Energetics

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Energetics
Exothermic and Endothermic reactions
Exothermic Reactions
Heat energy is given out to the surroundings so the H is negative
The energy needed to break the bonds is less than the energy needed to make the bonds
Endothermic Reactions
Heat energy is taken in from the surroundings so the H is positive
The energy needed to break the bonds is more than the energy needed to make the bonds
Standard Enthalpy of Formation
Standard Enthalpy of Formation (H f) - The enthalpy change when 1 mole of a compound is formed
.........................................................from its elements under standard conditions, 100kPa and 298K
.........................................................with all products and reactants in their standard states
Calculation
To calculate the enthalpy of formation use the following equation:
H = [ of Hf products ] ­ [ of Hf reactants ]
Example
Calculate the overall enthalpy change for the following reaction:
CH 4 (g)+ 2O2 (g) CO 2 (g)+ 2H
2O (l)
Hf
CH 4= - 75 CO 2 = -393 H2O = -286
H=
(-393 + (2 x -286))­ (-75)
H = -965 ­ (-75)
H=
- 890 KJ mol-1
Standard Enthalpy of Combustion
Standard Enthalpy of Combustion ( Hc) - The enthalpy change when 1 mole of a compound is
.............................................................completely burnt in oxygen under standard conditions,
............................................................100kPa and 298K, with all reactants and products in the
............................................................standard states
Calculation
To calculate the enthalpy of combustion use the following equation:
H = [ of Hf reactants ] - [ of Hf products ]
Example

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Calculate the overall enthalpy change for the following reaction:
C (s) + 2H 2(g) CH 4 (g)
Hc
C (s)= -393 H 2 (g)= -286 CH 4= -890
H=
(-393 + (2 x -286))­ (-890)
H=
-75
Hess's Law
Hess's Law - The enthalpy change of a reaction depends only on the initial and final states of the ...................…read more

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Bond H­H O=O C­C C=C C­H F­F H­O Br ­ Br H ­ Br
KJ mol-1 436 496 348 612 412 158 463 193 366
H­H + Br ­ Br 2 H ­ Br
H = 463 + 193
= 656
H = 2 x 366
= 732
H = 656 ­ 732
= - 76
H = 76…read more

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