Enthalpy Changes Flashcards
Enthalpy (H)
Heat content stored in a chemical system.
Chemical energy
A form of potential energy stored within the bonds in a substance.
Conservation of energy principle
In a closed system, energy cannot be created nor destroyed, only transferred from one form into another.
Enthalpy change (delta H)
Heat exchange with the surroundings during a chemical reaction at constant pressure.
N.b. Answer must be +/- kJmol^-1
Chemical system
The reactants & products involved in the reaction.
Surroundings
What is outside the chemical system.
Activation energy
The minimum energy required to start a chemical reaction by the breaking of bonds.
Combustion
A form of oxidation which releases energy. (If there’s a flame involved it’s burning)
Fuels
Substances that release energy on combustion. They’re oxidised, exothermic reactions.
Respiration
Exothermic oxidation reaction, where glucose is a fuel and produces energy.
Photosynthesis
Endothermic reaction, where glucose is made using the Suns energy. Conversion of CO2 & O2, using the Suns energy, into sugars.
Bond enthalpy
Enthalpy change that takes place when breaking, by homolytic fission, 1 mol of a given (specified) bond in the molecule of a gaseous species.
Mean/average bond enthalpy
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.
Standard conditions
Pressure: 100 kPa
Temperature: 298.15 K
All aqueous solutions at 1moldm^-3
Standard enthalpy change of reaction
Enthalpy change when the number of miles of substance, as written in the chemical equation, react under standard conditions of 100kPa and 298.15K. All reactants and products are in their standard state.
Standard enthalpy change of combustion
Enthalpy change when 1mol of the substance in its standard state reacts completely with O2 to form products in their standard state. The reaction occurs under standard conditions of 100kPa and 298.15K.
Standard enthalpy change of formation
Of a compound is the enthalpy change when 1mol of a substance is formed from its constituent elements in their standard States under standardised conditions (at 100kPa and 298.15K).
Specific heat capacity (shc)
The shc of a substance is the heat required to increase the temperature of 1g of a specified substance by 1C.
Units: Jg^-1C^-1
Hess’s law
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
