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8-Energetics 1
Enthalpy Change
Thermochemical Equations
Hess’s Law
Standard Enthalpies
Standard Enthalpy of Reaction (ΔHr ⦵)
Practical: Calculation of Enthalpy of Reaction from an Experiment using an Insulated
Container
Standard Enthalpy of Combustion (ΔHc ⦵)
Practical: Experimental Method to Find the Enthalpy of Combustion of a Liquid
Calculation of Standard Enthalpy of Formation from Combustion
Standard Enthalpy of Neutralisation (ΔHne u t ⦵)
Practical: Experimental Method to find the Enthalpy of Neutralisation of an Acid
Standard Enthalpy of Atomisation (ΔHa ⦵)
Bond Enthalpy (ΔHB )
Calculation of Enthalpy of Reaction from Average Bond Enthalpies
Enthalpy Titrations
Law of Conservation of Energy - Energy can neither be
created nor destroyed, converted from one form to another.
Unit of energy - joule (J) or kilojoule (kJ).
8-Energetics 1 1
, Heat Transferred = Mass x Specific Heat Capacity x Temperature
Change
Q=mcΔT
Specific heat capacity of water - 4.18Jg− 1o C− 1 .
Specific Heat Capacity:
The energy required to increase the temperature of one gram of a substance by
one degree kelvin. Units - J K− 1 g− 1 .
Example:
Calculate the heat required to increase the temperature of 100cm3 of water from
17.6o C to 50.5o C.
ΔT = 50.5-17.6 = 32.9o C
Q=mcΔT
Q = 100 x 4.18 x 32.9
Q = 13,800J / 13.8kJ
Enthalpy Change
Enthalpy (H) of a system:
The enthalpy change (Δr H) for a reaction is the heat energy change measured
out under conditions of constant pressure.
The release or absorption of heat energy by a system at constant pressure
is caused by a change in the enthalpy of the chemicals.
Absolute enthalpy values cannot be found.
ΔH = (enthalpy of products) - (enthalpy of reactants).
ΔH = Hp - Hr
Heat Energy Change in a reaction (Q):
Can be measured experimentally using Q=mcΔT, where m is the mass of the
substance that has a temperature change ΔT and a specific heat capacity c.
Calorimetry:
8-Energetics 1 2
, The measurement of the heat required or absorbed by chemical reactions.
Exothermic Reactions:
Reactants have a higher enthalpy than the products.
The system loses energy during the reaction.
The value of ΔH is negative.
Energy is transferred to the surroundings as heat - surrounding increase in
temperature.
Endothermic Reactions:
Reactants have a lower enthalpy than the products.
The system gains energy during the reaction.
The value of ΔH is positive.
Energy is transferred from the surroundings as heat - surroundings decrease
in temperature.
Thermochemical Equations
Thermochemistry:
8-Energetics 1 3
8-Energetics 1
Enthalpy Change
Thermochemical Equations
Hess’s Law
Standard Enthalpies
Standard Enthalpy of Reaction (ΔHr ⦵)
Practical: Calculation of Enthalpy of Reaction from an Experiment using an Insulated
Container
Standard Enthalpy of Combustion (ΔHc ⦵)
Practical: Experimental Method to Find the Enthalpy of Combustion of a Liquid
Calculation of Standard Enthalpy of Formation from Combustion
Standard Enthalpy of Neutralisation (ΔHne u t ⦵)
Practical: Experimental Method to find the Enthalpy of Neutralisation of an Acid
Standard Enthalpy of Atomisation (ΔHa ⦵)
Bond Enthalpy (ΔHB )
Calculation of Enthalpy of Reaction from Average Bond Enthalpies
Enthalpy Titrations
Law of Conservation of Energy - Energy can neither be
created nor destroyed, converted from one form to another.
Unit of energy - joule (J) or kilojoule (kJ).
8-Energetics 1 1
, Heat Transferred = Mass x Specific Heat Capacity x Temperature
Change
Q=mcΔT
Specific heat capacity of water - 4.18Jg− 1o C− 1 .
Specific Heat Capacity:
The energy required to increase the temperature of one gram of a substance by
one degree kelvin. Units - J K− 1 g− 1 .
Example:
Calculate the heat required to increase the temperature of 100cm3 of water from
17.6o C to 50.5o C.
ΔT = 50.5-17.6 = 32.9o C
Q=mcΔT
Q = 100 x 4.18 x 32.9
Q = 13,800J / 13.8kJ
Enthalpy Change
Enthalpy (H) of a system:
The enthalpy change (Δr H) for a reaction is the heat energy change measured
out under conditions of constant pressure.
The release or absorption of heat energy by a system at constant pressure
is caused by a change in the enthalpy of the chemicals.
Absolute enthalpy values cannot be found.
ΔH = (enthalpy of products) - (enthalpy of reactants).
ΔH = Hp - Hr
Heat Energy Change in a reaction (Q):
Can be measured experimentally using Q=mcΔT, where m is the mass of the
substance that has a temperature change ΔT and a specific heat capacity c.
Calorimetry:
8-Energetics 1 2
, The measurement of the heat required or absorbed by chemical reactions.
Exothermic Reactions:
Reactants have a higher enthalpy than the products.
The system loses energy during the reaction.
The value of ΔH is negative.
Energy is transferred to the surroundings as heat - surrounding increase in
temperature.
Endothermic Reactions:
Reactants have a lower enthalpy than the products.
The system gains energy during the reaction.
The value of ΔH is positive.
Energy is transferred from the surroundings as heat - surroundings decrease
in temperature.
Thermochemical Equations
Thermochemistry:
8-Energetics 1 3
