Chemical Reactions

1
Chemical Reactions

• Nature of Chemical Reactions

2
Characteristics of Chemical Reactions

• A chemical reaction is a process in which the
  physical and chemical properties of the original
  substances change as new substances with
  different physical and chemical properties are
  formed

3
Evidence Of Chemical Reactions

• Change in color
• Change in temperature

• Gas formation
• Solid formation

4

• Reactant Product
• Reactants changing into products
• Energy is either absorbed or released

5
Capacity to React

• It is the valence electrons that are involved in
  chemical reactions
• During a chemical reaction atoms can form
  molecules, molecules can break apart, or
  molecules can react with other molecules
• New substances are formed as bonds are broken,
  atoms are rearranged, and new bonds are formed

6
Conservation of Mass

• The Law of Conservation of Mass states that mass
  remains constant in a chemical reaction.. What
  goes in must come out!

7
Balancing Chemical Equations

• The meaning of chemical equations
• A mathematical equation x2x3x
• A chemical equation identifies the starting and
  finishing chemical as reactants and products
  reactants products
• Example (combustion of propane)
• C3H8 5O2 3CO2 4H20

8

• Unbalanced reaction
• H2 O2 H2O
• Balanced reaction, undetermined coefficients
• aH2 bO2 cH20
• Correct, balanced equation
• 2H2 O2 2H2O
• This reaction is balanced, but its coefficients
  are not least whole numbers
• 4H2 2O2 4H20

9

• Unbalanced equation
• NH3 O2 NO H2O
• Ammonia and oxygen yield nitric oxide and water.
• 4 NH3 5 O2 4 NO 6 H2O

10

• Another example of balancing
• Unbalanced equation
• PbO2 HCl PbCl2 Cl2 H2O
• Step 1 balance oxygen atoms
• PbO2 HCl PbCl2 Cl2 2 H2O
• Step 2 balance hydrogen atoms
• PbO2 4 HCl PbCl2 Cl2 2 H2O
• Step 3 balance chlorine atoms
• PbO2 4 HCl PbCl2 Cl2 2 H2O

11
Chemical Equations

• The number of atoms on the left side of the
  equations must equal the number of atoms on the
  right side of the equation.
• The equation must be balanced using coefficients
  - a number placed in front of the molecule or
  atom
• He H2O CO2
• four atoms nine atoms
  six atoms

A subscript is the number behind the chemical
symbol, it never changes.
12
Balancing

• H2 O2 H2O unbalanced
• Left side Right Side
• H2 O2 H2O balanced
• Left side Right side

H
H
H
H
O
O
O
O
H
H
O
H
H
O
H
H
O
H
H
13
Balancing Simple Equations

• Aluminium Oxygen Aluminium
  Oxide
• Al O2 Al2O3
• Left side             Right side
• Al O2 Al2O3
• 2 Al O2 Al2O3
• Left side Right Side
• Al O2 Al2O3
• Left side Right side

14
Chemical Reactions

• There are many types of chemical reactions.
• Each is related to the properties of
  combustibility and reactivity.
• In each type of reaction atoms are being
  rearranged and new substances are being formed

15
Combustion Reaction

• When oxygen reacts rapidly to release energy with
  the products of CO2 and H2O.
• CH4 2O2 CO2 H2O

16
Acid and Base Reaction

• Acid Base Salt H2O
• Remember this equation!

17
Energy of Chemical Reactions

• Exothermic reactions is one in which heat is
  released
• The original energy is stored in the molecules of
  the reactants because the products do not receive
  the energy, it is released as heat
• 2Na Cl2 2NaCl energy
• Exo three letters Hot

18

• Endothermic reactions is one in which energy is
  absorbed
• Energy is usually in the form of heat or light
• 2H2O energy 2H2 O2
• ENDO four letters Cold

19
Activation Energy

• It is the energy needed to begin the chemical
  reaction
• Striking a match to light it causes friction and
  this friction is the activation energy
• Ex friction, electric spark, light

20
Rates of Chemical Reactions

• Kinetics is the study of reaction rates
• Collision theory relates particle collisions to
  reaction rates and is affected by concentration,
  surface area, temperature, inhibitors, and
  catalysts

21
So How doChemical ReactionsHappen?
22
In Contact

• Reactions dont happen unless the substances are
  in contact.

23
Why?

• The particles of the reactants need to get
  together so that they can react.

24
How does that work then?
Reactant particles collide
REACTION
Product particles formed
25
Is it really that simple?

• Yes

Well, sort of.
not really.
OK, not quite!
26
Not all collisions are effective

• Paper burns
• Paper oxygen ? carbon dioxide water
  nitrogen
• The paper in this room isnt burning.
• It doesnt have enough energy to burn.
• If we make it hotter it will catch fire.
• Paper burns on its own at 250 ºC

27
A collision but with no effect
Reactant particles collide
Product particles not formed as there is not
enough energy
28
Activation Energy

• The amount of activation energy needed is
  different for each reaction.
• But
• Every reaction has activation energy, they all
  need a little push to get started.

• Reactions only happen if the particles have
  enough energy.
• The minimum amount of energy needed to start a
  reaction is called the Activation Energy

29
More than the activation energy.
Reactant particles collide
REACTION
Product particles formed
30
More than the activation energy.
C
Effective collisions, a reaction.
31
The Collision Theory

• Particles are constantly moving
• For a chemical reaction to take place the
  reactant particles must collide first
• For the collision to be effective the particles
  must have the right amount of energy
• The minimum amount of energy required for an
  effective collision is called the activation
  energy

32
Factors affecting rate

• Concentration
• Temperature
• Catalysts
• Surface Area

33
Concentration

• More particles in the same space means more
  collisions.
• More collisions means more effective collisions
• If we double the concentration we
  double the number of collisions

34
Temperature

• Particles turn heat energy into kinetic energy
• When they get hotter they move faster
• When they move faster they collide more often
• More collisions means more effective collisions

35
Catalysts
Activation energy with a catalyst

• Catalysts reduce the activation energy needed for
  a reaction
• They do this by offering an alternate route for
  the reaction to take
• Less activation energy means more effective
  collisions
• More effective collisions means faster rate

36
Surface Area/Particle Size

• Using smaller particles increases rate
• Increase in surface area allows more collisions
  at surface
• More collisions means more effective collisions
  means faster rate.

37
Collision Theory

• Concentration- the more concentrated a substance
  is the faster it will react
• An increase in surface area will increase the
  rate of a reaction
• An increase in temperature will increase the rate
  of a reaction

38
Inhibitors

• Slows down or stops a chemical reaction
• Ex preservatives and antibiotics

39
A catalysts is a substance that increases the
rate of reactions, but does not react with the
reaction ( it may just decrease the amount of
energy needed for a reaction to begin) ex
enzymes