Chemical energetics

Chemical Energetics

Learning outcomesexplain that some chemical reactions are accompanied by energy changes, principally in the form of heat energy; the energy changescan be exothermic (H, negative) or endothermicexplain and use the terms:(i) enthalpy change of reaction and standard conditions, with particular reference to: formation, combustion, hydration, solution, neutralisation, atomisation(ii) bond energy (H positive, i.e. bond breaking)Learning outcomescalculate enthalpy changes from appropriate experimental results, including the use of the relationshipenthalpy change = mcT

apply Hess Law to construct simple energy cycles, and carry out calculations involving such cycles and relevant energy terms, with particular reference to:(i) determining enthalpy changes that cannot be found by direct experiment, e.g. an enthalpy change of formation from enthalpy changes of combustion(ii) average bond energies

construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of the activation energy.Energy changeslots of chemical reactions give out energy as heat. Example: burning a fuel Splitting calcium carbonate into calcium oxide and carbon dioxide

Simple energy diagramsAny chemical reaction will involve breaking some bonds and making new ones.

Energy is needed to break bonds, and is given out when the new bonds are formed

Enthalpy, H is an indication of a substances total energy content and cannot be measured directly. However, enthalpy change, H, is measureable.

Enthalpy change, H Energy transferred between a system and its surroundings (constant pressure). unit: kJ mol

Depends on:- temperature- physical states- pressures- concentration of solution

Enthalpy change, HStandard conditions:

pressure (1 atm, 100 kPa) temperature (298 K) each substance in its normal physical state at 100 kPa and 298 K.

indicates the enthalpy change refers to a reaction carried out under standard conditions.

Simple energy diagramsFor an exothermic change:

Notice that in an exothermic change, the products have a lower energy than the reactants. The energy that the system loses is given out as heat. The surroundings warm up.Exothermic Reaction

A self-warming can of coffee.

The energy released heats the coffeeSimple energy diagrams

For an endothermic change:

This time the products have a higher energy than the reactants. The system absorbs this extra energy as heat from the surroundings.

Endothermic Reaction

Cold pack

When chemicals in the cold pack react, they take in energy as heat and the pack gets cold.A variety of enthalpy changes Enthalpy changes can be described according to the type of chemical reaction taking place.

a)Enthalpy change of formationb) Enthalpy change of combustionc) Enthalpy change of neutralisationd) Enthalpy change of solutione) Enthalpy change of atomisationf) Enthalpy change of hydrationHor

The standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation for the reaction, react under standard conditions to give the products in their standard states.Standard enthalpy change of reaction

Standard enthalpy change of reactionHof

The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions; both compound and elements are in standard states.Standard enthalpy change of formation

Enthalpy changes of formation can be exothermic or endothermic.

By definition, the standard enthalpy change of formation of any element in its standard state is zero.

Standard enthalpy change of formationHoc

The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.Standard enthalpy change of combustion

Other Standard Enthalpy ChangesStandard Enthalpy change of Hydration of an anhydrous saltEnthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard condition.

Na2S2O3 (s) + 5 H2O (l) Na2S2O3.5 H2O (s) H = -55.0 kJmol-1

Standard Enthalpy change of SolutionEnthalpy change when one mole of a solute dissolves in a large volume of solvent that addition of more solvent produces no further heat change under standard conditions

Hsol can be either positive or negative valueIf positive, compound is insolube in waterIf negative, compound is soluble in water

NaOH (s) + aq NaOH (aq)

Standard Enthalpy change of NeutralisationEnthalpy change when one mole of water is formed when acid and alkali is neutralised in an aqueous solution under standard conditions.

Hn is always negative.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Hn = -57.1 kJmol-1

For any acid-alkali reaction the ionic equation is:

H (aq) + OH (aq) H2O (l)

The other ions in solutions are spectator ions and take no part in the reaction.Standard Enthalpy change of Atomisation The enthalpy change of atomisation is the enthalpy change when one mole of gaseous atoms is formed from its element under standard conditions.

