Bonding & Energy Energy of Reactions K Warne

Bonding O O H H H H We know that atoms form bonds. And we also know that these bonds can break and new bonds can be formed when reactions take place.

Bonding O O H H H H

O H O H H H

O H O H H H

O H H H2OH2O O H H H2OH2O O O H H H H Two hydrogen molecules and one oxygen molecule react to form two water molecules H-H bonds O=O bond H-O bonds BONDS BROKEN x single bonds 1x O=O double bond BONDS FORMED H single bonds H 2 + O 2  H 2 O

Bonding O H H H2OH2O O H H H2OH2O O O H H H H 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 H 2 + O 2  2 H 2 O +

e-e- Bonding Theory Why do atoms join together? e-e- e-e- ? e-e- The nucleus of one atom attracts the electrons from the other atom.

Bonding Theory e-e- e-e- e-e- e-e- Atoms are attracted to each other as the ________ from one atom attract the ________ from the other atom. As two atoms approach each other the _______ of the system _______. When the atoms get close enough for the ________ to begin __________ the energy begins to ___________. If the atoms were forced closer the energy of the system would _________. e-e- e-e-

Bonding Theory e-e- e-e- 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- e-e-

Bond Energy & Length 1.Atoms are attracted to each other as the from one atom attract the from another atom. 2.As two atoms approach each other the energy of the system When the atoms get close enough for the to begin repelling the energy begins to If the atoms were forced closer the energy of the system would H-H bond length pm (X m)     

H-H bond length pm (X m)      Bond Energy & Length 1.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. X 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.

Bond Enthalpies + Whenever a bond is formed ENERGY is given out. Whenever a bond is BROKEN ENERGY is TAKEN IN. Bond FORMING – EXOTHERMIC. Bond BREAKING – ENDOTHERMIC. Bond Energy : The energy required to break a bond. (kJ.mol -1 ) Stronger bonds greater bond energy.

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

Exothermic Reactions Reaction ________________ the surroundings – _____________________!

Exothermic Reactions Reaction GIVES ENERGY to the surroundings – GET HOT. REACTANTS PRODUCTS ENTHALPY (Internal heat energy) kJ

Exothermic Reactions Reaction 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 ΔH r = ΔH P - ΔH R ΔH r for EXOTHERMIC is ALWAYS NEGATIVE!

Endothermic Endothermic Reactions Reaction takes energy from the surroundings – surroundings get cold!

Endothermic Endothermic Reactions Reaction ________________ the surroundings – _____________________! HW: Fill in all details on this profile lable: REACTANTS, PRODUCTS, ΔH r, ACTIVATION ENERGY, BOND BREAKING, BOND FORMING…

Endothermic Reactions Reaction takes energy from the surroundings – surroundings get cold!

(a) Energy taken in when reactant bonds break. Endothermic Reactions Reaction takes energy from the surroundings – surroundings get cold! Reactants Products Activated Complex (b) Energy released when product bonds form. (c) (a) (b) ΔH reaction = E products – E reactants = 540 – 140 = +400 kJ ΔH reaction > 0 (i.e. positive) ΔH reaction 140kJ 540kJ 600 kJ Activation energy 200 kJ 400 kJ ΔH r > 0 Overall more energy taken in than given out!

Exothermic Reactions Reaction gives off energy to the surroundings – surroundings get HOT! Energy needed to all bonds. Energy released when bonds ΔH reaction = E – E ΔH reaction (i.e ) ΔH reaction H = internal Heat Energy or ENTHALPY E = ENERGY ΔH r = E breaking – E forming = – = kJ 100 kJ 640 kJ

Exothermic Reactions Reaction gives off energy to the surroundings – surroundings get HOT! Energy needed to break all bonds. Energy released when new bonds form ΔH reaction = E BREAKING – E FORMING ΔH reaction < 0 (i.e. negative) ΔH reaction H = internal Heat Energy or ENTHALPY E = BOND ENERGY ΔH r = E breaking – E forming = 100 – 640 = -540 kJ 100 kJ 640 kJ

(a) Energy needed to all bonds. Endothermic Reactions Reaction takes energy from the surroundings – surroundings get cold! Reactants Products Activated Complex (b) Energy released when (c) (a) (b) ΔH reaction = E products – E reactants = – = kJ ΔH reaction (i.e ) ΔH reaction 140kJ 540kJ ΔH reaction = E breaking – E forming =......– = kJ ΔH reaction (i.e ) 600 kJ Activation energy 200 kJ 400 kJ Or ΔH r Overall energy taken in than given out!

