Unit 2: Chemical ReactionsUnderstanding Chemical Changes

LG: I can describe the condition for a chemical reaction to occur and I can represent chemical reactions using word

and chemical equations.

Recall: Signs of Chemical Change• What clues tell you a chemical reaction

has occurred?

Gas Produced

Energy Absorbed or Released

Precipitate Formed

New Colour

Representing Chemical Changes• Example: Cellular respiration is an important process

where glucose is broken down by oxygen to produce carbon dioxide and water.

Word Equation:Glucose + Oxygen Carbon Dioxide + Water

Chemical Equation:C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l)

Reactants Products

Kinetic Molecular Theory• KMT explain the behavior of particles in a substance.• KMT states:– All substances are made of particles that are in

constant motion– The particles constantly collide with each other and

the surroundings– As temperature increases, particles move faster,

increasing the rate of collisions

Collision Reaction Theory

• CRT states:– Particles must collide with the correct orientation

and sufficient energy to cause a chemical reaction• Correct orientation allows new bonds to form

simultaneously• Sufficient energy required to overcome valence shell

repulsion

• KMT and CRT explain why chemical reactions are affected by factors such as changes in temperature and pressure.

PEOE: Burning Methanol• Question: Will liquid methanol or gaseous

methane react faster with oxygen?– PREDICT!– EXPLAIN!– OBSERVE!– EXPLAIN!

Factors Affecting Reaction Rates1) Temperature – As temperature increases, particles

move faster, increasing the frequency and the energy of collisions

2) Concentration – more concentrated substances contain more reactant molecules, increasing the frequency of collisions

Factors Affecting Reaction rates3) Pressure – decreasing the size of a container,

increases the amount of reactant per unit volume, resulting in more frequent collisions

4) Surface Area – When particles are separated from one another, more particles are available to collide with other reactant molecules

Factors Affecting Reaction Rates5) Nature of Reactants – State reactants (solid, liquid,

or gas), and the strength of inter- and intramolecular forces determine how easily they react

6) Catalysts – chemicals that hold reactant molecules in the correct orientation, to reduce the amount of energy required for a reaction; the catalyst itself is not used up in the reaction

2 H2O2 2 H2O + O2

MnO2

Factors Affecting Reactions Rates

• http://www.youtube.com/watch?v=OttRV5ykP7A

Homework!• Complete Handout & attempt to balance

equations in #5