Chemistry: Chemical Energetics

What is enthalpy(H)?

The total heat content of a substance or system that is held a constant pressure

How do we calculate enthalpy change (ΔH)?

Change in H = Hproducts - Hreactants

What is the unit for change in H?

kJ/mol

What is an exothermic reaction?

A reaction that releases heat to the surroundings (ΔH<0)

What is an endothermic reaction?

A reaction that absorbs heat from the surroundings (ΔH>0)

Why are exothermic reactions more energetically favoured than endothermic reactions?

A lower enthalpy would mean a more stable system.

A higher enthalpy would mean a less stable system.

What is the relationship between the magnitude of ΔH & amount of reactant consumed in an equation?

They are DIRECTLY proportional

Formula to find heat change in the reaction

mcΔT

• m: mass of SOLUTION (in grams or cm³)

• c: specific heat capacity

• ΔT: change in temp. (final - initial temp)

Units for heat change formula

J g^-1 K^-1

Formula for enthalpy change of reaction

±(mcΔT/no. of mols of limiting reagent) x coefficient of limiting reagent

Define the standard enthalpy change of combustion

It is the energy released when one mole of a substance is completely burnt in oxygen under standard conditions of 298K & 1 bar.

Is standard enthalpy change of combustion an exo or endo reaction?

exothermic; heat is always involved in combustion

How do we calculate the standard enthalpy change of combustion?

ΔH°c = mcΔT/no. of mols of substance burnt

How do you calculate y efficiency of ΔH°c?

(mcΔT/no. of mols of substance burnt) x (100/y)

Define the standard enthalpy change of neutralisation (ΔH°n)

It is the enthalpy change when one mole of water is formed in the neutralisation between an aqueous acid and an alkali under standard conditions of 298K & 1 bar.

Is ΔH°n an exo or endo reaction?

Exothermic; for strong acids & alkalis

Formula for ΔH°n

ΔH°n = mcΔT/no. of mols of water formed

What is the ΔH°n for strong acids & bases?

-57.3 kJ/mol

Why is the ΔH°n for strong acids & bases constant?

Strong acids & bases dissociate in dilute solutions to form H= & OH- ions; which is the formation of water, thus the ΔH°n is more or less constant.

Why is the ΔH°n for weak acids & bases not constant (deviates from -57.3 kJ/mol)?

Weak acids & bases can only dissociate partially in dilute solutions.

Some heat released from the neutralisation is absorbed to ionise the weak acid/base completely; producing less heat & is less exothermic overall.

Define the bond energy of dissociation

It is the energy required to break one mole of a covalent bond between two atoms in the gaseous state under standard conditions of 298K & 1 bar.

Is bond breaking an exo or endo reaction?

Endo; energy must be absorbed to break the bonds

Is bond formation an exo or endo reaction?

Exothermic; energy is released when a bond forms between two atoms

Is the bond energy of dissociation an exo or endo reaction?

Endo; energy is required to break 1 mole of a covalent bond between two atoms int he gaseous state

Why are the bond energies in the Data Booklet said to be not accurate?

They are merely approximations of average bond energies & are not the true reflection of actual bond energies in the molecules

Define standard enthalpy change of formation

It is the enthalpy change when 1 mole of substance is formed from its elements under stp of 298K & 1 bar.

Is ΔH°f an exothermic or endothermic reaction?

exothermic; the substance has lower heat content than its elements and hence is energetically more stable than its constituent elements.

Define Hess’ Law

Th enthalpy change of a chemical reaction that depends only one the final & initial tates of the system & is independent of the reaction pathway taken.

Formula for Hess’ Law when working with ΔH°f

ΔH°r = (sum of ΔH°f of products) - (sum of ΔH°f of reactants)

Formula for Hess’ Law when working with ΔH°c

ΔH°r = (sum of ΔH°c for reactants) - (sum of ΔH°c for products)

Define the standard enthalpy change of hydration (ΔH°hyd)

The enthalpy change when one mole of gaseous ions is dissolved in such a large amount of water that the addition of more water produces no further heat change under stp of 298K & 1 bar.

Is ΔH°hyd an exothermic or endothermic reaction?

