IB Chemistry: Energetics HL

Kinetic Energy

The energy of motion

Potential Energy

Stored energy

Heat

Is the measure of total energy in a substance and depends on the substance present.

Temperature

Is a measure of average kinetic energy of particles in a substance

Enthalpy

The amount of energy locked up inside chemicals. Substances with lower enthalpy are more stable then those with higher.

Enthalpy Change

Is the amount of heat absorbed or released in a reaction. Comes from bonds breaking and forming.

Standard Conditions of Heat (Kelvin)

298K

Endothermic Reactions

A reaction that absorbs heat and makes outside feel colder so the products have more heat (energy) then the reactants. ΔH = +. Examples are Photosynthesis and cold packs

Endothermic Key points

Absorb Energy

Increase in enthalpy

Reactants more stable

Exothermic Reactions

A chemical reaction that releases heat to surroundings so the products have less heat (energy) then the reactants. ΔH = -. Examples are combustion and neutralization reactions.

Exothermic Key points

Give out energy

Decrease in enthalpy

Products are more stable

Enthalpy Level Diagrams (Endothermic)

ΔH is positive.

Activation energy is large.

Products less stable

Break stronger bonds make weaker ones.

Heat energy turned into chemical energy.

Enthalpy Level Diagrams (Exothermic)

ΔH is negative.

Activation energy is small.

Products more stable.

Break weaker bonds make stronger ones.

Chemical energy turned into heat energy.

Activation Energy

The minimum amount of energy that is required by the reactants for bonds to be broken and the reaction to proceed to the products

Specific Heat Capacity of Water

4.18

Equation of ΔH

q=mcΔT

q is heat (j)

m is the mass of the solution of water (g)

c is the specific heat capacity (Jg-1K-1)

ΔT is the change in temperature

Enthalpy of Combustion

The heat given out when one mole of a substance is burned in excess oxygen.

Enthalpy of Neutralisation

The heat change when one mole of H+ ions of an acid completely react with one mole of OH ions of a base.

Enthalpy of Formation

The heat change that occurs when one mole of a substance in its standard state is formed from the elements in their standard states.

Finding Enthalpy Change Method

1) Find Moles. Mass * Mr

2) Find ΔH

3) ΔH/moles

Bond Enthalpy

This is the enthalpy change required to break a covalent bond when all species are in their gaseous state.

ΔHrxn

ΣBonds broken - ΣBonds formed

Ozone

Absorbs UV radiation in our atmosphere. Blocks radiation which can cause damage to living tissues. The rate and formation of ozone is usually equal. CFC's and NOx's can disrupt the cycle destroying ozone and creating holes in the ozone layer.

Formation of Ozone

O2 -----> O + O

O + O2 -----> O3

Destruction of Ozone

O3 -----> O2 + O

O3 + O -----> 2O2

Bon Haber Cycle

Is an energy cycle for the formation of an Ionic compound

Lattice Enthalpy

Is the enthalpy change when one mole of solid ionic compound is broken down into gaseous ions under standard conditions.

Enthalpy of Atomization

Is the standard enthalpy change that occurs on the formation of one mole of seperate gaseous atoms of an element in its standard state.

Ionization Energy

Is the standard enthalpy change that occurs when 1 mole of electrons are removed from 1 mole of atoms or positively charged ions in the gaseous phase

Electron Affinity

Is the standard enthalpy change that occurs when one mole of electrons are added to one mole of atoms in their gaseous phase

Factors That Effect Lattice Enthalpy

High charge = Higher lattice enthalpy

High radius = Low lattice enthalpy

Enthalpy of Solution (ΔHsol)

The enthalpy change when one mole of an ionic substance dissolves in water to give a solution of infinite dilution.

Enthalpy of Hydration (ΔHhyd)

The enthalpy change when one mole of gaseous ions dissolve in water to give a solution of infinite dilution. (always exothermic)

ΔHsol =

ΔHlat + ΔHhyd

Factors that effect ΔHyd

Smaller the ion = the higher the ΔHhyd

The higher the charge = the higher the ΔHhyd

Entropy ΔS

The distribution of the amount of energy among the particles in the system. The amount of disorder in a substance.

Factors that Increase Entropy

Change of state from a liquid to a gas

Increased number of particles

Increase in the number of moles of gas

Factors that Decrease Entropy

Change of state from a gas to a liquid

Decrease in the number of particles

Decrease in the number of moles of gas

ΔS equation

ΔS(reaction) = ΣΔS(products) - Σ ΔS(reactants)

Spontaneous Reaction

ΔS(total) = ΔS(system) + ΔS(surroundings) > 0

ΔS(surroundings) equation

-ΔH(system)/T

Spontaneity

A process that occurs without adding energy. ΔG must be negative for a spontaneous reaction.

Gibbs Free Energy Equation

ΔG = ΔH - TΔS

ΔG = Gibbs free energy change measured in kJ mol

ΔH = Enthalpy change

T = Temperature in Kelvin

ΔS = Entropy change

Electron Affinity