Enthalpy & Hess

Bond breaking

Bond breaking is an endothermic process.

Bond dissociation enthalpy

Bond dissociation enthalpy is the energy required to break a particular chemical bond.

Enthalpy change of reaction using bond energies

The equation for calculating enthalpy change of reaction using bond energies is: ΔH = Σ(bonds broken) - Σ(bonds formed)

Average bond energy

Average bond energy is the energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds.

Exothermic reaction

A reaction is exothermic if more energy is released when new bonds are formed than the energy required to break bonds.

Endothermic reaction

A reaction is endothermic because more energy is required to break bonds than the energy released when new bonds are formed.

Enthalpy change of reaction calculation

The enthalpy change of reaction is: Bonds broken = (1 x 945) + (3 x 436) = +2253; Bonds formed= 6 x 391 = -2346; ΔHrꝋ = +2253 - 2346 = -93 kJ mol-1.

Hess's Law

Hess's Law states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.

Hess's Law calculation

Hess's Law allows us to calculate the standard enthalpy change of a reaction from known standard enthalpy changes.

Direct vs Indirect route in Hess's Law

In Hess's Law energy cycles, the enthalpy change of the direct route is equal to the enthalpy change of the indirect route.

Enthalpy change for conversion of graphite to diamond

The equation that would be used to calculate the enthalpy change for the conversion of graphite to diamond is ΔHr = ΔH1 - ΔH2.

Hess's Law and calorimetry

Hess's Law is used to calculate enthalpy changes which cannot be found experimentally using calorimetry.

Methods for solving Hess's Law problems

The two common methods for solving Hess's Law problems are: Using Hess's law cycles; Using equations.

Direction of arrows in Hess's Law cycles

When using cycles to solve Hess's Law problems, if you follow the direction of the arrow you add the quantity.

Adjusting for molar amounts in Hess's Law

In Hess's Law calculations, you always need to adjust for different molar amounts.

General equation for Hess's Law problems

The general equation for solving Hess's Law problems using equations is: ΔH(reaction) = Σ(ΔH products) - Σ(ΔH reactants).

Enthalpy change for the reaction

ΔHr = - ( - 544 x 2) + (- 1648) = - 560 kJ

Standard enthalpy of formation

The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

Enthalpy of formation cycles arrow direction

In enthalpy of formation cycles, arrows always point upwards because the definition of enthalpy of formation must go from elements to compounds.

Equation for calculating enthalpy changes from enthalpy of formation data

The equation used to calculate enthalpy changes from enthalpy of formation data is: ΔH = ΣΔHfꝊ(products) - ΣΔHfꝊ(reactants)

Enthalpy of combustion calculation

ΔH = ΔHf products - ΔHf reactants; ΔH = - 1996 - (+ 31.4) = -2027.4 kJ

Hess's law cycle for NH4NO3

Chemical equation: NH4NO3(s) + 1/2 C(s) -> N2(g) + 2H2O(g) + 1/2 CO2(g).

Enthalpy change for NH4NO3 reaction

ΔHrꝊ = +365 - 484 - 197 = -316 kJ mol-1

Enthalpy of formation of elements in standard states

The enthalpy of formation of elements in their standard states is always zero.

Enthalpy of formation value

The enthalpy of formation can be a positive or negative value.

Standard enthalpy of combustion

Standard enthalpy of combustion is the enthalpy change that occurs when one mole of a substance burns completely under standard conditions.

Enthalpy of combustion cycles arrow direction

In enthalpy of combustion cycles, arrows should be pointing downwards.

General equation for calculating ΔH using enthalpy of combustion data

The general equation for calculating ΔH using enthalpy of combustion data is: ΔH = ΣΔHcꝊ(reactants) - ΣΔHcꝊ(products)

Enthalpy of combustion process

The enthalpy of combustion is always an exothermic process.

Hess's law cycle for propanone

Chemical reaction showing 3C + 3H2 + 0.5O2 yielding CH3COCH3, and arrows pointing from the left, below to 3CO2 + 3H2O.

Enthalpy change of formation of propanone

Chemical reaction diagram: 3C(s) + 3H₂(g) + ½O₂(g) → CH₃COCH₃(l).

