Unit 2 - Energy Changes, Thermochemistry & Rates of Reaction

Bond dissociation energy (BDE) equation

Bonds BROKEN - Bonds FORMED or…

-bonds FORMED + bonds BROKEN

• Finds total energy change

Sublimation

Solid to gas

Desublimation

Gas to solid

Fusion

Solid to liquid

Solidification

Liquid to solid

Why doesn’t temperature change when reactant is changing states?

Energy goes into breaking intermolecular bonds between particles instead of increasing kinetic energy

Calorimeter constant equation

C_cal = q_cal / 𝝙T

Gibb’s Free Energy Equation

𝝙G = 𝝙H - T𝝙S (kJ or J)

Entropy of SURROUNDINGS equation

-𝝙H/T (J/K)

What 𝝙G value signifies a spontaneous reaction? What does a 𝝙G of 0 mean?

• When 𝝙G < 0, rxn is spontaneous

• When 𝝙G = 0, rxn is at equilibrium; neither occuring in forward or reverse directions

Entropy equation

S = k ln W (J/K) where:

• S = entropy

• k = Boltzmann constant

• W = microstates

Is the 𝝙S greater, lesser, or equal to zero?

𝝙S < 0

What factors increase entropy and why?

• Volume - more spatial configurations, more possible ways to distribute E

• Temperature - more kinetic E, more possible ways it can be partitioned

• # of molecules (MATTERS MORE THAN COMPLEXITY) - more molecules, more possible arrangements in space

• Complexity - more complex molecule, more degrees of freedom, more microstates

Enthalpy of formation definition

Energy change when one mol of product is generated from its elements in standard state.

Bond Energy definition

Energy required to break one mol of bond within a gaseous substance.

What is suggested about the entropy and enthalpy of this rxn if it’s spontaneous?

Entropy:

• 𝝙S approx equal to 0 or a little larger than zero

• Molecules amount or complexity does not change dramatically

Enthalpy:

• 𝝙H likely negative (exothermic)

• Probably largest contributing factor to spontaneity of this rxn

What is a reaction that is spontaneous where 𝝙G < 0 called?

EXERGONIC

What is a reaction that is not spontaneous where 𝝙G > 0 called?

ENDERGONIC

What factors affect rate of reaction?

Relating to collision theory:

• Concentration (more particles, more successful collisions, faster rxn)

• Surface Area (more reactant particles "exposed” to collide, faster rxn)

• Temperature (more kinetic E, more high E collisions, chance particles surpass E_A)

• Catalyst (lowers E_A, faster rxn)

Collision theory

For a rxn to occur, particles must collide with the proper:

• Energy (greater than E_A)

• Orientation (collision geometry)

Average rate formula

Slope of concentration-time graph:

• -(𝝙[A] / 𝝙T)

negative bc 𝝙[A] is negative, but rate is traditionally positive.

Rate law equation

Rate = k[A]^x[B]^y

where:

• Rate in mol/(L x S)

• k = rate constant

• [A] and [B] = reactant concentration (mol/L)

• x and y = reactant order, NOT stoich coeff

Why does the rate constant k depend on temperature?

Define rate law and units of k for this reaction, where A + B → C

Rate law: rate = k[A]¹[B]²

k = L²/(mol² x S)