Chemistry Energetics Notes SL and HL

8/19/2019 Chemistry Energetics Notes SL and HL

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Defne an exothermic reaction.•ΔH negative•Temperature rises•Energy is transferred to surroundings (heat released)•Product enthalpy lower than reactant

Defne an endothermic reaction.•ΔH positive•Temperature decreases•Energy is absorbed from surroundings (heat released)•Reactant enthalpy lower than products

Defne standard enthalpy change o reaction(ΔH°).tandard enthalpy change is the heat energytransferred under standard conditions!pressure "#"$%&Pa' temperature * +$ ,nly ΔH can be measured' notH for the initial or -nal state of a system$

Give examples o exothermic reactions.•.ombustion•/eutralisation•,0idation•Respiration•ΔH condensation•ΔH solidification

Give examples o endothermic reactions.•1issolving•Reduction•ΔH fusion2melting•ΔH vaporisation2evaporation•3oiling•Photosynthesis•Thermal decomposistion

Apply the relationship beteen temperat!rechange" enthalpy change.45mcΔTΔH546moles

Ded!ce" rom an enthalpy level diagram" therelative stabilities o reactants and prod!cts"and the sign o the enthalpy change or thereaction.7ower enthalpy2more stable$7ess stable to more stable28ve ΔH

9ore stable to less stable2:ve ΔH



#alc!late the heat energy change hen thetemperat!re o a p!re s!bstance is changed.45mcΔT

Design s!itable experimental proced!res or

meas!ring the heat energy changes oreactions.Reactions in a;ueous solution and combustion reactions$<se of the bomb calorimeter and calibration ofcalorimeters will not be assessed$

#alc!late the enthalpy change or a reaction!sing experimental data on temperat!rechanges" $!antities o reactants and mass oater.45mcΔTΔH546moles

%val!ate the res!lts o experiments todetermine enthalpy changes.=ware of the assumptions made and errors due to heatloss$

Determine the enthalpy change o a reactionthat is the s!m o to or three reactions ith&non enthalpy changes.

<se simple enthalpy cycles and enthalpy level diagramsand to manipulate e;uations$ /ot re;uired to stateHess>s law$

%nthalpy change or ormation o molec!les.tabilisation reaction which is e0othermic$

%nthalpy change or brea&ing o bonds.E0othermic as energy is needed$

Defne activation energy.9inimum energy re;uired to start a reaction$

'tate Hess a.Enthalpy change for any chemical reaction isindependent of the route provided the startingconditions and the -nal conditions and reactants andproducts are the same$

Defne the term average bond enthalpy.

=verage energy re;uired to brea& one mole of gaseouscovalent bonds$



.alculated by ta&ing the average of the enthalpies forthat speci-c bond obtained from a number of similarcompounds$ The average bond enthalpy provides anindication of the strength of a chemical bond$?!@ (g)A? (g) : @ (g)

Ho do yo! calc!late ΔH reactionΔH5um of enthalpies of bonds bro&en8sum ofenthalpies of bonds formed$

1e-ne standard state$• temperature of a substance at e0actly BC.$• pressure of gaseous substance are e0actly " atm$• li;uids and solids are pure$• a;ueous solutions are e0actly " molDdmF

Defne standard enthalpy o ormation..hange in enthalpy that accompanies the formation ofone mole of a compound from its elements with allsubstances in standard states$• E0ampleG /I (g) : ,I (g)2 /,I (g) ΔHf 5 : %J&K6mol for /,I (g)

Defne standard enthalpy o comb!stion..hange in enthalpy that accompanies the formation ofone mole of a compound from its complete reaction with

o0ygen gas under standard conditions$• E0ampleG . (s) : ,I (g) 2 .,I (g) ΔHc 5 8%% &K6molfor .,I (g)

Defne and apply the terms lattice enthalpy.Enthalpy change when one mole of a solid ioniccompound is separated into its gaseous ions understandard conditions$9?(s) 2 9L(g) : ?(g) (e0othermic)•:ve brea&ing bonds (endo)•8ve forming lattice (e0o)

*hat a+ects the strength o latticeM•.harge of ion•iNe of ionic radius

%xplain ho the relative si,es and the chargeso ions a+ect the lattice enthalpies o di+erentionic compo!nds.•Oonic lattice with higher enthalpy considered morestable' harder to brea& apart' have higher melting and

boiling point$•The relative value of the theoretical lattice enthalpy



increases with higher ionic charge and smaller ionicradius due to increased attractive forces5&;Q;I6r

#onstr!ct a -ornHaber cycle or gro!p / and 0

oxides and chlorides and !se it to calc!late anenthalpy change."$ =tomisation of solid metal where metal goes intogaseous state (endo)$ Oonisation of gaseous metal where the metal loseselectrons to form a cation (endo)%$ =tomisation of molecular non8metal into atoms wherenon8metal goes into gaseous state and is bro&en intoindividual atoms$(endo)J$ Electron aSnity where non8metal atoms gainelectrons to form anions (e0o)

Disc!ss the di+erence beteen theoretical andexperimental lattice enthalpy val!es o ioniccompo!nds in terms o their covalent character.= signi-cant dierence between the two valuesindicates covalent character$

Defne %lectron a1nity.Enthalpy change when one mole of gaseous atomsattract one mole of electrons to form one mole of

gaseous ions with a 8" charge?(g):e2?(g)

Defne %ntropy.1egree of randomness or disorder of a system$ Tends toincrease over time in a closed system

'tate and explain the actors that increase theentropy in a system.•1isorder of a system is highest when the particles arein the gaseous state and lowest when they are in thesolid state$•1issolutionG entropy increases when a solid is dissolvedin a li;uid$•.hange of stateG Entropy of a substance increases asgo from olidU7i;uidUVas•1ecompositionG leads to increase in number of particlesin the system•Oncrease in temperatureG &inetic energy of particlesincreases

2redict hether the entropy change (Δ') or agiven reaction or process is positive or negative.



Δ positive2entropy increase' more disorderΔ negative2entropy has decreased' less disorder

#alc!late the standard entropy change or areaction ( Δ'°) !sing standard entropy val!es

('°).ΔC has units K+W or K+WmolW$ =lways measured at"$#"0"#&Pa and *+$ΔC5Cproducts8Creactants

2redict hether a reaction or process ill bespontaneo!s by !sing the sign o ΔG°.•V is Vibbs free energy symbol' which indicates whethera reaction will ta&e place on its own$•pontaneous when ΔV is 8ve•ΔV5ΔH8TΔΔV5V(products)8V(reactants)

2redict the e+ect o a change in temperat!re onthe spontaneity o a reaction !sing standardentropy and enthalpy changes and the e$!ation$ΔVC5ΔHCXTΔC$Yhen temperature increases TΔC also increases$ Of Δis negative' then increasing temperature will cause thereaction to become less spontaneous$

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Chemistry Energetics Notes SL and HL