H, is the heat content that is stored in a chemical system.
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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 (^H-ve).
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Endothermic
refers to a reaction in which the enthalpy of the products is greater that the enthalpy of the reactants, resulting in heat being taken in from the surroundings (^H+ve)
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enthalpy profile diagram
a diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products
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Activation energy
the minimum energy required to start a reaction by the breaking of bonds.
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Standard conditions
a pressure of 100kPa (1 atmosphere), a stated temperature, usually 298K (25'C) and a concentration of 1.0moldm-3 (for reactions with aqueous solutions).
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Standard state
the physical state of a substance under the standard conditions of 100kPa (1 atmosphere) and 298K (25'C).
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standard enthalpy change of reaction
^Hr, is 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.
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standard enthalpy change of combustion
^Hc, is 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.
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standard enthalpy change of formation
^Hf, of a compound is 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.
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Specific heat capacity
c, is the energy required to raise the temperature of 1g of substance by 1'C.
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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.
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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.
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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.
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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.
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rate of reaction
the change in concentration of a reactant or a product in a given time.
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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.
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Homogeneous catalysis
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.
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Boltzman distribution
the distribution of energies of molecules at a particular temperature, often shown as a graph.
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le Chatelier's principle
states that when a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change
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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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Card 2
Front
Exothermic
Back
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 (^H-ve).
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