energy & bonding

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Bonding & Energy

K Warne

http://www.warnescience.net/



Bonding

O

H2O

O

H2O

OO

Two hydrogen molecules and one oxygen molecule react to form two water

molecules.

Two H-H

single bonds

One O=O

double bond

Four H-O

single bonds

BONDS BROKEN

2x H-H single bonds

1x O=O double bond

BONDS FORMED

4x O-H single bonds

2 H2 + O2 2 H2O

+





e-

Bonding Theory

Why do atoms join together?

e- e-

?

e-

The nucleus of one atom attracts the electrons from the other atom.





Bonding Theory

e- e-

e- e-

• Atoms are attracted to each other as the protons from one atom attract the electrons from another atom.

• As two atoms approach each other the energy of the system drops.

• When the atoms get close enough for the nuclei to begin repelling the energy begins to increase.

• If the atoms were forced closer the energy of the system would rise sharply.

e-e-




SAMPLE ONLY SAMPLE ONLY SAMPLE ONLY74 100 200

H-H bond length pm (x10-12 m)

0

-100-

-200-

-300-

-400-

-432-

-500-

Bond Energy & Length1. Atoms are attracted

to each other as the protons from one atom attract the electrons from another atom.

2. As two atoms approach each other the energy of the system drops.

3. When the atoms get close enough for the nuclei to begin repelling the energy begins to increase.

4. If the atoms were forced closer the energy of the system would rise sharply.

The bond is stable when

the minimum energy

system is reached.





Bond Enthalpies

+

Whenever a bond is formed ENERGY ____________.

Whenever a bond is BROKEN ENERGY is _________.Bond FORMING – _______________.

Bond BREAKING – ________________.

Bond Energy: The energy __________ to break a bond. (kJ.mol-1)

Stronger bonds _______________ bond energy.




Exothermic Reactions

Reaction gives off energy to the surroundings – surroundings get HOT!


http://images.google.co.za/imgres?imgurl=http://sci_wiz.tripod.com/SpontaneousCombustion.jpg&imgrefurl=http://sci_wiz.tripod.com/Spontaneous_Combustion.htm&h=253&w=338&sz=11&hl=en&start=19&tbnid=SyUqj-AnQ5TBGM:&tbnh=89&tbnw=119&prev=/images%3Fq%3Dexothermic%2Breactions%26svnum%3D10%26hl%3Den%26lr%3D

http://images.google.co.za/imgres?imgurl=http://sci_wiz.tripod.com/SpontaneousCombustion.jpg&imgrefurl=http://sci_wiz.tripod.com/Spontaneous_Combustion.htm&h=253&w=338&sz=11&hl=en&start=19&tbnid=SyUqj-AnQ5TBGM:&tbnh=89&tbnw=119&prev=/images%3Fq%3Dexothermic%2Breactions%26svnum%3D10%26hl%3Den%26lr%3D




Exothermic Reactions

Reaction ________________ the surroundings – _____________________!





Exothermic ReactionsReaction gives off energy to the surroundings – surroundings get HOT!

540 kJ

Energy

140 kJ

Energy released

= 140 – 540 = -400 kJ

Energy of REACTION = ENERGY OF PRODUCTS– ENERGY OF REACTANTS

ΔHr = ΔHP - ΔHR

ΔHr for EXOTHERMIC is ALWAYS NEGATIVE!





Exothermic ReactionsReaction gives off energy to the surroundings – surroundings get HOT!

Energy

needed to

...................

all bonds.

Energy

released

when

...............

bonds

...................

ΔHreaction = E............................ – E.................................

ΔHreaction ....... 0 (i.e. ...................)

ΔHreaction

H = internal

Heat Energy

or

ENTHALPY

E = ................

ENERGY

ΔHr= Ebreaking – E forming = ........– ........= ......... kJ

10

0 k

J

64

0 k

J





(a) Energy

needed to

................ all

bonds.

