Download Chemical Reactions Notes I. Variables review The independent

Chemical Reactions Notes
I.
Variables review
The independent variable is changed by the experimenter. We call it the “I”
change it variable, because as the experimenter, “I” change the independent
variable from the controlled set up versus the experimental set up. For example,
if we are testing to see which white substance reacts with vinegar what I will
change in the various set ups will be the different white substances.
The dependent variable is what is measured to determine the outcome of the
experiment. We call it the “D” variable because it is the result we are trying to
determine or discover. The controlled variable, sometimes called a constant, is any
factor that could change but is intentionally kept the same. We call it “C” variable
because we keep it constant or the same.
II. Physical and chemical properties
Physical properties are characteristics that can be observed without changing
the composition of a substance. These include: appearance such as observations
that can be made with the senses – color, shape, luster, texture, etc. Volume,
mass, and density are measurements that can describe physical properties. The
states of matter – solid, liquid, or gas are physical properties. So are the
temperatures when changes between states occur, the melting point, boiling point,
and other state changes are physical properties of a substance.
A chemical property is a characteristic that cannot be observed without
altering the substance. These include the ability to burn, the tendency to rust, and
the reaction to other substances such as acids and bases.
III. Physical and chemical changes
During a physical change, form or appearance of matter changes, but the
composition of the substance stays the same. The change does not produce a new
substance. The shape can change, but substance does not. Dissolving a solid into a
liquid is a physical change. Changing states from solid to liquid to gas is a physical
change.
Chemical changes result in a change in the substance’s composition. These are
signs that a chemical change has occurred: Energy may be gained or released, color
can change, substances may change odor or form a gas, and/or a solid is produced
as a precipitate.
IV. Chemical Reactions
When chemical changes happen, a new substance with a new chemical formula is
produced. A chemical reaction occurs when one or more substances are changed
into one or more new substances with a new set of physical and chemical
properties. An example of this is when iron combines with oxygen to form rust.
The rust has new properties compared to the iron and oxygen. A chemical equation
represents the chemical combining of elements and/or compounds by showing the
reactants and their products.
2KI + Pb(NO3)2 
2KNO3
+
PbI2
A coefficient is a number that indicates how many compounds or elements are
present in a chemical reaction. The coefficient will be written in front of the
formula or chemical symbol. A subscript is a smaller number written after a
symbol telling how many atoms are present in a compound.
In a chemical reaction reactants produce products. The reactants are the
substances that enter into the reaction and are written on the left side of the
equation. The products are the new substances that are produced in the reaction
and are written on the right side of the equation. The arrow in an equation may be
read as yields or produces, therefore reactants yield products.
V. Signs of a Chemical Reactions
There are several signs or indicators that a chemical reaction has occurred.
Heat may be released or absorbed causing a temperature change. There may be an
unexpected color change. There may be a release of a gas in the form of an odor or
bubbles. A precipitate, which is the formation of a solid in a solution, may be
produced.
VI. Law of Conservation of Mass
During a chemical reaction, mass will be conserved, meaning the mass of the
reactants will be the same as the mass of the products. This is the law of
conservation of mass. In a chemical equation, the number and kind of atoms must
be equal on both sides. Atoms are never lost or created during a chemical reaction;
however, they do change partners.
VII.
Balancing equations
We must balance equations, meaning that the numbers and kinds of atoms on
both sides of the equation are same, because of the law of conservation of mass.
There are four steps:
1. Write a correct equation using correct symbols and formulas to show reactants
and products.
2. Count the numbers of atoms on both sides of the arrow.
3. Balance the numbers of atoms by using coefficients in front of the symbols and
formulas.
4. Check to see if the numbers of all the atoms on both sides of the arrow are
the same.
VIII. Energy Reactions
During a reaction, energy may be released or absorbed. We call these
reactions exothermic and endothermic. Thermic refers to heat energy.
The prefix exo refers to going out, such as an exit. An exothermic reaction is a
reaction in which energy is released during the reaction. The products will have
less energy than the reactants. Exothermic reactions feel warm. In a chemical
equation, energy will be written with the products for an exothermic reaction.
The prefix endo refers to going in, such as an entrance. An endothermic reaction
is a reaction in which energy is absorbed during the reaction. The products will
have more energy than the reactants. Endothermic reactions fee cold. In a
chemical equation, energy will be written with the reactants for an endothermic
reaction.
Exothermic
Endothermic
Energy
Energy
Heat is absorbed
Heat is released
Reactants
Products
Reactants  Products + Energy
Reactants
Reactants + Energy  Products
Products