll have water instead. Hydrogen peroxide decomposes
into water and oxygen gas. 2 H₂O₂ → 2 H₂O + O₂
What are Three Types of Chemical Reactions?
•
Replacement – when one element replaces another element in a compound,
or if two elements in different compounds trade places, the reaction is called
a replacement.
Single replacement - 2 Cu₂O + C →4 Cu + CO₂
Double Replacement – FeS + 2 HCl → FeCl₂ + H₂S