Exothermic; bond formation — ion-dipole attractions are being formed, thus heat is evolved

What is the relationship between the hydration energy of an ion & the ionic charge/radius of the ion

They are directly proportional to each other

ΔH°hyd ∝ charge(q)/ radius

The higher the charge & smaller the radius, the more exothermic the reaction

Define the standard enthalpy change of solution (ΔHsol°)

The enthalpy change when one mole of substance is dissolved in such a large volume of solvent that the addition of more solvent produces no further heat change under stp of 298K & 1 bar.

Is ΔHsol° an exothermic or endothermic reaction?

Both; depends on the nature of the reaction

By Hess’ Law, what is the equation that is used to represent energy cycle

ΔHsol° = -lattice energy + ΣΔHhyd°

Define Lattice Energy

The lattice energy of an ionic solid is the energy evolved when one mole of the ionic solid is formed from this separate gaseous ions

Is lattice energy an exo or endo reaction?

Exothermic; heat is released when the ions come together to form ionic bonds

The more energy released, the more exothermic the reaction is & the stronger the ionic bonds

Define the standard enthalpy change of atomisation (ΔHat°)

It is the enthalpy change when one mole of separate gaseous atoms is formed from its elements under stp of 298K & 1 bar.

Is ΔHat° an exo or endo reaction?

Endothermic; energy is absorbed to poll the atoms far apart from each other & to break the bonds between them

What is the difference between the enthalpy change of vaporization & atomisation?

• ΔHvap° — breaking of intermolecular bonds

• ΔHat° — breaking of inter & INTRAmolecular bonds

What is the ΔHat° of noble gases?

0

Why is the ΔHat° of noble gases 0?

Th elements already exist in the form of separate gaseous atoms under standard conditions

Define ionisation energy (I.E)

The energy required to remove one mole of electrons from one mole of gaseous atoms (or ions) to produce one mole of gaseous CATIONS.

Is I.E an exothermic or endothermic reaction?

Endothermic; energy is absorbed in ionisation

Define electron affinity (E.A.)

The energy change when one mole of electrons is added to one mole of gaseous atoms to produce one mole of gaseous anions.

It is the measure of the attraction of the atom for additional electrons.

Is electron affinity an exo or endo reaction?

• First E.A: Exothermic; the incoming e- is attracted to the nuclear charge of the atom.

• Second E.A: Endothermic: heat must be absorbed because the second electron added must have enough energy to be able to overcome the repulsion from the negatively charged anions.

Define Entropy (S)

It is the measure of disorder of the system; the extent to which particles & energy are distributed within the system

(high entropy means high degree of disorder in the system)

How do we calculate change in entropy (ΔS)?

ΔS system = ΔSproducts - ΔSreactants

ΔS>0 — increase in disorder

ΔS<0 — decrease in disorder

Units for ΔS

kJ K-1 mol-1

Factors affecting entropy in a system

1. Change in Temperature

2. Change in Phase

3. Change in the No. Of Particles

How does the change in temperature affect entropy of a system?

• When temp. of system increase, the kinetic energy of the system also increases

• With increased energy, there are more ways of arranging energy quanta in the hotter system

• Thus, resulting in an increase in disorder; resulting in a increase in entropy

How does the change in phase affect entropy of a system?

• There is a change in phase from (initial state) to (final state).

• There are now more ways of arranging the particles in the (final state).

• This increases the disorder & hence increases the entropy of the system.

Out of all the different states, which state has the highest entropy?

Gas » Lqiuid > Solid

How does change in the number of particles affect the entropy of a system?

• When there is an increase in the no. of particles, there are more ways of arranging the particles.

• Leading to an increase in the disorder of the system; increase in entropy.

What is the Gibbs Free Energy?

It is a function connecting the system’s enthalpy & entropy.

Define the standard Gibbs Free Energy change (ΔG°)

It is the measure of spontaneity of a process under standard conditions of 298K & 1 bar.

What is the formula for the Gibbs Free Energy Change (ΔG°)?

ΔG° = ΔH° - TΔS°

Units representing ΔG°?

kJ/mol

What does ΔG<0 mean?

The forward reaction is spontaneous

What does ΔG>0 mean?

forward reaction is non-spontaneous

What does ΔG=0 mean?

The system is at equilibrium for a reversible reaction & there is no net change