Chemistry Definitions Module 3.2
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Activation energy
The minimum energy required for a reaction to take place
Average Bond Enthalpy
The energy required to break one mole of gaseous bonds. Considers a particular bond in a range of molecules.
Enthalpy (H)
A value that represents the heat content of a system
Enthalpy change (ΔH)
The change in the heat content of a system during a reaction.
Endothermic
A reaction that takes in energy (ΔH is positive). More energy is required to break bonds than is released by making bonds
Exothermic
A reaction that gives out energy (ΔH is negative). More energy is released by making bonds than is required to break bonds
Enthalpy Change of Combustion (ΔcH)
The enthalpy change that takes place when one mole of a substance is completely combusted
Enthalpy Change of Formation (ΔfH)
The enthalpy change that takes place when one mole of a compound is formed from its elements.
Enthalpy Change of Neutralisation (Δ neut H)
The enthalpy change that takes place when one mole of water is formed from a neutralisation reaction.
Enthalpy Change of Reaction (Δ r H)
The enthalpy change that is associated with a particular chemical equation
Enthalpy Profile Diagram
Shows the difference in the enthalpy of reactants and products as well as the activation energy of a reaction.
Hess’ Law
The enthalpy change of a reaction is independent of the route it takes
Standard Conditions
A pressure of 100kPa and temperature of 298K (25C)
Standard State
The physical state of a substance under standard conditions
Boltzmann Distribution
A graph showing the distribution of the energies of molecules in relation to the activation energy
Catalyst
A substance that speeds up the rate of reaction without being used up. Allows the reaction to proceed via a different route with a lower activation energy
Collision theory
The theory which states that molecules must collide with sufficient energy at the correct orientation for a reaction to occur
Concentration
The amount of a substance that is dissolved per unit volume of solution.
Heterogeneous Catalyst
A catalyst that is in a different state to the reactants
Homogeneous catalyst
A catalyst which is in the same state as the reactants
Pressure
The force that a gas exerts on the walls of a container
Rate of reaction
A measure of how quickly a reactant is used up/a product is formed. Rate can be determined by measuring concentration, volume of gas produced or mass lost over time
Compromise
Balancing several different factors in order to get the best possible outcome.
Dynamic equilibrium
A closed system in which the rate of the forward reaction is equal to the rate of the backward reaction