and Chemistry?

andand Chemistry? Chemistry?

Joules or calories Joules or calories

Joules or calories Joules or calories

How much energy is stored in How much energy is stored in glucose?glucose?

CC66HH1212OO66

Chemical EnergyChemical Energy

When chemical reactions occur, bonds are When chemical reactions occur, bonds are first broken and then formed.first broken and then formed.

The amount of energy released / absorbed The amount of energy released / absorbed during a chemical reaction can be measured during a chemical reaction can be measured and calculated from the bond energiesand calculated from the bond energies

Bond EnergyBond Energy

Energy is absorbed by atoms when their bonds break.

(+∆H) - bond dissociation

Energy is released when bonds form between atoms.

(-∆H) - bond formation

Bond Energy - AnalogyBond Energy - Analogy

Imagine stretching a rubber band until it breaks. You must do work to stretch the band because the tension in the band opposes your efforts. You lose energy; the band gains it. Something similar happens when bonds break in a chemical reaction.

The energy required to break the bonds is absorbed from the surroundings.

Bond EnergyBond Energy

BondEnergy (kJ/mol)

H - H 436

C - H 413

N - H 393

436 kJ is required to disrupt H-H

436 kJ is released when H-H is formed

The higher the bond energy, the more work is required to break the bond, the more stable it is

Bond EnergyBond Energy

BondEnergy (kJ/mol)

BondEnergy (kJ/mol)

H - H 436 N - N 160

C - H 413 N = O 631

N - H 393 N triple N 941

P - H 297 N - O 201

C - C 347 N - P 297

C - O 358 O - H 464

C - N 305 O - S 265

C - Cl 397 O - Cl 269

C = C 607 O - O 204

C = O 805 C - F 552

O = O 498 C - S 259

Bond EnergyBond Energy

Energy is absorbed when bonds break.

Energy is released when bonds form.

Energy is absorbed or released when the heat capacities of the products and reactants differ.

Calculating Energy of a ReactionCalculating Energy of a Reaction

H2 + Cl2 2HCl

Breaking H bond = 436 kJ/mol

Breaking Cl bond = 242 kJ/mol

H-Cl bond forming = -431 kJ/mol

Reactions and EnergyReactions and Energy

If heat is generated during a reaction ≡ exothermic

If heat is absorbed during a reaction ≡ endothermic

Calculating Energy of a ReactionCalculating Energy of a Reaction

H2 + Cl2 2HCl

Moles of Bonds Moles of Bonds BrokenBroken

Energy absorbed Energy absorbed (kJ)(kJ)

Moles of Bonds Moles of Bonds FormedFormed

Energy released Energy released (kJ)(kJ)

1 H-H @ 431.2 1 H-H @ 431.2 kJkJ

436436 2 H-Cl @427 kJ2 H-Cl @427 kJ 854854

1 Cl-Cl @ 1 Cl-Cl @ 243243

679679 854854

Heat of Reaction is 679 – 854 = -175kJ; energy is released during the Heat of Reaction is 679 – 854 = -175kJ; energy is released during the reactionreaction

Endothermic or exothermic?

Chemical Reactions and EnergyChemical Reactions and Energy

H2 + Cl2 Bonds are broken HCl

Energy releasedEnergy released

Calculating chemical energyCalculating chemical energy

N2 + O2 2NO

Breaking one N bond = 946 kJ/mol

Breaking one O bond = 498 kJ/mol

Forming 2 N-O bonds = 2 x 631 kJ/mol = 1262 kJ/mol

Net energy released = (946 + 498) + 2(-631) = +182 kJ/mol

Endothermic or exothermic?

Chemical Reactions and EnergyChemical Reactions and Energy

N2 + O2 Bonds are broken NO

Energy absorbedEnergy absorbed

Calculating chemical energyCalculating chemical energy

2H2 + O2 2H2O

Endothermic or exothermic?

Calculating chemical energyCalculating chemical energy

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l)

Endothermic or exothermic?