Chapter 9 Enthalpy Flashcards
enthalpy H
The heat content that is stored in a chemical system.
enthalpy change ΔH
The difference between the enthalpy of the products and the enthalpy of
the reactants.
ΔH= H(products)- H(reactants)
law of conservation of energy
energy cannot be created or destroyed
exothermic reaction
A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (ΔH is negative).
energy transfer from the system to the surroundings
temperature of the surroundings increase as they gain energy> temp rise on thermometer
endothermic reaction
A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (ΔH is positive).
energy transfer from the surroundings to the system
temperature of surrounding decreases as they lose energy > temp fall on thermometer
enthalpy profile diagram
A diagram for a reaction to compares the enthalpy of the reactants with the enthalpy of the products.
activation energy Ea
The minimum energy required to start a reaction by the breaking of bonds.
standard conditions
A pressure of 100 kPa, a stated temperature, usually 298 K (25 °C) and a concentration of 1 mol dm–3 (for reactions with aqueous solutions).
standard enthalpy change of reaction ΔrHΘ
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
standard enthalpy change of formation ΔfHΘ
The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
standard enthalpy change of combustion ΔcHΘ
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
standard enthalpy change of neutralisation ΔneutHΘ
The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O(l), under standard conditions, with all reactants and products in their standard states.
specific heat capacity c
The energy required to raise the temperature of 1 g of a substance by 1 °C.
average bond enthalpy
The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species.
- Energy is always required to break bonds
- Bond enthalpies are always endothermic
- Bond enthalpies always have a positive enthalpy value
Bond breaking and making
- Energy is required to break bonds, bond breaking is endothermic ^H is positive
- Energy is released when bonds form, bond making is exothermic ^H is negative
Exothermic~ the energy released when making bonds is greater than the energy required when breaking bonds
Endothermic~ The energy required when breaking bonds is greater than the energy released when making bonds
