JMJ
Chemical Reactions
Theme: Changes and Reactions
Physical Properties (Review)
Color, melting point, boiling point, electrical conductivity, specific heat, density, state
(solid, liquid, or gas)
Physical Change (Review)
Changes in physical properties - melting, boiling, condensation, freezing
No change occurs in the identity of the substance
Chemical Change
Atoms in the reactants are rearranged to form one or more different substances
Old bonds are broken; new bonds form
Main Ideas
Chemical Reactions are represented by Chemical Equations.
Chemical Equations are balanced to show the same number of atoms of each element on
each side.
The Law of Conservation of Matter (Mass) says that atoms won’t be created or destroyed
in a chemical reaction. That is why chemical equations must be balanced!
Vocabulary
Reactants
Products
Chemical Formula
Chemical Equation
Coefficients
Chemical Reaction
A process in which at least one new substance is produced as a result of chemical change.
How do you know when a chemical reaction takes place?
Color Change, Precipitate Formation, Gas Formation, Odor, Temperature Change,
Change in Acidity
Representing Chemical Reactions – Chemical Equations
Chemical Equations
Reactants produce Products
Reactants

Products
Solid Sodium combines with Chlorine gas to make solid Sodium Chloride:
2Na (s) + Cl2 (g)  2NaCl
Reactant A + Reactant B  Product
The reactants are used up in forming the product
The arrow  shows the direction of the reaction
Writing a Chemical Equation
Chemical symbols give a “before-and-after” picture of a chemical reaction
Law of Conservation of Matter (Mass)
Matter cannot be created or destroyed. However, it can change form.
The mass of the reactants must equal the mass of the products in a chemical reaction.
Balancing Chemical Equations
The total number of each type of atom must be the same on both sides of the equation.
Understanding Chemical Formulas
Subscripts
BaF2 the 2 in this formula is called the subscript. It refers only to the element
preceding it. In this case the F (fluorine).
Parentheses
Al(NO3)3 in some chemical formulas it is necessary to use parentheses. The
subscript outside the parentheses refers to all the elements inside the parentheses.
In this example there are: one Al (aluminum), three N (nitrogen), and nine O
(oxygen).
Coefficients
3 BaF2 the 3 in this formula is called the coefficient. It refers to each element that
follows. In this case there would be 3 Ba (barium) and 3 F2 (a total of 6 fluorine).
Reaction Rates
Increasing Temperature increases reaction rate
Surface Area – increasing the surface area increases the rate of reaction.
Concentration – amount of reactants in a given volume.
Stirring increases reaction rate
Catalyst – speeds up a reaction but does not change during the reaction
Catalysts do not get used up in a reaction
HCl
CH3COCH3 + I2  CH3COCH2I + HI
HCl is a catalyst, necessary for the reaction but not used up in the reaction, that is why it
is written above the reaction arrow.
Increasing reactants increases the rate of reaction and increases the products made
Reactions will occur until at least one of the reactants is all used up
Mass of all the reactants is equal to the mass of all the products
Spectrophotometer measures absorbance or transmission of light
Can be used to measure the disappearance or appearance of products or reactants in a
chemical reaction
Summary
As the amounts of the reactants increase, the amounts of the products produced also
increase. In Investigation 1, as the amounts of the NH4OH and the HCl increased, the amount of
heat produced increased.
Reactants must be present in the reaction in equivalent amounts to produce the maximum
amount of all products. In Investigation 2, the HCl was present in a greater amount than the
magnesium since all the magnesium was consumed by the reaction to produce H2 gas. When the
amount of magnesium was increased from 0.5 to 1 gram, the excess HCl was able to react and
produce more H2.
No matter is lost from a chemical reaction even if the reactants are not present in
equivalent amounts. Every atom of every reactant can be found in either the products or in
2
reactants that are not consumed. In Investigation 2, the atoms of the HCl were found either in
the unreacted HCl or in the H2 produced. In every reaction of the Investigation, all of the
magnesium atoms were converted to the product, MgCl2.
As the amount of a reactant increases, the rate of the reaction increases. The rate can be
measured either as an increase in the rate of disappearance of a reactant or an increase in the rate
of production of a product. Therefore, more reactant added to a reaction, results in more
products produced at a faster rate. Investigation 3, as the concentration of the reactant acetone
increased, the reaction rate increased as evidenced by the more rapid disappearance of the
colored reactant, I2.
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