Upload
arkadiy-popov
View
24
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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 Presentation
Citation preview
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?