Chemistry R1 & R2 definitions

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35 Terms

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Exothermic

A reaction in which energy is transferred from the system to the surroundings

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Endothermic

A reaction in which energy is transferred from the surroundings to the system

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Specific heat capacity

The amount of energy required to increase the temperature of a 1 kg substance by 1 degree K (or C)

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Standard enthalpy change of combustion (2 marks!)

The enthalpy change when 1 mole of substance burns (combines vigorously with oxygen) under standard conditions

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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

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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

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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

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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

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Bond enthalpy

The energy required to break 1 mole of chemical bonds in the gaseous state

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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

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(In)complete combustion

A reaction is complete when it has reached equilibrium (when the concentrations of the reactants and products are no longer changing)

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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

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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

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Standard enthalpy change of solution

The enthalpy change when 1 mole of an ionic substance is dissolved sufficient water to form an indefinite dilution

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Entropy

A measure of the amount of disorder of the particles in a system.

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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

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Limiting reagent

The substance which is completely consumed in a chemical reaction and determines the yield of the product formed

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Neutralisation

When an acid and base react together to form a salt and water

(e.g. HCl + NaOH → NaCl + H2O)

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End

point

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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

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Percentage yield

The experimental yield actually produced by the reaction compared to the theoretical yield, expressed as a percentage

Actual / theoretical x 100

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Rate of reaction

The change over time of the concentration of a reactant or a product of a reaction

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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

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Activation energy (Ea)

The minimum amount of kinetic energy needed by particles for successful collisions to occur

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Catalyst

A substance that changes the rate of reaction and is not used up or changed by it

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Average rate

The change over time of the concentration of a reactant or a product of a reaction

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Instantaneous rate

The rate of reaction at a particular moment in time

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Initial rate

The rate of reaction when t = 0

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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

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Order of reaction

The power to which the concentration of that reactant is raised to in the rate equation

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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

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Molecularity

The number of reactant species taking part in any reaction

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Rate determining step (RDS)

The slowest step of a chemical reaction that determines the rate at which the overall reaction proceeds

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Reaction quotient (Q)

The ratio between the concentration of products and reactants in a reversible reaction not at equilibrium

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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