SCH 4U1 - Quiz Ls 6 - 8 --> Rates &

Chemical Kinetics

Branch of chemistry concerned with the rates of chemical reactions

Rate

Change in a measurable quantity over time

Average rate of reaction

How much product is formed (or reactant used) in a given period of time

r

Rate of a reaction

q

Quantity (e.g., temperature, mass, pH, concentration, conductivity, colour)

t

Time (change in time)

Average rate during a certain period of time

Slope of a secant line drawn between those times (a line that touches two points on the curve)

Instantaneous rate (at a certain time)

Slope of a tangent line (a line that touches one point on the curve)

What determines if a collision is successful?

1. Proper orientation of particles when they collide

2. Need to have a minimum amount of kinetic energy

Activation Energy (E_a)

• The energy required to reach the transition state/stage.

• Bonds are being broken and reformed, and atoms are being rearranged.

• The intermediate state is called the activated complex.

Where is Activation Energy (E_a) measured?

From reactants to the activated complex

Where is enthalpy measured?

From reactants to products

Factors that increase the rate of reaction

• Temperature

• Concentration

• Pressure (for gas reactants)

• Catalyst

• Nature of the reactants

• Surface area

How does temperature increase reaction rate?

• increased # of molecules with the required Ea

• Increases average kinetic energy

• Increases speed of particles

• Increases frequency of collisions

How does concentration increase reaction rate?

• Increases frequency of collisions

• Only applies to aqueous (aq) and gaseous (g) reactants

How does surface area increase reaction rate?

• Increased surface area leads to increased collisions

• Example: Baking a whole potato vs. fries

Catalyst

• A substance that increases the rate of a reaction without being used up

• Requires less activation energy

• Increases the rate of BOTH forward and reverse reactions

How does the nature of reactants affect reaction rate?

• Ions typically have more rapid reactions.

• Molecules are usually slower.

How does the pressure of gas reactants increase reaction rate?

Increasing the pressure leads to increasing the frequency of collisions.