H2 (g) H(g) Hat = +218 kJmol-1

Measuring Enthalpy Changes

A polystyrene cup can act as a calorimeter for finding some enthalpy changes

The energy transferred as heat (the enthalpy change) is given by the relationship:

q = mcT

Example25cm3 of 2.0M HCl was added to 25cm3 of 2.0M NaOH in an insulated beaker. The initial temperature of both solutions was 20C. The highest temperature reached by the solution was 33C. Calculate the Molar Enthalpy of Neutralisation. [The specific heat capacity (c) of water is 4.18 J K -1 g -1]

NaOH + HCl > NaCl + H2O

Temperature rise (DT) = 13CVolume of resulting solution= 25 + 25 = 50cm3 Equivalent mass of water= 50g

(density is 1 g per cm3)

Heat absorbed by the water (Q)= m x c x DT = 50 x 4.18 x 13= 2717 J

MEASURING ENTHALPY CHANGES NaOH + HCl > NaCl + H2O

Moles of HCl reacting= 2 x 25/1000= 0.05 molMoles of NaOH reacting= 2 x 25/1000= 0.05 molMoles of water produced= 0.05 mol

Enthalpy change per mol (DH)= heat energy / moles of water = 2717 / 0.05= 54340 J mol -1 = - 54.34 kJ mol -1 MEASURING ENTHALPY CHANGESExample calculationWhen 0.18g of hexane underwent complete combustion, it raised the temperature of 100g (0.1kg) water from 22C to 47C. Calculate its enthalpy of combustion.

Heat absorbed by the water (Q) = m C DT = 100 x 4.18 x 25 = 10450 J

Moles of hexane burned= mass / Mr= 0.18 / 86= 0.00209

Enthalpy change = heat energy / moles= 10450/ 0.00209= 5000000 J mol -1 DH = - 5000 kJ mol -1MEASURING ENTHALPY CHANGESMEASURING ENTHALPY CHANGES

Learning Check50 cm3 of HCl(aq) added to 50 cm3 of NaOH (aq), concentration 1 mol dm-3, temperature rise 6.2 C.

Calculate the enthalpy change of combustion for propanol

HESSS LAWEnergy cycles

Consider the reaction:Reactants A Product B

Example:

Hesss Law says that the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place.If we take the example above then the enthalpy change of route 1 would equal the total of the enthalpy changes for route 2:

Enthalpy Change of Reaction From Enthalpy Changes Of Formation

An energy cycle for calculating standard enthalpies of reaction from standard enthalpies of formation

Enthalpy Change of Reaction From Enthalpy Changes Of Formation

Enthalpy Change of Reaction From Enthalpy Changes Of Formation

Learning CheckEnthalpy Change of Formation From Enthalpy Changes Of Combustion

An energy cycle for calculating standard enthalpies of formation from standard enthalpies of combustion:

H1 = H2 - H3Enthalpy Change of Formation From Enthalpy Changes Of CombustionCalculate the enthalpy change of formation of propane, C3H8 at 298 K given the following standard enthalpies of combustion.

Propane, Hoc [C3H8 (g)] = -2220 kJ mol -1 Carbon, Hoc [C (graphite)] = -393 kJ mol -1 Hydrogen Hoc [H2 (g)] = -286 kJ mol -1

Enthalpy Change of Formation From Enthalpy Changes Of CombustionSolution:Calculating the enthalpy change of hydration of an anhydrous salt

An enthalpy cycle to calculate the enthalpy change of hydration of an anhydrous salt.Calculating the enthalpy change of hydration of an anhydrous salt

Enthalpy change of hydration of anhydrous sodium thiosulfate

Bond Energies and Enthalpy ChangesBond breaking and Bond making

Energy is needed to break bonds, and is given out when the new bonds are formed.

In a chemical reaction:

Energy is needed to break bonds < energy released when new bonds are formed (Exothermic)

Energy is needed to break bonds > energy released when new bonds are formed (Endothermic)

Bond Energies and Enthalpy ChangesBond Energy- The amount of energy needed to break a specific covalent bond is called the bond dissociation energy (also called bond energy or bond enthalpy).

Example: 2H2 (g) + O2 (g) 2H2O (g)Bond Energies and Enthalpy ChangesWhen H and O react, more energy is released in making four new O-H bonds in the two H2O molecules than in breaking the bonds in two H2 molecules and one O2 molecule.

So, the overall reaction is exothermicBond Energies and Enthalpy ChangesAverage Bond Energy

The average bond energy of the C H bond in methane can be found using the enthalpy changes of atomisation of carbon and hydrogen and the enthalpy change of combustion or formation of methane.

- The average C H bond energy can be found by dividing the value of H on the diagram by 4.

Calculating enthalpy changes using bond energiesUse mean bond enthalpies to estimate the enthalpy of formation of hydrazine, N2H4 .

Ans: Write out the equation for the reaction (show all the atoms and bonds).