1. UNIT 4:
CHEMICAL REACTIONS & SPECIAL
TOPICS

2. BILL NYE THE SCIENCE GUY
 http://www.youtube.com/watch?v=PlwuxpMh8nk

3. 7.1: DESCRIBING REACTIONS
The most useful way of describing reactions (or a
chemical change) is by stating what was present
before and after the chemical change.

4. CHEMICAL CHANGE
 chemical reaction
 process in which one or more substances are changed into new
substances
 reactants
 substance that reacts
 product
the new substance that is formed

5. CONSERVATION OF MASS
The French chemist Antoine Lavoisier
established that the total mass of the products
always equals the total mass of the reactants.
 This is called the Law of Conservation of Mass (mass is
not created nor destroyed in a chemical reaction).

6. WRITING EQUATIONS
chemical equation
shorthand method to describe a chemical
reaction using chemical formulas and
other symbols
Reactants
HgO(s) 
Products
Hg(l) + O2(g)

7. WRITING EQUATIONS
(aq) – aqueous (substance dissolved in water)
(s)– solid
(l) – liquid
(g) – gas
coefficients – the numbers to the left of the formulas used to help
balance the equation

8. BALANCING EQUATIONS
 If you notice that the number of atoms on the left side does
not equal the number of atoms on the right side then we
must balance the equation.
 Since mass is conserved before and after a chemical reaction, an equation MUST BE
balanced in order for it to be true.

9. BALANCING EQUATION RULES
1. You should NEVER change the subscripts in a
formula.
2. Start by, counting the number of atoms of each
element on each side of the equation.
3. Change one or more coefficients until the
equation is balanced

10. EXAMPLE
 Balance the following equation
NiCl2(aq) + NaOH(aq)  Ni(OH)2(s) + NaCl(aq)

11. EXAMPLE
HgO(s)  Hg(l) + O2(g)

12. 7.2: TYPES OF REACTIONS
 Reactions
are classified by the type of reactant or
the number of reactants and products.
There are 4 different types of reactions that we
will discuss
 Synthesis
 Decomposition
 Single-Replacement
 Double-Replacement

13. SYNTHESIS
A synthesis reaction is a reaction in which two or
more substances react to form a single substance.
 The product synthesized is always a compound
 Examples
A + B  AB
2Na + Cl2 2NaCl

14. DECOMPOSITION
 The opposite of synthesis
A decomposition reaction is a reaction in which a
compound breaks down into two or more simpler
substances.
 The reactant MUST BE a compound.
 Examples
AB A + B
2H2O 2H2 + O2

15. SINGLE REPLACEMENT
A single replacement reaction is a reaction in
which one element takes the place of another
element in a compound.
Example Form:
A + BC B + AC
Cu + 2AgNO3  2Ag + Cu(NO3)2

16. DOUBLE REPLACEMENT
A double replacement reaction is one in which
two different compounds exchange positive ions
and form two new compounds.
Example Forms:
AB + CD AD + CB
Pb(NO3)2 + 2KI PbI2 + 2KNO3

17. 7.3: ENERGY CHANGES IN REACTIONS
Chemical Energy – the energy stored in the
chemical bonds of a substance.
Chemical Reactions involve the breaking of
chemical bonds in the reactants and the formation
of chemical bonds in the products.

18. BREAKING BONDS
Breaking Bonds REQUIRES energy.
 This means we need to ADD energy in order to break
the bonds of reacting molecules in order to get the
reaction started.

19. FORMING BONDS
The formation of chemical bonds RELEASES
energy.
 When new chemical bonds are formed, a bit of energy is
released usually in the form of heat or light.

20. ENERGY IN REACTIONS
During a chemical reaction, energy is either
ABSORBED or RELEASED.
 We describe these reactions in two different ways either
Exothermic or Endothermic.

21. EXOTHERMIC REACTIONS
A chemical reaction that RELEASES energy is called
an exothermic reaction.
 The energy released as the products form is greater
than the energy required to break the bonds in the
reactants.
 Think of it as energy is EXITING the reaction
 EXiting _ Exothermic

22. ENDOTHERMIC REACTIONS
A chemical reaction that absorbs energy from its
surroundings is called an endothermic reaction.
 This means that there is more energy require to break
the bonds of the reactants than is released by the
formation of the products.

23. CONSERVATION OF ENERGY
The total amount of energy BEFORE a reaction is
EQUAL to the total amount of energy AFTER a
reaction.
 This is called the Conservation of Energy.

24. 8.1: FORMATION OF SOLUTIONS
A solution is a mixture that forms when
substances dissolve and form a homogeneous
mixture
 In order for a solution to form, one substance must
dissolve in another.

25. DISSOLVING
Every solution has two components
 A solute is a substance who particles are dissolved in a
solution
 A solvent is the substance in which the solute dissolves
in.
There are three ways that substances can dissolve
into water: dissociation, dispersion, ionization

26. DISSOCIATION
 In order for a solution to form, the attractions that hold the solute
together and the solvent together must be overcome.
The process in which an ionic compound separates
into ions as it dissolves is called dissociation.
 Example: Sodium Chloride & Water

27. DISPERSION
Sugar dissolves into water by dispersion, or
breaking into small pieces that spread throughout
the water.
 Example: Sugar & Water

28. IONIZATION
The process in which molecules gain or lose
electrons is known as ionization.
 Example: Ions are formed by the reaction of the solute
and solvent particles.

