IB Chem Reactivity Mix

Heat

A measure of the total energy of a substance and therefore depends on the amount of substance present.

Always transfers/flows from a higher temperature object to a lower temperature object.

Absolute temperature (K)

A measure of the average kinetic energy of the particles in a substance.

Exothermic reaction

Heat flows from the system to the surroundings.

The temperature of the reaction mixture and the surroundings increases as heat is released.

ΔH = negative < 0

Endothermic reaction

Heat flows from the surroundings into the system.

The temperature of the reaction mixture and the surroundings decreases as heat is absorbed.

ΔH = positive > 0

Enthalpy (H)

The heat absorbed or released in a chemical reaction that takes place at constant pressure.

Can be thought of as the heat content if a substance.

Cannot be measured.

Enthalpy change (ΔH)

Can be measured.

The change in enthalpy that occurs when a chemical reaction takes place.

q=mcΔT

q - heat (J)

m - mass (g)

c - specific heat capacity (J g^{-1 o}C^-1)

ΔT - temperature change

Average bond enthalpy

The energy required to break one mole of bonds in a gaseous molecule averaged over similar compounds.

ΔH=∑(bonds broken) - ∑(bonds formed)

(ΔH=reactants - products)

Enthalpy change of formation

One mole of a compound is formed from its elements in their standard states under standard conditions.

ΔH^Ꝋ=∑ΔH_f^Ꝋ(products) - ∑ΔH_f^Ꝋ(reactants)

Entropy (S)

The distribution of available energy among the particles in a system.

Standard entropy change (ΔS^Ꝋ)

ΔS^Ꝋ(reaction) = ∑ΔS^Ꝋ(products) - ∑ΔS^Ꝋ(reactants)

Spontaneous process

A process that occurs without adding energy (other than the energy required to overcome the energy barrier).

The change in entropy of ΔS(total) must be positive.

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

Gibbs free energy (ΔG)

ΔG=ΔH-TΔS

Spontaneous reaction: -ΔG

Non-spontaneous reaction: +ΔG

Atom economy

% atom economy = molar mass of desired product / molar mass of all reactants x 100

The higher the atom economy for a chemical reaction, the less waste is produced and the more efficient the reaction is.

Gibbs free energy change (ΔG^Ꝋ)

ΔG^Ꝋ=∑ΔG_f^Ꝋ(products) - ∑ΔG_f^Ꝋ(reactants)

Data booklet: Table 12

Elements have a standard Gibbs free energy of formation value of zero

ΔG^Ꝋ = -RT ln K

ΔG^Ꝋ = standard change in Gibbs free energy in J

R = universal gas constant

T = temperature in kelvin

ln K = natural log of K

Rate of reaction

Unit: mol dm^-3 s^-1

Increase in product conc. over time OR decrease in reactant conc. over time

Factors that affect the rate of reaction

• Temperature

• Concentration

• Particle size / surface area

• Pressure

• Catalysts

Activation energy: exothermic reaction

Activation energy: endothermic reaction