Enthalpy values in Hess's law cycle

Arrows show enthalpy values (-1182, -858, -1821) leading to 3CO₂ + 3H₂O.

Enthalpy change of formation of propanone

ΔHfꝊ = -1182 - 858 + 1821 = -219 kJ mol-1

Sign of enthalpy change of combustion

The sign for enthalpy change of combustion values will always be negative.

Enthalpy of combustion and formation

Enthalpy of combustion can be used to calculate enthalpy of formation using Hess's Law.

Enthalpy of formation of propane

ΔH = (3x -393)+ (4 x -286) - (-2220) = -103 kJ mol-1

Significance of enthalpy of combustion

Enthalpy of combustion is significant in practical applications as it helps determine the energy content of fuels and food.

Symbol for enthalpy change of combustion

The symbol to represent the enthalpy change of combustion under standard conditions is ΔHcꝊ.

Born-Haber cycle

A Born-Haber cycle is a specific application of Hess's Law for ionic compounds that enables the calculation of lattice enthalpy.

First ionisation energy

The first ionisation energy is always an endothermic process.

Lattice enthalpy

Lattice enthalpy is the energy required to separate one mole of an ionic compound into its constituent gaseous ions.

Enthalpy of atomisation

The enthalpy of atomisation is the energy required to convert one mole of an element in its standard state to gaseous atoms.

Electron affinity

The energy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous anions is known as electron affinity.

Electron affinity process

While electron affinity is often exothermic, it can be endothermic for some elements.

Arrow in Born-Haber cycle for NaCl

In a Born-Haber cycle for NaCl, the arrow from Na (s) to Na (g) represents the enthalpy of atomisation of sodium.

Arrows in Born-Haber cycles

Arrows pointing upwards in a Born-Haber cycles represent endothermic reactions.

First electron affinity of chlorine

An equation to represent the first electron affinity of chlorine is: Cl (g) + e- → Cl- (g)

Second ionisation energy of magnesium

An equation to represent the second ionisation energy of magnesium: Mg+ (g) → Mg2+ (g) + e-

Step 1 enthalpy change

The enthalpy change shown in Step 1 is the enthalpy change of formation.

Step 2 enthalpy change

The enthalpy change shown in Step 2 is the enthalpy of atomisation of potassium.

Enthalpy of atomisation of potassium

The enthalpy change shown in Step 2.

Enthalpy of atomisation of fluorine

The enthalpy change shown in Step 3.

First ionisation energy of potassium

The enthalpy change shown in Step 4.

First electron affinity of fluorine

The enthalpy change shown in Step 5.

Lattice enthalpy of formation

The enthalpy change shown in Step 6.

Lattice enthalpy of KF

ΔHθlattice KF = -563-90-79-419+328 = -823 (kJ mol−1).

Enthalpy of formation LiF

ΔHθf LiF = 216 + 79 + 520 + (-348) + (-1061) = -594 (kJ mol-1).

Enthalpy of atomisation of chlorine

The enthalpy of atomisation of chlorine is 121 kJ mol-1.

Enthalpy change for Ca (s) → Ca2+ (g) + 2e-

ΔHθat Ca + ΔHθIE1 Ca + ΔHθIE2 Ca = 178 + 590 + 1145 = (+)1913 (kJ mol-1).

First electron affinity of chlorine for MgCl2

When performing a Born-Haber cycle calculation for MgCl2, the value for the first electron affinity of chlorine would need to be doubled.

Enthalpy change for Cl2 (g) + 2e- → 2Cl- (g)

ΔHθBE Cl2 + (2 x ΔHθEA Cl) = 242 + (2 x -349) = -456 (kJ mol-1).

Lattice enthalpy calculation equation

ΔHꝋlat = - ΔHꝋf + ΔHꝋat + ΔHꝋat + ΔHꝋIE + ΔHꝋEA.

Lattice enthalpy of CaCl2

ΔHθlattice CaCl2 = -814 - 178 - 590 - 1145 - 242 + (349 x 2) = -2271 (kJ mol−1).