foundation physical and inorganic chemistry

8/16/2019 Foundation Physical and Inorganic Chemistry

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Foundation Physical and

Inorganic Chemistry

The energy of reactions


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Contents

• Energetics

• Kinetics

• Equilibria• Redox Reactions

• Grou !II" The #alogens


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Energetics

• Enthaly Change $%H &

• Calorimetry

• #ess's (a)• Calculation of *ond Enthalies

• +sing *ond Enthalies


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Enthaly change $%H &

• ,ll chemical reaction in-ol-e a transfer of energy. Chemists call thisenergy change as the enthaly change of the reaction.

• Exothermic reactions ha-e a negati-e enthaly change" that is theytransfer energy to their surroundings.

• Endothermic reactions ha-e a ositi-e enthaly change" that is they

ta/e in energy from their surroundings.• The standard enthaly change of combustion is the enthaly

change )hen 0 mole of a comound is burnt comletely in oxygenunder standard conditions $123K and 044/Pa&" all reactants androducts being in their standard state.

• The standard enthaly change of formation is the enthaly change

)hen 0 mole of a comound is formed from its elements understandard conditions $123K and 044/Pa&" all reactants and roductsbeing in their standard state.


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Calorimetry

• The enthaly change can be calculated from thetemerature change in a reaction using the equation5 6 q 7 mc %T

• q is the enthalpy change (J), m is the mass (g) c is the specific heatcapacity J g -1K -1, %T is the temperature change in K.

• Examle5 excess of magnesium )as added to 044cm8 of19 Cu:;


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#ess's (a)

• #ess's la) states that" if a reaction can ta/e lace by

more than one route and the initial and final conditions

are the same for each route.

• B1$g& 8#1$g& 1B#8 $g&

1B$g& D#$g&

• %#f 7 %#0 %#1

route 0

route 1

%#f

%#0 %#1


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Calculation of *ond Enthalies

• *ond enthalies are a measure of the energy

required to brea/ a bond. They are al)ays

negati-e.

• The -alues quoted are usually a-erage -alues"as the strength of a bond bet)een t)o atoms is

deendant on the molecule.

• They are difficult to measure directly they are

usually calculated from data enthaly changes of

combustion of se-eral comounds.


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+sing *ond Enthalies

• B1$g& 8#1$g& 1B#8 $g&

1B$g& D#$g&

• *ond enthalies can be used to calculate the enthaly change for a reaction.

• %#f 7 %#0 %#1 6 %#0 7 the enthaly for brea/ing bonds

6 %#1 7 the enthaly for ma/ing bonds• For this reaction

6 %#f 7 8xE$#6#& E$B≡Ν& 6 DxE$B6#&

• here E$@6@& reresents the bond enthaly for @@

route 0

route 1

%#f

%#0 %#1


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Kinetics

• Collision Theory

• 9ax)ell?*oltmann istribution


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Collision Theory

• Reactions can only occur )hen reacting articles collide )ith enoughenergy. This energy is called the acti-ation energy. 9ost collisions do notresult in reaction.

• Increasing the temerature of the reaction increases the seed at )hicharticles mo-e. There are more collision and the collisions ha-e a higher

energy. This increases the number of effecti-e collisions. Therefore the rateof reaction is increased

• Increasing the concentration of reactants increase the number of articlesa-ailable to react. 9ore collisions occur. Therefore the rate of reaction isincreased. ecreasing the sie of articles increases the surface area. Ifthere is a larger surface a-ailable for articles to collide )ith there )ill be

more collisions. Therefore the rate of reaction is increased.

• Catalyst ro-ide a different route for the reaction to haen. This route hasa lo)er acti-ation energy. This means that more of the collisions )ill beeffecti-e. The rate of reaction is increased


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9ax)ell?*oltmann istribution

Bumber

ofarticles

)ith

Energy E

Energy E

Ea

These articles do not ha-e

enough energy to react

These article ha-e enough

Energy to react

•*elo) is a 9ax)ell?*oltman distribution cur-e. It sho)s the distribution ofmolecular energies in a gas.• ,s the temerature increases the ea/ of the line mo-es lo)er and to the right.

$From the blue to the red on the icture.&•The area under the grah reresents the total number of articles and stays the

same )hate-er the temerature.

•Ea is the acti-ation energy" only collisions occurringabo-e this energy result in reaction.•Increasing the temerature increases the number of

collisions occurring abo-e the acti-ation energy.•The change from blue to red is only an increase in

temerature of 04HC" ho)e-er the number of

articles that ha-e enough energy to react has

increased significantly


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Equilibria

• The ynamic Bature of Equilibria

• (e Chatelier's Princile.

