QUALITATIVE ANALYSIS

• PRE-LAB

2

Students will be able to: • Describe how the reaction coordinate can be used to

predict whether a reaction will proceed including how the potential energy of the system changes.

• Describe what affects the potential energy of the particles and how that relates to the energy graph.

• Describe how the reaction coordinate can be used to predict whether a reaction will proceed slowly, quickly or not at all.

• Use the potential energy diagram to determine: –The approximate activation energy for the forward and

reverse reactions. –The sign difference in energy between reactants and

products. • Draw a potential energy diagram from the energies

of reactants and products and activation energy.

Learning Goals

Up to now...• We have been able to determine reaction rates from

our study of Chemical Kinetics• We know how to find the equilibrium constant for a

reaction at a fixed temperature.–REMEMBER:

(1) Equilibrium means that the reaction has reached a point where the rate of converting reactants to products is equal to that of the rate of converting products to reactants; and,

(2) that both reactants and products are maintaining constant concentrations

Up to now...

• We can tell what direction the reaction will proceed by comparing the ratio of concentrations between reactants and products (Qc) to the equilibrium constant (Kc)

What Don't we know at this point?

How do you speed up or slow down a reaction?

• FIRST, we must look at the reaction process?The activation Energy can be a thought of as a barrier to any reaction.

We need something to happen to get over this hump...

to do this requires:- a collision, or...- a sudden increase in temperature, or...- a higher concentration of reactants, etc., etc., etc...WE NEED SOMETHING!

Factors affecting reaction rates

Factors Affecting Reaction Rates• 1. Chemical Nature• 2. Concentration• 3. Pressure• 4. Temperature• 5. Surface Area• 6. Catalysts

FACTOR 1

FACTOR 2

FACTOR 3• A change in pressure (or volume) will result in

an attempt to restore equilibrium by creating more or less moles of gas.

• For example, if the pressure in a system increases (or the volume decreases), the equilibrium will shift to favor the side of the reaction that involves fewer moles of gas.

• Similarly, if the volume of a system increases (or the pressure decreases), the production of additional moles of gas will be favored

FACTOR 4

FACTOR 4

FACTOR 5

FACTOR 6

FACTOR 6

16

Which reaction would probably appear to be quickest?

A B C D

17

A.Increase the number of & B. Increase the number of &C. Have no effect

Using the heater would

18

What would best describe what is in the container after several minutes have passed ?

19

Answer choices

A. Container will have mostly & B. Container will have mostly &C. Container will have a mixture of all four

with nearly equal amountsD. No reaction will occur since the products

and reactants have the same energy

Post-Lab

• Chemical reactions occur as a result of collisions between reacting molecules.

• Collision Theory of chemical kinetics...

rate ὰ (number of collisions)/second

• The rate of a reaction is directly proportional to the number of molecular collisions per second

BUT...• An activation energy is require

d to start a reaction.

When the potential energy of the products is greater than the reactants, ΔH is positive. Positive means heat or energy is absorbed during the reaction.

The stability of reactants and products are determined by how easily they can crest the hump of the "activated complex."

ΔH is the difference between the final potential energy of the products and the initial potential energy of the reactants

interpreting energy diagrams• An activation energy is require

d to start a reaction.

When the potential energy of the reactants is greater than the products, ΔH is negative. Negative means heat or energy is released during the reaction.

The stability of reactants and products are determined by how easily they can crest the hump of the "activated complex."

ΔH is the difference between the final potential energy of the products and the initial potential energy of the reactants

Stability• An activation energy is requ

ired to start a reaction.

• The endothermic reaction on top has very stable reactants because it takes a lot of activation energy to start the reaction.

• It takes less energy to have the reaction go from right to left

• The reactants are more stable!

Stability• An activation energy is required to st

art a reaction.

• The opposite is true for exothermic reactions. The distance between the starting potential energy and the crest of the "activated complex" is much less than that of the potential energy of the products. It will take more activation energy to get the reaction to go to the left than it would to get the reaction to go to the right.

• The products are more stable!