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Exothermic
A reaction in which energy is transferred from the system to the surroundings
Endothermic
A reaction in which energy is transferred from the surroundings to the system
Specific heat capacity
The amount of energy required to increase the temperature of a 1 kg substance by 1 degree K (or C)
Standard enthalpy change of combustion (2 marks!)
The enthalpy change when 1 mole of substance burns (combines vigorously with oxygen) under standard conditions
Standard enthalpy change of reaction (2 marks!)
The enthalpy change when molar quantities of reactants in their standard states react to form products in their standard states under standard conditions
Standard enthalpy change of formation (2 marks!)
The enthalpy change when 1 mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions
Standard enthalpy change of neutralisation (2 marks!)
The enthalpy change when solutions of an acid and alkali react together under standard conditions to produce 1 mole of water
Hess’ law
States that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same
Bond enthalpy
The energy required to break 1 mole of chemical bonds in the gaseous state
Average bond enthalpy
The enthalpy change when 1 mole of molecules is broken in the gaseous state and averaged for the bond in similar compounds
(In)complete combustion
A reaction is complete when it has reached equilibrium (when the concentrations of the reactants and products are no longer changing)
Standard enthalpy of atomisation
The enthalpy change when 1 mole of an element in its standard state is atomised to produce 1 mole of gaseous atoms
Standard enthalpy change of hydration
The enthalpy change when 1 mole of gaseous ions is dissolved in water to give 1 mole of aqueous ions and a solution of indefinite dilution
Standard enthalpy change of solution
The enthalpy change when 1 mole of an ionic substance is dissolved sufficient water to form an indefinite dilution
Entropy
A measure of the amount of disorder of the particles in a system.
Gibbs free energy
The energy associated with a chemical reaction that can be used to do work, and is equal to ΔH−TΔS where enthalpy is H, T is temperature in K, and S is the entropy of the system
Limiting reagent
The substance which is completely consumed in a chemical reaction and determines the yield of the product formed
Neutralisation
When an acid and base react together to form a salt and water
(e.g. HCl + NaOH → NaCl + H2O)
End
point
Atom economy
A measure of reaction efficiency which compares the total mass of reactant atoms which end up as desired products, represented as a percentage
Mass of desired products / mass of total products x 100
Percentage yield
The experimental yield actually produced by the reaction compared to the theoretical yield, expressed as a percentage
Actual / theoretical x 100
Rate of reaction
The change over time of the concentration of a reactant or a product of a reaction
Collision theory
An explanation of how reactions take place, which states that for reactions to happen, the particles must collide with a minimum amount of energy, and the collisions must have the correct collision geometry
Activation energy (Ea)
The minimum amount of kinetic energy needed by particles for successful collisions to occur
Catalyst
A substance that changes the rate of reaction and is not used up or changed by it
Average rate
The change over time of the concentration of a reactant or a product of a reaction
Instantaneous rate
The rate of reaction at a particular moment in time
Initial rate
The rate of reaction when t = 0
Dynamic equilibrium
When the rate of the forward reaction equals the rate of the reverse reaction in a closed reaction, and the concentrations of the products and reactants are constant
Order of reaction
The power to which the concentration of that reactant is raised to in the rate equation
Rate law / expression
The mathematical expression that describes the relationship between the rate of a chemical reaction and the concentration of its reactants
rate = k[A]x[B]y
Molecularity
The number of reactant species taking part in any reaction
Rate determining step (RDS)
The slowest step of a chemical reaction that determines the rate at which the overall reaction proceeds
Reaction quotient (Q)
The ratio between the concentration of products and reactants in a reversible reaction not at equilibrium
Standard change in entropy (2 marks!)
The entropy of 1 mole of a substance measured at standard conditions (298 K and 101 kPa). Units are JK