(a) Energy needed to break all bonds. Endothermic Reactions Reaction takes energy from the surroundings – surroundings get cold! Reactants Products Activated Complex (b) Energy released when new bonds form (c) (a) (b) ΔH reaction = E products – E reactants = 540 – 140 = +400 kJ ΔH reaction > 0 (i.e. positive) ΔH reaction 140kJ 540kJ ΔH reaction = E breaking – E forming = 600 – 200 = +400 kJ ΔH reaction > 0 (i.e. positive) 600 kJ Activation energy 200 kJ 400 kJ Or ΔH r > 0 Overall more energy taken in than given out!

BondLength (pm) Energy (kJ/mol) BondLength (pm) Energy (kJ/mol) H--H74436H--C C--C154348H--N N--N145170H--O96463 O--O148145H--F92568 F--F142158H--Cl Cl-Cl199243H--Br Br-Br228193H--I I--I267151C--C C--C154348C=C C--N CCCC C--O143360O--O C--S O=O C--F135488N--N C--Cl177330C=O C--Br Can you identify any TRENDS or PATTERNS in this data?

Bondlength (pm) and bond energy (kJ/mol) BondLengthEnergyBondLengthEnergy H--H74436H--C C--C154348H--N N--N145170H--O96463 O--O148145H--F92568 F--F142158H--Cl Cl-Cl199243H--Br Br-Br228193H--I I--I267151C--C C--C154348C=C C--N CCCC C--O143360O--O C--S O=O C--F135488N--N C--Cl C--Br194288

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 ___________. Potential Energy Distance between nuclei Bond Length: d (nm) The distance between ___________ _____________________________. _________ when bonds STRONGER. ___________ when bonds WEAKER. e-e- e-e- e-e- e-e-

Bond Energy & Length Bond FORMING – EXOTHERMIC. Bond BREAKING – ENDOTHERMIC. Bond Energy: E (kJmol -1 ) The amount of energy needed to break a bond. Given out when a bond is FORMED. Taken in when bond is BROKEN. Potential Energy Distance between nuclei Bond Length: d (nm) The distance between the nuclei of two covalently bonded atoms. Shorter when bonds STRONGER. Longer when bonds WEAKER. d E e-e- e-e- e-e- e-e-

Energy Changes During Reactions

ΔH r = H prod - H React Potential Energy H prod H react H prod H react A CATALYST speeds up a reaction by lowering the ACTIVATION ENERGY of the reaction through providing a different reaction pathway. ΔH r > 0 ΔH r < 0 Biological catalysts are called ENZYMES

Energy Changes During Reactions H breaking ΔH r = H prod – H React = E breaking – E forming Potential Energy H prod H react H prod H react You can determine ΔH r in TWO different ways. The equation you use depends on the information you are given. ΔH r > 0 ΔH r < 0 H breaking H forming H breaking H forming Given ENTHALPYS given BOND ENERGIES

Bond Energy Calculations H 2 + Cl 2  2H-Cl H H Cl H H H H Activated complex Bonds Breaking/Forming ΔH reaction = E breaking – E forming = = …… – ……. = …….kJ ΔH reaction > …… (i.e. ………….) Look up the required bond energies: E (H-H) = E (Cl-Cl) = E (H-Cl) =

Bond Energy Calculations H 2 + Cl 2  2H-Cl H H Cl H H H H Activated complex Bonds Breaking/Forming ΔH reaction = E breaking – E forming = E(H-H)+E(Cl-Cl) – 2E(H-Cl) = –(2x432) = -185kJ Δh reaction < 0 (i.e. EXOTHERMIC) Look up the required bond energies: E (H-H) = 436 kJ.mol -1 E (Cl-Cl) = 243 kJ.mol -1 E (H-Cl) = 432 kJ.mol -1

