chemical reaction class for junior science

1. Chemical Reactions
• Fireworks are a result of
chemical reactions

2. Chemical Reactions
• Chemical Reactions form new
substances by breaking and
making chemical bonds
• Chemical reactions change the
way the atoms are arranged

3. Difference between
Physical and Chemical
Changes
• Physical
• A change in the
state
• Solid to liquid
• Liquid to gas
• Gas to liquid
• Atoms/molecules
of the
substances do
not change
• Chemical
• Atoms/molecules
broken down or
combined to
form new
substances
• Properties of
new substances
are different
than properties
of the
substances that
make them up

5. Reactants and Products
• Reactants: present at the beginning
of the reaction
• Products: are the substances formed
by the chemical reaction
• Example burning natural gas
• CH4 + 2O2 CO2 +2H2O
• Reactants Products

6. Evidence of Chemical
Reactions
• Color Change
• Formation of Precipitate
(solid formed from two
liquids)
• Formation of gas
• Temperature change:
endothermic (temp down)
exothermic (temp up)
• Change in smell
www.learner.org

7. Classification of
Chemical Reactions
• Synthesis: a
new compound
is formed by the
combination of
simpler
reactants
• Smog formed
when nitrogen
and oxygen
combine
• N2 +2O2 > 2 NO2
oceanworld.tamu.edu

8. Classification of Chemical
Reactions
• Decomposition: A
reactant breaks
down into simpler
products (reverse
of synthesis)
• Water can be
decomposed into
hydrogen and
oxygen
• 2 H2O > 2H2 +O2 www.blewbury.co.uk

9. Classification of
Chemical Reactions
• Combustion: One reactant is
always oxygen and another
reactant often contains carbon
and hydrogen
• Burning of methane
• CH4 + 2O2 > CO2 +2H2O
earthguide.ucsd.edu

10. The Rates of Chemical
Reactions can vary
• Concentration: measures the number of
particles present in a certain volume.
• A high concentration of reactants means
there is a large number of particles that
can collide and react
• Turning the valve on a gas stove increases
the methane molecules that can combine
with oxygen and results in a bigger flame
and faster combustion reaction

11. The Rates of Chemical
Reactions can vary
• Surface Area: the exposed surface of
a substance.
• To increase reaction break a large
piece of the material so that there is
more surface area of the material
• Increase surface area=Increase
reaction rate

12. Temperature
• Rate of reaction can be increased
by an increase in temperature
which increases how fast the
particles are moving and increases
the number of collisions
• Remember the cold and hot water
distribution of food coloring

13. Catalyst
• Catalyst: is a substance that
increases the rate of chemical
reaction but is not consumed in the
reaction
• Enzymes are catalysts that are used
by living things to cause a chemical
reaction. Resulting in a new product
being made and the catalyst not
being changed.

14. The masses of reactants and
products are equal
• Law of conservation
of mass: states that
in a chemical
reaction atoms are
neither created nor
destroyed. (Antoine
Lavoisier)
• All atoms present in
the reactants are
also present in the
products www.iuav.it

15. Chemical Equations
• Reactants > Products
• Balancing Chemical
Equations
• Use conservation of
mass to balance: atoms
can not be created or
destroyed.
• Reactants = products mooni.fccj.org

16. Word equations
• A word equation represents a
chemical reaction using the names
of the substances involved.
• Word equations do not show
any chemical symbols or formulae
• Eg. Sodium reacts with oxygen to
gives sodium oxide.
• Sodium + oxygen sodium oxide

17. Formula equation
• represents the reactants and
products of a chemical reaction by
their symbols or formulas.
• Eg. CH4(g) + O2(g) → CO2(g) + H2O(g)
• (not balanced)

18. Balanced equation
• A chemical equation in which
there are equal numbers of
atoms of each element on both
sides of the equation.
• Eg.

19. Chemical equations
• CH4 + O2 CO2 + H2O
• Balanced?
• No
• Reactants: C=1, H=4, O=2
• Products: C=1, H=2, O=3
• H and O not equal
• Use coefficients to balance equations
• CH4 + 2O2 CO2 + 2H2O
• C=1, H=4, O=4 > C=1, H= 4, O=4
• Equation is now balanced!

