Reaction Rates

What is the definition of a reaction rate?

A change in reactant or product concentration over a given time interval

Are reaction rates constant?

• They are NOT constant; they change as the reaction proceeds

How do we express reaction rates in words? what can we express them in terms of?

• Are always expressed as POSITIVE values

• Can be expressed in terms of any reactant or product

How can reaction rates be determined?

• Can be determined by monitoring changes in pressure, volume, colour, mass, pH or conductivity 

What factors affect reaction rates?

• CHEMICAL PROPERTIES

• PHYSICAL PROPERTIES

• CONCENTRATION

• SURFACE AREA

• TEMPERATURE

• CATALYST

What does Collision Theory state?

“Chemical reactions can occur only if reactants collide with proper ORIENTATION and with enough KINETIC ENERGY to break reactant bonds and form new product bonds”

What does Collision Geometry state?

• Reacting particles MUST collide with the correct orientation

Define an “effective collision”.

• one in which reactants are converted to product

Define Activation Energy.

• The minimum energy that reactant molecules must possess for a reaction to be successful

What is activation energy used for?

• It is used to overcome the electrostatic repulsive forces between colliding entities

• It is used to weaken bonds of the reactants

What is an activated complex? What is another name for it?

• The unstable arrangement of atoms containing partially formed and unformed bonds

• Also called the TRANSITION STATE

What does an activated complex represent?

• Represents the maximum energy point in the change

• Activation energy represents the energy difference between the reactants and the activated complex

Does the rate of reaction depend on the physical properties and chemical nature of reactants?

• Yes

How does temperature affect the rate of reaction?

• An INCREASE in temperature increases the rate of reaction

• Increasing the temperature will increase the kinetic energy of the reactants

• The reactants will have a quantity of kinetic energy that is the SAME or GREATER than the activation energy

• The number of collisions will increase

How does surface area affect the rate of reaction?

• For reactants that are in different states of matter, an INCREASE in reactant surface area increases the rate of reaction

• Increasing the surface area increases the total number of collisions which increases the total number of EFFECTIVE collisions

What is a catalyst and what does it do?

• A substance that alters the rate of a chemical reaction without itself being permanently changed

• Provides an alternative pathway for the reaction which has a LOWER activation energy
  
  

What are the three different types of catalysts?

• BIOLOGICAL: produced by a living system (e.g. enzymes)

• HETEROGENEOUS: exists in a different state of matter from the other reactants

• HOMOGENEOUS: exists in the same phase as the reactants

Does a catalyst lower activation energy?

• No

• It provides an alternative pathway for the reaction which has a LOWER activation energy; it might add intermediate steps if it is too energy intensive to go directly to the product, things like that.

What DONT catalysts do?

• DO NOT increase the number of collisions between reactants

• DO NOT increase the kinetic energy of the reactants

• DO NOT affect the energy difference between products and reactants

Recall the average rate formula.

What does [ X ] indicate?

• Indicates the entity’s (X’s) concentration in mol/L. [ ] just means concentration

What are some rules to keep in mind for the average rate formula?

• A NEGATIVE sign indicates the CONSUMPTION of a reactant

• A POSITIVE sign indicates the PRODUCTION of a product

• The numerical value of the reaction rate will ALWAYS be positive

The average rate is represented by the slope of what type of line?

• A SECANT line

What is a secant?

• A line that intersects two points on a curve

We draw secants on the ______ of a curve.

• Inside

We draw tangents on the _______ of a curve

• Outside

What is an instantaneous rate of reaction?

• The rate of a  chemical reaction at a single point in time

What is a tangent?

• a straight line that touches a curve at a single point and does not cross through the curve.

The instantaneous rate at any time t is equal to the _____ of the tangent to the curve at that time.

• slope

What 2 points can we use to calculate the instantaneous rate of reaction?

• The specific point you want and another point on its line

• 2 different points on the line, passing through the specified point

• As long as the 2 points are on the tangent line, they work, because the slope is specific to that point on the curve and not another.