Endothermic ReactionsReaction takes energy from the surroundings – surroundings get cold!

Reactants

Products

Activated Complex

(b) Energy released

when

..................................

(c)

(a)

(b)

ΔHreaction = Eproducts– Ereactants

= .........– .........= .......kJ

ΔHreaction .... 0 (i.e. ...............)

ΔHreaction

140kJ

540kJ

ΔHreaction = Ebreaking– Eforming

= ......– ........= ..........kJ

ΔHreaction ...... 0 (i.e. ...............)

60

0 k

JActivation energy

20

0 k

J

40

0

kJ

Or

ΔHr ....... 0

Overall ...........

energy taken in

than given out!




Bond Length (pm) Energy

(kJ/mol)Bond Length (pm)

Energy

(kJ/mol)

H--H 74 436 H-C 109 413

C-C 154 348 H-N 101 391

N-N 145 170 H-O 96 463

O-O 148 145 H-F 92 568

F-F 142 158 H-Cl 127 432

Cl-Cl 199 243 H-Br 141 366

Br-Br 228 193 H-I 161 298

I-I 267 151 C-C 154 348

C-C 154 348 C=C 134 614

C-N 147 308 CC 120 839

C-O 143 360 O-O 148 145

C-S 182 272 O=O 121 498

C-F 135 488 N-N 145 170

C-Cl 177 330 C=O 120 799

C-Br 194 288

Can you identify any TRENDS or PATTERNS in this data?





Bond Energy & Length

Bond FORMING – EXOTHERMIC.

Bond BREAKING – ENDOTHERMIC.

Bond Energy: E (kJmol-1)

• The amount of energy ____________

______________________________

• Given out when a bond is _________.

• Taken in when bond is ___________.

Po

ten

tia

l E

ne

rgy

Distance between nuclei

Bond Length: d (nm)

• The distance between ___________

_____________________________.

• _________ when bonds STRONGER.

• ___________ when bonds WEAKER.

e- e-

e- e-





Energy Changes During Reactions

ΔHr = Hprod – HReact = Ebreaking– Eforming

Po

ten

tial E

nerg

y

Po

ten

tial E

nerg

y

H prod

HreactH prod

H react

You can determine ΔHr in TWO different ways. The equation you

use depends on the information you are given.

ΔHr > 0 ΔHr < 0

HbreakingHforming

HbreakingHforming

Given ENTHALPYS given BOND ENERGIES





Bond Energy Calculations

H2 + Cl2 2H-Cl

H H

Cl Cl

H H

Cl Cl

H H

Cl Cl

Activated complexBonds Breaking/Forming

ΔHreaction = Ebreaking– Eforming

=

= …… – ……. = …….kJ

ΔHreaction > …… (i.e. ………….)

Look up the required bond energies:

E (H-H) =

E (Cl-Cl) =

E (H-Cl) =





Bond Energy Examples

Use data from the bond energy table to

calculate the enthalpy of reaction for the

following reactions: (per mol of the first element in each

reaction)

1. O2 + 2 H2 2H2O (no 4 TT)

2. CH4 + O2 CO2 + H2O

3. CH3CH2OH + O2 CO2 + H2O

OC C

H

H

H

H

H

H

C

H

H

H

H




Calculate Hr




Born Haber Cycle

A large amount of energy (lattice) is released when the gaseous ions bond together into the ionic crystal lattice.

Ionic compounds are therefore very stable and require large amounts of energy to break the bonding.

Ionic compounds have HIGH MELTING POINTS we say they are thermally stable.

Na(s) + 1/2 Cl2(g) NaCl(s)

Na(g) + 1/2 Cl2(g)

Na(g) + Cl(g)

Na+(g) + e- + Cl(g)

Na+(g) + Cl

-

(g)Ionisation Energy

Dissociation Energy

Sublimation Energy

Electron

Affinity

Lattice Energy





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WarneScience





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