1. Chemical Reactions
Ch 9

2. Chemical reactions
Chemical Reaction (RXN): when a substance undergoes chemical changes, forming a new substance
Involve a rearrangement of atoms and CANNOT be reversed
The new substance will have DIFFERENT chemical and physical properties than the original

3. Chemical reactions
There are some signs that provide clues a chemical reaction is happening
Bubbles (formation of gas)
Form a solid – precipitate
Color change – (bleach on fabric)
Change in energy (NRG) – gain or loss

4. Chemical equations 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

5. Chemical Equations
Chemical reactions are represented by chemical equations
Reactants  Products
The arrow stands for yield

6. Chemical equations
Due to the Law of Conservation of Mass, the amount of atoms in the products must be EQUAL to those of the reactants
This is achieved by balancing the chemical equation - changing ratios of molecules/atoms so both sides have the lowest possible same whole number and type of atoms

7. Balancing equations
A coefficient is a number that shows the relative amount of a compound in a reaction
You CAN change the coefficient in front of any atom/molecule
You CANNOT change the subscript (this would change the formula of the molecules)

8. Balancing equations Count the number of atoms for each reactant
Count the number of atoms for each product
Use coefficients to balance the equation

9. C6
H12 O6 +
6O2 6C +  O2
6H2 O
Carbon - 6
Carbon - 6
Hydrogen - 12
Hydrogen - 12
Oxygen - 18
Oxygen - 18

10. _____H2 + _____ O2  ____ H2O ____ H2 + ____ N2  ____ NH3
Let’s Practice!
_____H2 + _____ O2  ____ H2O ____ H2 + ____ N2  ____ NH3

11. Chemical reactions
Chemical reactions involve breaking old bonds followed by the making of new ones
Breaking bonds REQUIRES energy
Forming bonds RELEASES energy
Energy stored in bonds = chemical energy
Activation Energy: the minimum amount of energy needed to start a chemical reaction

12. Chemical reactions Exothermic Reactions: reactions that RELEASE energy
Energy in the bonds is greater than in the products

13. Chemical reactions
Endothermic Reactions: reactions that ABSORB energy during a reaction
More energy needed to break the bonds than is released
Example: photosynthesis

14. Types of reactions
Patterns can be used to identify kinds of chemical reactions and to predict the products

15. Types of reactions
Synthesis: 2 or more substances combined to form a new compound
A + B  AB
Decomposition: substances are broken apart
AB  A + B

16. Types of reactions
Single Displacement: elements trade places, usually forms a precipitate
AX + B  BX + A
Double Displacement: two compounds exchange ions
AX + BY  AY + BX

17. Types of equations Combustion: must contain oxygen in the reaction
Water is also common in combustion
Redox: transfer of electrons (free radicals)
Substances accept e- = reduced
Substances give e- = oxidized

18. Types of reactions S8 + O2  SO2 6CO2 + 6H2O  C6H12O6 + 6O2
KClO3  KCl + O2
Zn + HCl  ZnCl2 + H2

19. Reaction rates
Reaction Rate: how quickly the concentration of products increases, or how quickly the concentration of reactants decrease
Reaction rate can be changed by changing the concentration or temperature of the reactants.

20. Reaction rates Molecules must collide in order to have a reaction
Increasing temperature makes molecules move faster, creating more forceful collisions
Increasing concentration puts more molecules in a given volume, making collisions more probable

21. Reaction rates
A third way to increase reaction rate is to add a catalyst
Catalyst: Any substance that increases the rate of a chemical reaction by lowering its activation energy
It does NOT change what is made or how much, just how fast

22. entropy Entropy: a measure of random activity in a system
Energy and temperature are directly proportional
If you increase the temperature of a system, entropy increases (particles move faster)
If you decrease temperature, entropy decreases (particles slow down)

23. Entropy
As a gas expands in a system, entropy increases. This one is also easy to visualize. If an atom has more space to bounce around, it will bounce more. Gases and plasmas have large amounts of entropy when compared to liquids and solids.

24. Gas laws Properties of Gases:
fluids with particles that move in all directions
expand to fill their containers, have low densities, and are compressible
Gas Laws describe how the behavior of gases is affected by pressure, volume, and pressure.
The laws will help you understand and predict the behavior of gases in specific situations

25. Boyle’s law
For a fixed amount of gas at a constant temperature, the volume of a gas increases as the pressure decreases, and vice versa.

26. Gay-Lussac’s Law
The pressure of a gas increases as the temperature increases, if the volume of the gas does not change.

27. Charles’s law
For a fixed amount of gas at a constant pressure, the volume of the gas increases as the gas’s temperature increases.