Chemistry 12
Chapter 6
Collision theory
- explains rates on the molecular level
- before molecules can react, they must collide.
H2
+
I2
2HI
+
first
later
successful collision ( reaction )
later still
How collision theory explains:
1. Effect of concentration
low conc. both
low chance
of collision
(slow reaction)
high conc. blue
low conc. red
high conc. both
higher chance
of collision
(faster reaction)
very high chance
of collision
(much faster reaction)
2. Effect of temperature
- when molecules move faster  more collisions per unit time  faster rate
- also - when they move faster they collide with more kinetic energy. (hit harder)
Chemistry 12
Chapter 6
Enthalpy (H) & enthalpy change (H )
Enthalpy
- the “heat content” of a substance
or - the total KE & PE of a substance at const. pressure.
Chemists interested in enthalpy changes (H )
Exothermic reaction energy is released into the surroundings. The environment will get
warmer and the reaction will be colder.
If the reaction gets colder then it lost heat so the H is negative
H + H → H2 + heat
H+H
Enthalpy (H)
Heat is released to surroundings.
Exothermic H is negative (-)
H
H2
Reaction Proceeds
Endothermic reaction energy is absorbed from the surroundings. The environment will get
colder and the reaction will be warmer.
If the reaction gets warmer then it gained heat so the H is positive
O2 + Heat → O + O
O + O
H
Enthalpy (H)
O2
Heat is absorbed from the surroundings.
Endothermic H is positive (+)
Reaction Proceeds
Chemistry 12
Chapter 6
Equations and heat
H = - 20 KJ
( -ive H means exothermic)
 shown H2 + S → H2S
6C + 3H2 → C6H6
H = + 83 KJ ( +ive H means endothermic)
beside
Thermochemical equations:
(“Heat Term” is in the equation. NO “H” shown beside the equation!)
- “heat term” shown on left side of arrow - endothermic (“it uses up heat like a reactant”)
eg.
CH3OH + 201KJ  C(s) + 2H2(g) + ½ O2(g)
-“heat term” shown on right side of arrow -exothermic ( “it gives off heat like a product”)
eg.
S(g) + O2(g)  SO2(g) + 296 kJ
pg 119 # 70 - 80