Chemical Reactions —the means by which elements and compounds form different substances

in living systems: metabolism
in industry: production of food, cloth, materials, fertilizers…

Chapter story: recovery from methanol poisoning with ethanol
(Reminiscent of the recovery of CO poisoning with oxygen)

The speed of an object: Change in distance per unit of the time,
e.g., mile/hour, cm/s, m/s, km/h, etc.

"Rate": Change of "something" with respect to "time"
e.g., food rotting, radionuclides decay, draining a tank, etc.

Chemical reaction ratethe amount of reactants react in a given time period to form products (or the amount of products produced in a given time period) or the change in reactant (or product) concentration per unit of time, which is studied with "Chemical kinetics".

    e.g., A ® B

"Average Rate" = moles of C appearing/elapsed time = – (moles of A disappearing/elapsed time)
                      = D [B]/s = – D [A]/s

If 2.5 mol reactant is used up in 10 minutes, then the average reaction rate is 2.5 mol/10 min = 0.25 mol/min, or 4.17 x 10–3 mol/s.

If 5 g of a reactant (molecular mass 20 g/mol) is used up in 5 minutes, what is the average reaction rate of this reaction in mol/s?

What are the factors that affect chemical reaction rate?
Answer: Activation energy, temperature, catalyst, concentrations, particle size, and surface area

Reactive collision: Chemical reaction takes place only when two reactants collide under the right conditions. The higher the temperature the more reactants with kinetic energy above the activation threshold (the activation energy) to form products upon effective collision.

  Figure 8.2

Activation energy: The minimum energy the colliding particles must have in order to react, i.e., Ea in the bottom two figures.

Exothermic reaction
Heat is generated (i.e., given away by the system) in a reaction.  Exo: out, away...; therm:  heat

Endothermic reaction
Heat is needed (i.e., taken by the system) in a reaction.  Endo: in, inside...

Effect of temperature—increasing temperature to overcome the activation energy barrier and to increase entropy
The influence of temperature on biological processes: ectothermic animals, thermophilic bacteria and archea, polar fish…

Effect of catalysts—to lower the activation energy
e.g., vegetable oil + H2 ® no reaction!
       vegetable oil + H2 + catalyst ® margarine!

Catalysts do not alter the results of a reaction, but only increase the speed.

Catalysis is what enzymes are doing, in which the rate acceleration can even reach >10 billion times! In the case of urea hydrolysis by urease, the rate acceleration reaches 10 trillion times!!

Effect of concentration—increasing colliding frequency
What’s going to happen when cigarette in put in pure oxygen?


Chemical equilibrium
Reversible reactions: The conversion of reactants into products and the conversion of products back into reactants occurs simultaneously. (Why should this occur? Any thing related to energy?)

Example: What the difference/similarities in the following two reactions?
1) 2SO2(g) + O2 ® 2SO3 at 725 °C
2) 2 SO3 ® 2SO2(g) + O2 at 725 °C

Chemical equilibrium—a dynamic state in which the forward and the reverse reactions are taking place at the same rate, but without change in the actual amounts of reactants and products in the system.

  There is no change in [A], [B], and [C] at equilibrium!

What will happen when one of the compounds is added to the above equilibrium A + B h C?

Equilibrium constant (Keq): a value that relates the relative amounts of reactants and products in a chemical reaction at equilibrium.

aA + bB h cC + dD         Keq = [C]c[D]d/[A]a[B]b

[A], [B], [C], and [D]: concentration of A, B, C, and D

The larger the Keq value, the larger the amounts of products relative to reactants at equilibrium.

Write and calculate an equilibrium constant (Examples 8.3–6)
The question in "8.2 Chemistry in Depth, P. 216" is more challenging for students who wants to know equilibrium constant in more detail!

Le Châtelier's principle: If an equilibrium system experiences a disturbance, the system changes to relieve the stress.
e.g., If pressure, heat, additional amount of reactants/products are applied at equilibrium, the system will decrease those disturbances.

Change in concentrations in the equilibrium (What’s going to happen if more CO2 (like in soda cans), water, and H2CO3 are added, respectively, to the following equilibrium?
        CO2 + H2O h H2CO3 (carbonic acid)

Change in temperature in the equilibrium:
        CaCO3(s) + 158 kJ h CO + CO2
        2SO2 + O2 h 2 SO3 + heat

Change in pressure (and temperature)
        N2 + 3H2 h 2NH3 + 98kJ

Industrial production of NH3: high pressure and relatively low temperature (but high enough to overcome the activation energy!!)
        N2O4(g) h 2NO2(g)