Module 3 Definitions

Question	Answer
Enthalpy	The heat content that is stored in a chemical system
Exothermic	Refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings
Endothermic	Refers to 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
Enthalpy profile diagram	A diagram for a reaction to come the enthalpy of the reactants with the enthalpy of the products
Activation energy	Minimum energy required to start a reaction by the breaking of bonds
Standard conditions	A pressure of 100kPa (1 atmosphere) and a stated temperature, usually 25°C and a concentration of 1.0 mol dm⁻³
Standard enthalpy change of reaction	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 combustion	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 formation	Of a compound is when one mole of a compound is formed from its constituent elements inn their standard states under standard conditions
Specific heat capacity	Is the energy required to raise the temperature of 1g of a substance by 1°C
Bond enthalpy	The enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species
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
Hess' law	States that, 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
Enthalpy cycle	A diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy changes using Hess' law
Rate of reaction	The change in concentration of a reactant or a product in a given time
Heterogeneous catalysis	Catalysis of a reaction in which the catalyst has a different physical state from the reactants; frequently, reactants are gases whilst the catalyst is a solid
Homogeneous catalysts	Catalysis of a reaction in which the catalyst and reactants are in the same physical state, which is most frequently the aqueous or gaseous state
Boltzmann distribution	The distribution of energies of molecules at a particular temperature, often shown as a graph
le Chatelier's principle	States that when a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise the change
Dynamic equilibrium	The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction

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	Created by abenabiney over 9 years ago