and chemistry?
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Energy. and Chemistry?. Joules or calories. Joules or calories. Joules or calories. Joules or calories. How much energy is stored in glucose?. C 6 H 12 O 6. Chemical Energy. When chemical reactions occur, bonds are first broken and then formed. - PowerPoint PPT PresentationTRANSCRIPT
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andand Chemistry? Chemistry?
Joules or calories Joules or calories
Joules or calories Joules or calories
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How much energy is stored in How much energy is stored in glucose?glucose?
CC66HH1212OO66
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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
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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
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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.
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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
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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
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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.
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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
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Reactions and EnergyReactions and Energy
If heat is generated during a reaction ≡ exothermic
If heat is absorbed during a reaction ≡ endothermic
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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?
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Chemical Reactions and EnergyChemical Reactions and Energy
H2 + Cl2 Bonds are broken HCl
Energy releasedEnergy released
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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?
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Chemical Reactions and EnergyChemical Reactions and Energy
N2 + O2 Bonds are broken NO
Energy absorbedEnergy absorbed
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Calculating chemical energyCalculating chemical energy
2H2 + O2 2H2O
Endothermic or exothermic?
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Calculating chemical energyCalculating chemical energy
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l)
Endothermic or exothermic?