29. PROPERTIES OF LIQUID SOLUTIONS
Conductivity: ability to conduct electric current
Boiling Point: temperature needed for solution to
change from liquid phase to gas phase
Freezing Point: temperature needed for solution to
turn from liquid phase to solid phase.
Solutions can also be described as endothermic or
exothermic depending upon whether energy is
released or absorbed.

30. 8.2: SOLUBILITY & CONCENTRATION
The maximum amount of solute that dissolves in a
given amount of solvent at a constant temperature
is. called solubility
 Depending upon the amount of solute in a solution,
solutions can be described as either saturated,
unsaturated or supersaturated.

31. SATURATED
A saturated solution is one that contains as much
solute as the solvent can hold at a given
temperature.

32. UNSATURATED
A solution that has less than the maximum amount
of solute that can be dissolved is called an
unsaturated solution.

33. SUPERSATURATED
A supersaturated solution is one that contains
more solute than it can normally hold at a given
temperature.
 Usually very unstable.

34. FACTORS AFFECTING SOLUBILITY
Polarity of the solvent
 “like dissolves like”
 Solution formation is more likely to happen when the solute
and solvent are either both polar or both nonpolar.
Temperature
 The solubility of a solids increases as the solvent
temperature increases
Pressure
 Increasing pressure on a gas increases solubility in a liquid.

35. CONCENTRATION
The concentration of a solution is the amount
solute dissolved in a specified amount of solution.
 Can be expressed as percent by volume, percent by
mass and molarity.

36. SOLUBILITY CURVE
 Each line on the graph is
called a solubility curve for a
particular substance.
 You can use a solubility
curve to figure out how
much solute will dissolve at
any temperature given on
the graph.

37. 10.1: RADIOACTIVITY
Henri Becquerel
 1896
 left uranium salt in a drawer with a photographic
plate
 when he developed the plate, he found an outline
of the clumps of the uranium salt
 he hypothesized that the uranium salt emitted some
sort of energy
Marie and Pierre Curie
 students of Becquerel
 2 years later, they discovered Po and Ra while
studying uranium ore “pitch blende”

38. THEY DISCOVERED …
 Radiation
 release of matter and energy from nucleus
 Light energy (electromagnetic spectrum)
 all forms of radiation

39. STRONG FORCES
Protons are held together by strong
forces
short range force
as the distance increases, the force weakens
causes protons and neutrons to be attracted to
each other

40. STRONG FORCES
 to hold a nucleus together tightly, the
nucleus can decay and give off matter and
energy
 stable nucleus
 stays together permanently
 unstable nuclei
 radioactive!!!!
 nucleus does not stay together; emits matter and energy

41. RADIOACTIVE ELEMENTS
 radioactivity
 the process of nuclear decay
 elements after #83 are radioactive
 all elements after #92 are synthetic and
decay soon after they are created

42. REVIEW !!!!!!!
 Mass number = # protons + # neutrons
 Atomic number = # of protons
Example
(12 = mass #)
(6= atomic #)
12
6
C

43. REVIEW: ISOTOPES
 most elements have at least one radioactive
isotope
 isotope
 same element with a different number of neutrons
 Example: Carbon-12 (stable)
Carbon-14 (unstable)

44. RADIOACTIVE DECAY
Transmutation
 the process of changing one element into another through
nuclear decay
Radioactive decay
 occurs until a stable nucleus is formed

45. ALPHA DECAY
alpha decay (α)
 releases alpha particle (a helium nucleus)
 a helium nucleus consists of 2 protons and 2 neutrons
 atomic mass is 4
 atomic number is 2
4
2He
 Ex:
238
92
U
234
90
Th +
4
2He

46. BETA DECAY
Beta Decay (β)
 release beta particle it occurs when a neutron breaks down
into 1 electron and 1 proton
the result is an atom with 1 more proton
Ex:
14
6
C
14
7
N+
0 e
-1

47. PRACTICE: ALPHA & BETA DECAY
214
84
222
86
214
82
234
92
Po
Rn
Pb
U
210
82
Pb ______
______
4
2
______
0
1
234
93
Np
He
e
______

48. NUCLEAR REACTIONS
 FISSION
 process of splitting a nucleus into several smaller nuclei

49. NUCLEAR REACTIONS
FUSION:
 Two nuclei with low masses are combined to form one
nucleus of larger mass

50. HALF LIFE
half-life
 the amount of time it takes for
half the nuclei in a sample of the
isotope to decay
the nucleus left after the
isotope decays is called the
daughter nucleus
some half-lives are seconds,
others are millions of years

51. EXAMPLE: HALF LIFE
 Assume a 20g sample of Ba-139 has a
half-life of 86 minutes.
how much Ba-139 remains after 86 minutes?
 after 172 minutes?
 how many ½ lives leave 1.25g of Ba-139?
