Chem12- Reaction Rates

Unit 1

Formula for a reaction rate

amount/time

Reaction Properties

Colour Change,(colour intensity/time)

Temperature Change, (temperature/time)

Pressure Change, (pressure/time)

Mass Change (mass/time)

Factors affecting reaction rates

Temperature: As temperature INCREASE, rate INCREASE

Concentration: As (reactant) concentration INCREASE, rate INCREASE

Pressure: As (gaseous) reactant pressure INCREASE, rate INCREASE

Nature of Reactants: More bonds present, rate DECREASE

Surface Area: As surface area INCREASE, rate INCREASE

Catalyst: As catalyst is added, rate INCREASE

Inhibitor: As inhibitor is added, rate DECREASE

Fastest to slowest(phases)

(aq) > (g), (l) > (s)

Collision theory

Molecules bounce off each other and transfer energy(during collisions)

(# collisions/second INCREASE = Reaction rate INCREASE)

To break a bond(bond energies)

Energy equal to a bond energy must be ADDED

To form a bond(bond energies)

Energy equal to a bond energy must be RELEASED

What is enthalpy in terms of energy, in a system?

H

KE+PE in a system, at constant pressure

How can “H” be calculated?

H= H(prod)- H(react)

How can “H” tell us if a reaction is endo or exothermic?

Endothermic if H > 0

Exothermic if H < 0

Where would we place an “H” value in a balanced equation?

If “H” is (+), LEFT hand side

If “H” is (-), RIGHT hand side

Energy distribution proof?

At a given temperature, some molecules REACT and some DONT. Therefore there MUST be a CONTINUOUS DISTRIBUTION of ENERGIES among the molecules.

What is the relationship between average energy and temperature?

As average energy INCREASES, temperature INCREASES. Therefore more molecules have the ME required to react, so rate INCREASES.

****Increased reaction rate is due to INCREASE in molecules and not so much the # of collisions****

What is an activated complex?

An arrangement of atoms when reactants are in the process of forming products (re -arrangement)

What is activation energy? (Ea)

Minimum PE required to change the reactants into the Activated Complex

How can molecules surpass the “barrier” made by the required activation energy?

Molecules must gain enough PE

(PE is gained by converting KE gained into PE and adding this to their existing energy)

\-this allows for bonds to be made/broken

\-this allows for the formation of the Activated Complex

When two molecules approach each other, spending their KE to buy more PE, what are three cases that can occur?

Case 1: KE is less than needed to buy an amount of PE equal to the activation energy (INEFFECTIVE COLLISION)

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Case 2: KE(total) = ME

(KE is spent, reaction is possible, but not guaranteed)

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Case 3: KE(total) > ME

(KE is spent, reaction is possible, reacting molecules with be locked in and forced to move as a unit)

How can total energy change?

Total energy NEVER changes.

KE + PE = Constant

Is activation energy endo/exo ?

Activation energy is always ENDOthermic

\-energy must be added to get on the top of the energy hill

What is Ea(f) and Ea(r)?

Ea(f) = activation energy for a FORWARD reaction

Ea(r) = activation energy for a REVERSE reaction

Formula for endothermic reactions?

Ea(f) = Ea(r) + H

**H > 0**

Formula for exothermic reactions?

Ea(f) = Ea(r) + H

**H < 0**