Unit 10B - Bond Enthalpy and Enthalpy of Reactions

latent heat

the amount of heat energy required to push a substance through a phase change (overcome intermolecular forces)

latent heat equation

q = mΔH

ΔHf

heat of fusion (energy required to melt 1g of ice)

= 334 J/g or 79.9 cal/g

ΔHv

heat of vaporization (energy required to boil 1g of water)

= 2260 J/g or 540 cal/g

heat capacity (C)

the amount of energy required to increase the temperature of 1 g of a substance 1 degree Celsius

Cice

2.05 J/gºC or 0.5 cal/gºC

Cwater

4.18 J/gºC or 1 cal/gºC

Csteam

2.01 J/gºC or 0.48 cal/gºC

phase changes (when to use latent heat vs. heat capacity)

slope = changing temperature

no slope = phase change

when reactants form products

energy is released

when products are broken into reactants

energy is taken in

chemical bond

when two atoms are joined through the sharing of electrons

enthalpy (ΔH)

stored (potential) chemical energy in the form of heat

state function

a function (variable) whose value depends only on the current state of the system, and whose value is independent of the pathway

enthalpy is an example of a state function

G

gibbs free energy (overall energy)

H

heat

S

order

equation for gibbs free energy

ΔG = ΔH - TΔS

bond energy

the energy required to break a chemical bond

homolytic dissociation

electrons are equally split between two atoms, electrons in the bonding pair “move” independently

(bond energies are lower than heterolytic dissociation)

heterolytic dissociation

electrons are not equally split between two atoms, electrons in bonding pair “move” together

(bond energies are higher than homolytic dissociation)

exothermic

system releases more energy than it takes in

endothermic

system releases less energy than it takes in

enthalpy is positive when

breaking a bond, endothermic, putting energy in

enthalpy is negative when

forming a bond, exothermic, releasing energy

enthalpy of formation

ΔHrxn for the formation of a compound from its elements in their natural state

the enthalpy of formation of any pure element in its natural state

0