• The #aber Process


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The ynamic Bature of Equilibria

• 9any reactions are re-ersible. That is they canroceed in both directions at the same time.

• ,n equilibrium can be established if a reaction

occurs in a closed system• Reactions do not sto at equilibrium" but the

concentration of roducts and reactants remainsconstant because the reaction is roceeding at

the same rate in both directions.• This does not mean that there are equal

amounts of roducts and reactant though.


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(e Chatelier's Princile

• (e Chatelier's rincile states that5 6 The position of the equilibrium of a system changes to

minimise the effect of any imposed change in conditions.

• This means5

6 Increasing the temerature fa-ours the endothermic reaction 6 Increasing the ressure in a gaseous reaction fa-ours the side of

the reaction )hich has the fe)est number of gaseous molecules"because this )ill educe the ressure.

6 #o)e-er adding a catalyst does not effect the osition of the

equilibrium.


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The #aber Process

• B1$g& 8#1$g& 1B#8$g& %H=-92Jm!l -1 • Increasing the temperature will move the equilibrium in

favour of the reactants, in the endothermic direction.• Increasing the pressure will favour the products

because this side has the fewer number of gaseousmolecules.• Lowering the temperature will favour the formation of

the products, but will mean that reaction will happenslowly. A compromise temperature is used.

• Too high a pressure would have high energy costs and

expensive thicer walled pipes and reactor so acompromise pressure is used.• A !nely divided iron catalyst is used to speed up the

reaction, and the unconverted reactants are recycledd


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Redox Reactions

• ;xidation and Reduction

• ;xidation :tates

• Redox Equations.


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;xidation and Reduction

• The term redox is used for thesimultaneous rocesses of oxidation andreduction.

• ;xidation is the loss of electrons.• Reduction ids the gain of electrons.

• ,ny reaction )hich in-ol-es reduction also

in-ol-es oxidation.• ,n oxidising agent remo-es electrons" a

reducing agent donates electrons.


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;xidation :tates

• ;xidation states sho) ho) oxidised or reduced anelement is )ithin a comound or ion.

• If the oxidation state increases the substance is oxidised

• If the oxidation state decreases the substance is

reduced.• Rules for oxidation numbers5

6 For atoms in element the oxidation state is al)ays 4

6 For atoms in simle ions the oxidation state is the same as the

charge 6 In comounds the total of all the oxidation states is 4

6 In olyatomic ions the total of all the oxidation states is the sameas the charge on the ion


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Redox Equations

• :ometimes it is easier to sho) )hat is beingoxidised and reduced by )riting half equations.

• E.g Ca Cl1 CaCl1

• This can be )ritten as5 6 Ca Ca1C 1e?

6 and Cl1 1e? 1Cl?

• Remember only one element loses or gains

electrons.• The equations must balance for atoms and

charge.


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The #alogens

• Trends

• Key facts


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Trends

Proerty Trend do)ngrou

Exlanation

Electronegati-ity decreases because sie of atoms increase

*oiling oint increases because temorary diole induceddiole interactions increase

;xidising ability decreases because the ability to accet anelectron decreases do)n the grou

Reducing ability ofhalide

increases because the ability to donate anelectron from the halide increasesdo)n the grou


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Key Facts

• ,ll halogens form diatomic molecules

• , halogen )ill dislace a halogen )hich is belo) it fromits salts because reacti-ity decreases do)n the grou. ie

Chlorine )ill dislace bromine.• The reaction of sil-er nitrate )ith the halide is a test for

halides5 ,gCl 6)hite" ,g*r 6cream" and ,gI 6yello).

• :il-er halides are used in hotograhy as they dar/en in

the resence of light.• Chlorine is used extensi-ely for disinfection.


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:ummary

• Energetics 6 ,ll chemical changes in-ol-e the transfer of energy. The o-erall enthaly transfer

during a chemical change is indeendent of the route ta/en.• Kinetics

6 The rate of reaction is effect by factors that effect the o-erall collision rate. ecan use the 9ax)ell?*oltman distribution to sho) the change in distribution ofmolecular /inetic energy )ith increasing temerature.

• Equilibria 6 9any reactions are re-ersible and in a closed system reach an equilibrium. The

osition of the equilibrium of a system changes to minimise the effect of anyimosed change in conditions. The nature of equilibria means that sometimescomromises need to be made in industrial rocesses.

• Redox Reactions 6 These are reactions )hich in-ol-e simultaneous oxidation and reduction. e use

the concet of oxidation number to indicate ho) oxidised or reduced elementsare.

• Group VII, The alogens 6 The halogens ha-e many similar roerties. They are all reacti-e and this

reacti-ity decreases do)n the grou. Chorine is used as a disinfectant.

foundation physical and inorganic chemistry

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