Bond Energy Calculations H 2 + Cl 2  2H-Cl H H Cl H H H H Activated complex Bonds Breaking/Forming E (H-H) = 436 kJ.mol -1 E (Cl-Cl) = 243 kJ.mol -1 E(H-Cl) = 432 kJ.mol -1 ΔH reaction = E breaking – E forming = E(H-H) + E(Cl-Cl) – 2E(H-Cl) = – 2(432) = kJ ΔH reaction > 0 (i.e. EXOTHERMIC)

Bond Energy Examples Use data from the bond energy table to calculate the enthalpy of reaction for the following reactions: 1.2 H 2 + O 2  2H 2 O (no 4 TT) 2.CH 4 + O 2  CO 2 + H 2 O 3.CH 3 CH 2 OH + O 2  CO 2 + H 2 O

Bond Energy Examples Use data from the bond energy table to calculate the enthalpy of reaction for the following reactions: 1.2 H 2 + O 2  2H 2 O (no 4 TT)

Bond Energy Examples Use data from the bond energy table to calculate the enthalpy of reaction for the following reactions: 2.CH 4 + O 2  CO 2 + H 2 O

Bond Energy Examples Use data from the bond energy table to calculate the enthalpy of reaction for the following reactions: 3.CH 3 CH 2 OH + O 2  CO 2 + H 2 O

EXAM QUESTIONS TRUE or FALSE 1.The C-H bond is stronger than the C-C bond because an H-atom is bigger than a C-atom. 2.Energy is released in all chemical reactions.

EXAM QUESTIONS - ANSWERS TRUE or FALSE 1.The C-H bond is stronger than the C-C bond because an H-atom is bigger than a C-atom. FALSE – longer = weaker/lower bond energy (H atom is smaller) 2.Energy is released in all chemical reactions. FALSE – only released in exothermic reactions

EXAM QUESTIONS 1. 2.

EXAM QUESTIONS C B

Calculate  H  r

Formation of Ionic Bond 1.Write down all the steps that need to take place for this reaction to take place. 2.Try and place the steps in order. 3.Decide which steps would be endothermic and which would be exothermic. Remember NaCl is actually Na + Cl - Na (s) + Cl 2(g) NaCl (s) Eg: sodium atoms need to separate from each other: Na (s)  Na (g)

Formation of Ionic Bond 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 Cl 2(g) NaCl (s) Na (g) + 1 / 2 Cl 2(g) Na (g) + Cl (g) Na + (g) + e - + Cl (g) Na + (g) + Cl - (g)

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 Cl 2(g) NaCl (s) Na (g) + 1 / 2 Cl 2(g) Na (g) + Cl (g) Na + (g) + e - + Cl (g) Na + (g) + Cl - (g) Ionisation Energy Dissociation Energy Sublimation Energy Electron Affinity Lattice Energy

MUST BE LEARNT BY HEART! ONETWOTHREE Hydrogen H + Beryllium Be 2+ Aluminium Al 3+ Lithium Li + Magnesium Mg 2+ Iron(III) Fe 3+ Sodium Na + Calcium Ca 2+ Potassium K + Barium Ba 2+ Silver Ag + Lead Pb 2+ Copper(I) Cu + Zinc Zn 2+ Ammonium NH 4 + Iron(II) Fe 2+ Oxonium H 3 O + Copper(II) Cu 2+ VALENCY TABLE 1

VALENCY TABLE 2 Negative Ions Fluoride F - Oxide O 2- Nitride N 3- Chloride Cl - Sulphide S 2- Phosphate PO 4 3- Bromide Br - Carbonate CO 3 2- Iodide I - Sulphate SO 4 2- Hydroxide OH - Nitrate NO 3 - Hydrogencarbonate HCO 3 - Hydrogensulphate HSO 4 - Permanganate MnO 4 - Ethanoate CH 3 COO - The trivial names for HCO 3 - and HSO 4 - are bicarbonate and bisulphate, respectively.