1. Chemical Reactions
Chapter 7 Section 1

2. Chemical Reactions
Occur when substances go through chemical changes to form new substances…
You can often tell because you can see the changes
A chemical reaction is rearrangement of atoms in the reactants to make new substances in the products.

3. What is a chemical reaction
Reactants- Starting materials in a reaction
Products- Ending materials in a reaction
The products of a reaction are made up of the same number and type of atoms as were present in the reactants (just in a different arrangement)

4. For example
2 H2 + O2 2 H2O
Same number and kind of atoms on each side
They are just in different arrangements
Both reactants are gases, the product is a liquid
It has changed into a different molecule.

5. Evidence for a chemical change
Some observations that suggest a chemical change has taken place…
Evolution of energy as heat, light or sound
Production of a gas

6. More Evidence… Formation of a precipitate (“stuff forming”)
Change in color
This is not conclusive proof though, it needs to be proven that some new compound is present.

7. Activation Energy
In order to start a chemical reaction you must break bonds (covalent, ionic or metallic)
To do this takes energy
The energy required to break enough bonds for a reaction to start is the activation energy of a reaction

8. Exothermic Exothermic reactions give off energy..
When the bonds reform they will release energy
If more energy is released than is used to break more bonds (to continue) the reaction is exothermic

9. Exothermic energy graph
Activation energy
energy
reactants
products
time
It ended up giving energy
To get to the products
More energy came out, than
was put in

10. Endothermic
If more energy is required to make the reaction than is released the reaction is endothermic
Endothermic take in energy.

11. Endothermic energy graph
Activation energy
Energy
products
reactants
time
Overall the reaction still took energy. It required
Energy to get to the products

12. Balancing Equations
Chapter 7 section 2

13. Reminder: All MATTER is conserved, but it can change forms..
What goes in must come out, but it can be rearranged!
You have to account for all the “stuff” that goes into the reaction

14. What is balancing an equation?
Balancing an equation is changing the ratios of molecules/atoms to make the lowest possible same whole number and type of atoms on each side of the equation.
Na + Cl2  NaCl
What goes in must come out
Na + Cl2  NaCl 2 2

15. Describing Reactions
A chemical equation uses symbols to represent a chemical reaction and shows the relationship between the reactants and products of a reaction….

16. What you can not do!
You can NOT change the formula of the molecules I give you.
2H2 + O2  2H2O
If the number is one, you
don’t have to write it!

17. What you can not do!
you cannot change water to H2O2 (hydrogen peroxide)
So, you can change COEFFICIENTS, not SUBSCRIPTS!
you have to increase the
amount of H on this side
and this will make 2
water molecules H H O O H O  H H H O H H

18. What you are allowed to do
change the coefficient in front of any molecule/atom (changing the ratio of atoms)
___ Ag + ___ Cl2  ___ AgCl
you can only write on those lines 2 2

19. Balancing Equations
The coefficient (the number in front) is used to make the number of atoms on each side of the arrow equal (you do not use the subscript for this)

20. Problem solving strategies
It helps to make a list of all atoms present
HCl + Ba(OH)2  BaCl H2O 2
Ba – 1
H – 3
Cl – 1
O – 2
Ba – 1
H – 2
Cl – 2
O – 1 4 2 4 2 2

21. Problem solving strategies
It helps to make a list of all atoms present
HCl + Ba(OH)2  BaCl H2O 2 2
Ba – 1
H – 3
Cl – 1
O – 2
Start making the
numbers agree
one at a time
(pick something,
that occurs in only
one molecule 1st)
~not H in this case
Ba – 1
H – 2
Cl – 2
O – 1
Now going
to O 4 2 4 2
Now everything
is balanced!
starting w/ Cl

22. Another to try CH4 + O2  H2O + CO2 2 2 C- 1 C- 1 H- 4 H- 2 O- 2 O- 3
Everything is balanced now!
This takes a lot of trial and error.
Start with H
now O