20. Chemical Equations
4 Al(s) + 3 O2(g) ---> 2
Al2O3(s)
This equation means
4 Al atoms + 3 O2
molecules ---
produces--->
2 molecules of Al2O3
AND/OR
4 moles of Al + 3 moles of
O2 ---produces--->

22. There are four basic steps to balancing a chemical
equation.
1. Write the correct formula for the reactants and
the products. DO NOT TRY TO BALANCE IT YET!
You must write the correct formulas first.
**And most importantly, once you write them correctly
DO NOT CHANGE THE FORMULAS!
2. Find the number of atoms for each element on the
left side. Compare those against the number of
the atoms of the same element on the right side.
3. Determine where to place coefficients in front of
formulas so that the left side has the same
number of atoms as the right side for EACH
element in order to balance the equation.
4. Check your answer to see if:
− The numbers of atoms on both sides of the
equation are now balanced.
− The coefficients are in the lowest possible
whole number ratios. (reduced)
Steps to Balancing Equations

23. Some Suggestions to Help You
Some helpful hints for balancing equations:
• Take one element at a time, working left to
right except for H and O. Metals, then
nonmetals are a good way, too. Save H for
next to last, and O until last.
• IF everything balances except for O, and
there is no way to balance O with a whole
number, double all the coefficients and try
again. (Because O is diatomic as an
element)
• (Shortcut) Polyatomic ions that appear on
both sides of the equation should be
balanced as independent units

24. Balancing Equations
___ H2(g) + ___ O2(g) ---> ___ H2O(l)
2 2
What Happened to the Other
Oxygen Atom?????
This equation is not balanced!
Two hydrogen atoms from a hydrogen
molecule (H2) combine with one of the
oxygen atoms from an oxygen molecule
(O2) to form H2O. Then, the remaining
oxygen atom combines with two more
hydrogen atoms (from another H2 molecule)
to make a second H2O molecule.

25. Balance this equation!
Na + Cl2 NaCl
Na- 1 Na-1
Cl-2 Cl-1
**note that the number of
sodiums balance but the
chlorine does not. We will have
to use coefficients in order to
balance this equation.

26. Inserting subscripts
Na + Cl2 NaCl
Na- 1 Na- 1 2
Cl-2 Cl- 1 2
** Now the chlorine balances but
the sodium does not! So we go
back and balance the sodium.

27. Finally balanced!
2Na + Cl2 2 NaCl
Na- 1 2 Na-1 2
Cl-2 Cl-1 2
**Since the number of each
element on the reactant side
and the product side of the
equation are equal, the equation
is balanced.

28. Balancing
Equations
___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)
2 3

29. Balancing Equations
Sodium phosphate + iron (III)
oxide  sodium oxide + iron (III)
phosphate
Na3PO4 + Fe2O3 ---->
Na2O + FePO42
2
3

30. Chemical Reactions
Involve Energy Changes
• It takes energy to break
chemical bonds. This
energy is called bond
energy.
students.ed.uiuc.edu

31. Exothermic Reactions
• Exothermic
reaction: energy
is released and
temp goes
up…more energy
is released
when products
are formed.
• Space shuttle on
take off: white
clouds of water
vapor are
formed www.sanchezcircuit.com

32. Exothermic Reactions
• Exothermic reactions release
energy!
• All common combustion reactions
are exothermic
• Fireflies release energy in the form
of light as an exothermic reaction
www.learner.org

33. Endothermic reactions
• Endothermic reactions: produce a
decrease in temperature because
the bond energies of the reactants
are greater than the bond energies
of the products
• All endothermic reactions absorb
energy!
• Alka Seltzer and water…temperature
goes down.

34. Endothermic Reactions
• Photosynthesis
(general formula)
• 6CO2 +6H2O > C6H12O6 +6O2
• plants absorb energy from
sunlight to turn carbon
dioxide and water into
oxygen and glucose (sugar).
• Energy is stored in the
glucose molecule to be used
when needed.
www.osovo.com

35. Life and Industry depend
on Chemical Reactions
• Living things require chemical reactions
• Respiration: living things get energy from
glucose by respiration. (combustion of glucose)
• Photosynthesis
• 6CO2 +6H2O + energy> C6H12O6 +6O2
• Respiration
• C6H12O6 +6O2 > 6CO2 +6H2O + energy
• Respiration is the opposite of
photosynthesis!