3/7/17
C.O.
SW be able to recognize
chemical reactions
based on specific
characteristics.
Warm up:
How do we know a chemical reaction is going on?
What is a Chemical
Reaction?
 A chemical reaction is the process by which one or
more substances are changed into one or more
different substances
 The original substances are known as reactants and
the resulting substances are known as products
 Chemical reactions can be described using
chemical equations
 Chemical equations represent reactions by using
symbols, formulas, and the amount of reactants and
products in a chemical reaction
How Do We Know a Chemical
Reaction is Occurring?
 To know a chemical reaction is occurring, there has to be evidence that
one or more substances has undergone a change
 Evolution of energy as heat and light: a change in matter that releases
energy as heat and light is a strong indication that a chemical reaction has
occurred
 https://www.youtube.com/watch?v=JOHdZsQXw7I
 Production of a gas: the evolution of gas bubbles when two or more
substances react is good evidence of a reaction
 Formation of a precipitate: a solid that is produced as a result of a
chemical reaction in a solution and can be separated from the solution
 Color change: a change in color can indicate a chemical reaction
Characteristics of Chemical
Equations
1.
The equation must represent all the known facts. It has to
have all the reactants and products identified
2.
The equation must contain the correct formulas for the
reactants and products. Make sure to know the oxidation
rules to correctly write the equations. Remember that many
pure elements are made by bonding with two atoms of the
same element such as H2
3.
The law of conservation of mass must be satisfied. Atoms are
not created or destroyed during reactions so make sure to
include all of them in the reactants and products. Make sure
to add coefficients, numbers to represent the amount of
compounds, when you are balancing equations
Word Equations
 Writing word equations are easy to help identify what is going
on in a reaction
 Word equations are qualitative representations of what is
going on in a reaction by using the names of the reactants
and products, they tell us what is happening, but not how
much of the compounds are involved
 A good example is the reaction between methane and
oxygen:
Methane + oxygen
carbon dioxide + water
 The arrow shows what is being produced from the reaction
Formula Equations
 A formula equation represents the reactants
and products of a reaction by their symbols
and formulas
 There are specific symbols to represent the
state of each compound and how they are
reacting
 When you are writing the products, make sure
to balance the equation
 A good example is the reaction of methane
and oxygen
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
Significance of a Chemical
Equation
1.
The coefficients of a chemical reaction indicate relative, not
absolute amounts of reactants and products. A chemical
equation usually shows you the smallest ratios of the
compounds to satisfy the law of conservation of mass
2.
The relative masses of the reactants and products of a
chemical reaction can be determined from the reaction’s
coefficients. We know that when two chemicals react and
create a new compound, the compound will have the
equal parts of each compound that was part of the reaction
that went into it
3.
The reverse reaction for a chemical reaction has the same
relative amounts of substances as the forward reaction. It
shows that the reactants and products are going to have
the same amount of each compound used in the reaction.
Balancing Chemical Equations
 When we write chemical equations, we need to make sure they
are accurate by balancing them by using four steps
1.
Identify the names of the reactants and the products, and write a
word equation
2.
Write a formula equation by substituting correct formulas for the
names of the reactants and the products
3.
Balance the formula equation according to the law of
conservation of mass
4.
Count the atoms to be sure that the equation is balanced
 Let’s try to balance the decomposition of water
Practicing Balancing Chemical
Equations
 The reaction of zinc with aqueous hydrochloric
acid produces a solution of zinc chloride and
hydrogen gas. Write word equation and a
balanced chemical equation for the reaction.