foundation physical and inorganic chemistry
TRANSCRIPT
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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.