enthalpy changes

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Enthalpy changes

The enthalpy change for a process is the heat energy exchanged with

the surroundings at constant pressure.

Enthalpy changes are frequently measured under standard conditions, i.e. 298 K and 100 kPa. If an enthalpy change is measured under standard conditions, the symbol ө is used in superscript, ∆Hө.

Enthalpy change is given the symbol ∆H. The units are kJ mol–1.


Enthalpy changes


Examples of enthalpy changes


Bond enthalpies

When a chemical reaction takes place, bonds are broken in the reactants and bonds are formed in the products. Breaking bonds is an endothermic process. Making bonds is an exothermic process.

The mean bond enthalpy is the average (mean) bond dissociation enthalpy for a particular bond in

a range of different compounds.

Precisely, it is the average enthalpy change for breaking 1 mole of a particular bond in a range of

different compounds in the gas phase.

The enthalpy change for a reaction can be calculated by working out the enthalpy changes for bonds made and bonds broken during the reaction using mean bond enthalpies.


Calculations using bond enthalpies


What is a Born–Haber cycle?


Constructing a Born–Haber cycle


Born–Haber cycle for MgCl2

Mg(s) + Cl2(g)

Mg(g) + Cl2(g)

Haө (Mg) = +150

Mg(g) + 2Cl(g)

2 ×Haө (Cl) =

2 × (+121)

Mg+(g) + 2Cl(g) + e–

Hi(1st)ө (Mg) = +736

Hi(2nd)ө (Mg) = +1450

Mg2+(g) + 2Cl(g) + 2e–

Mg2+(g) + 2Cl–

(g)

2 ×Heaө (Cl) =

2 × (–364)

MgCl2(s)

Hlө (MgCl2) = –2493

Hfө (MgCl2)


Can you construct a Born–Haber cycle?


Born–Haber cycle questions


Lattice formation enthalpies

When an ionic lattice is formed, the oppositely charged ions are attracted to each other. The stronger the attraction, the higher the lattice formation enthalpy.

Two factors increase the attraction and therefore the lattice formation enthalpy:

high charge

small size.

N3–O2–F–

Li+Na+K+

increasing lattice formation enthalpy


Polarization and lattice enthalpy


Fill in the missing words: lattice enthalpy


Enthalpies of solution and hydration

The standard enthalpy of solution (Hsol

ө) is the enthalpy change when one mole of an ionic

compound is dissolved in water to produce aqueous ions.

Na+(g) Na+

(aq)

The standard enthalpy of hydration (Hhydө) is the

enthalpy change when one mole of gaseous ions is converted to one mole of aqueous ions.

NaCl(s) Na+(aq) + Cl–

(aq)


Calculating enthalpies of solution


Enthalpy of solution calculations


Factors affecting enthalpy of hydration

The size of the enthalpy of hydration depends on:

incr

easi

ng s

ize

The charge on the ion. The larger the charge on the ion, the larger the enthalpy of hydration.

The size of the ion. The smaller the ion, the larger the enthalpy of hydration.

Li+

Na+

K+

–519

–406

–322

F–

Cl–

Br –

–506

–364

–335

Ion Hhyd (kJ mol–1) Ion Hhyd (kJ mol–1)

Ion Hhyd (kJ mol–1) Ion Hhyd (kJ mol–1)

Fe2+ Fe3+–1950 –4430


What is a spontaneous reaction?


Entropy

Entropy is a measure of disorder, and is given the symbol S. The units of S are: J K–1

mol–1.

ordered disordered

low entropy high entropy

regular arrangement of particles

random arrangement of particles


Entropy change for reactions


Predicting entropy changes


Calculating entropy changes

Standard entropy changes for any chemical reaction or physical change can be calculated using the following simple expression:

Remember the following points:

entropies of elements are not zero like Hf values, so they should be included in calculations.

the units of entropy, S, are J K–1 mol–1

S = Sөproducts – Sө

reactants


Calculating entropy changes


Entropy change calculations


Entropy changes in the surroundings


Gibbs free energy

Whether a reaction is spontaneous depends on:

The change in a quantity called the Gibbs free energy provides a measure of whether a reaction is spontaneous. The Gibbs free energy change is given the symbol G and can be calculated for a reaction using the expression:

A reaction will be spontaneous if G < 0.

the temperature.

the enthalpy change of the system

the entropy change of the system

G = H – TS


How to calculate ∆G


Calculating ∆G


Feasibility of reactions

Even if G is positive at room temperature, there may be a higher temperature at which a reaction becomes feasible.

If S is positive, there may be a point at which TS is big enough to outweigh the enthalpy factor.

G = H – TS

positive

negative

positive

negative

positive

positive

negative

negative

makes TS > H

makes G more negative

no effect: G always positive

unlikely to make TS > H

yes, above a certain temp.

always

never

usually

H S As temp. increases… Feasible?


Finding the temperature

Consider the reduction of aluminium oxide with carbon:

If G = 0, then T = H / S

The temperature at which this reaction becomes feasible can be calculated. This will be when G = 0.

As both H and S are positive, G will become negative if TS > H.

H = +1336 kJ mol–1

Al2O3(s) + 3C(s) 2Al(s) + 3CO(g)

S = +581 J K–1 mol–1

T = 2299 K

T = 1336 / (581/1000)


When is a reaction feasible?


Solubility

In the same way that reactions are only feasible if G < 0, a substance will be soluble in water at a specific temperature if Gsol < 0.

Gsol = Hsol – TSsol

positive

negative

positive

negative

positive

positive

negative

negative

Hsol Ssol G / feasibility

feasible if T is large enough to make G negative

always feasible

never feasible

usually feasible (T is unlikely to be large enough to make G positive)


Thermodynamics vs. kinetics

Just because a reaction is spontaneous does not mean that it appears to happen. It may be that the reaction is so slow or has such a high activation energy that it is not generally observed.

A reaction that is thermodynamically feasible is not necessarily kinetically feasible.

This reaction has a negative value for G, so is feasible, but a piece of carbon does not spontaneously burn if left on the table. Energy would need to be put in for the reaction to begin.

S = +3 J K–1 mol–1H = –394 kJ mol–1

C(s) + O2(g) CO2(g)

Consider the reaction involving the combustion of carbon:


Glossary


What’s the keyword?


Multiple-choice quiz

enthalpy changes

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