Chpt 17 Chemical Reactions
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Transcript Chpt 17 Chemical Reactions
Chemical formulas and Equations
Rates of Chemical Reactions
Chemical Formulas and Equations
Physical Change
Changes that only affect its physical properties like
size, shape, or whether it is a solid, liquid or gas
There is no change of material
You still have the same substance that you began
with
Ie: crumple paper, cut up wood, changes of water
from ice, to liquid to a gas
Chemical Formulas and
Equations
Chemical Change
A change that produces a new substance with
properties different from those of the original
substances
Things that show that a chemical reaction has
occurred
Precipitate forms (when 2 liquids combine the new
substance will drop out of solution..is not soluble)
Odor given off
Change of color
Temperature change
Chemical Formulas and
Equations
Chemical reaction occurs when a chemical change
takes place
Anytime you have a new substance formed
Burning paper is a result of a chemical reaction
Folding paper is a result of a physical change because
you still have paper, only change of shape
Chemical Equations
Using chemical symbols of elements and chemical
formula you can write a shorthand of a chemical
reaction
The substances you begin with in a chemical reaction
is called the reactants
The new substances that are formed is called the
product
In a chemical reaction you can write the reactants and
resulting products in a chemical equation using arrows
(and is read as produces)
Chemical Equations
Baking soda + vinegar
2NaHCO3 + H4C2O2
gas + white solid
2NaH3C2O2 + H2O + CO2
Sodium hydroxide plus Sulfuric acid produces sodium sulfate and
water
2NaOH + H2SO4
Na2SO4 + H2O
Conservation of Mass
Antoine Lavoisier: mass of the product must be the
same as the mass of the reactants
Chemical equations must show that there is the same
kind and number of atoms that you began with as was
formed
Every atom that appears on the reactant side of the
equation is found on the product side
Balancing Equations
Chemical equations must show the conservation of
mass by having same kind and same numbers of atoms
on both side of a chemical reaction
Ag + H2S
Ag2S + H2
(Silver tarnishes in presence of sulfur)
Notice that the number of atoms aren’t the same on
both side
Need to count the atoms… you can’t change the
formula but you can add a coefficient in front of the
formula to balance equations
Balancing Equations
Ag + H2S
Ag2S + H2
Notice there is 2 Ag’s on product side and only one on
reactant side…. Add a coefficient of 2 in front of
reactant silver
2Ag +
H2 S
2 Ag’s, 2 H’s and 1 S
It is balanced
Ag2S + H2
Balancing Equations
Al
+ O2
Al2O3
Aluminum and oxygen produces aluminum oxide
Not balanced (can only change the coefficient)
4Al + 3O2
2Al2SO3
Balancing Equations
http://education.jlab.org/elementbalancing/index.html
Energy in
Chemical Reactions
Energy released
Heat, light, sound or other energy is given off
When bonds break in a chemical reaction new bonds
form in the products and energy is given off
Energy found on product side of chemical reaction
Called an exothermic reaction
2H2 + O2
2H2O + energy
Energy in
Chemical Reactions
Energy is Absorbed
Energy is required to form new products
Can absorb light, sound, heat
Called an endothermic reaction
Energy is placed on the reactant side of equations
2H2O + energy
2H2
+
O2
Ice packs are example of endothermic reactions
Rates of Chemical reaction
Activation Energy: starting a reaction
To start a chemical reaction a minimum amount of
energy is required to begin the process
That energy required is its activation energy
For gasoline to burn energy needs to be introduced to
begin the chemical reaction…. That energy (match) will
begin the chemical process
Reaction Rate
Rate refers to a change over time
Reaction rate refers to the speed at which chemical
reactions takes place over time
Reactions rates in some industries is important
because the faster the rate, the less the cost
Some faster reaction rates can hurt industries
Ripening fruit gives less time from field to markets
The slower the ripening rate increases the time it can be
on shelves
Reaction Rate
Temperature changes rate
Lowering the temperature slows down reaction rates
Meat in freezers, bacteria grows faster in higher
temperatures, lower movement of molecules, less
chance of causing chemical changes
Temperature affects Rate
Most chemical reactions speed up with increase in
temperature because of faster moving molecules
Lowering temperature usually slows down reaction rate
Reaction Rate
Concentration Affects Rate
Closer the reactant atoms and molecules to each other,
the greater the chance of collisions between them and
faster the reaction rate
Amount of substance present in a certain volume is
called the concentration of that substance
Increasing the concentrations increases the number of
particles of a substance per unit of volume
Reaction Rate
Surface Area Affects Rate
If you increase the surface area you will
increase its chance of reacting with other
substances
Twigs broken up compared to logs to begin a fire
Crushing up materials into smaller pieces
increases surface area
Slowing down Reactions
Inhibitors slow down chemical reactions
Makes the process of chemical change longer
BHT on cereal packages slow down the spoilage
of cereal and increase its shelf life
Speeding up Reactions
Catalysts are substances that speed up a chemical
reaction without entering into the chemical reactions
It produces the same amount of product faster
Catalytic Converters utilize metals (platinum) to
produce CO2 and water , which is less harmful in cars
fuel combustion.
Enzymes are catalysts that are large protein molecules
that speed up reactions needed in cells to work
properly
They help speed up conversion of food to fuel, build
bone and muscle tissue, and converting energy by fitting
structures together properly so that reactions can occur
faster
Speeding up Reactions
Proteases is a special type of enzyme
that helps break down large protein
molecules
Meat tenderizers
Contact lenses cleaning solutions breaks down
protein from your eyes that collect on your
lenses and cloud your view