# Unit 4.1 Energetic Definitions

Taken from the A level edexcel specification

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Activation energy
The minimum energy needed for a reaction to start and have successful collisions.
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Exothermic reaction
Σ Bond enthalpies of the reactants < Σ Bonds enthalpies of the products
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Endothermic reaction
Σ Bond enthalpies of the reactants > Σ Bond enthalpies of the products
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All elements
In their standard states have a value of zero enthalpy for their standard enthalpies of formation.
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Enthalpy Change (ΔH)
The heat energy change measured under conditions of constant pressure.
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Standard enthalpy change (ΔH°)
Is stated for 1 mole at a pressure of 1 atm and a stated temperature. Unless the temperature is stated, the standard condition for temperature is 298K.
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Standard enthalpy change of formation (ΔH°f )
Is the enthalpy change when one mole of a substance in its standard state is formed from its elements in their standard states under standard conditions of 1 atm pressure and specified temperature (usually 298K).
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Standard enthalpy change of reaction (ΔH°r)
Is the enthalpy change when amounts of reactants react together under standard conditions to give products in their standard states.
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Standard enthalpy change of combustion (ΔH°c)
Is the enthalpy change when one mole of a substance is completely burned in excess oxygen, all reactants and products in their standard states under standard conditions of 1 atm and specified temperature.
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Standard enthalpy change of atomisation of an element (ΔH°at)
Is the enthalpy change when 1 mole of gaseous atoms are formed from the element in its standard state at 1 atm pressure and 298K.
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Standard enthalpy change of neutralisation
The enthalpy change when one mole of water is formed from reaction of an acid with a base under standard conditions.
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Hess's Law (ΔH1 = ΔH2 + ΔH3)
The total enthalpy change for a reaction is independent of the route taken provided the initial and final conditions are the same.
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## Other cards in this set

### Card 2

#### Front

Σ Bond enthalpies of the reactants < Σ Bonds enthalpies of the products

#### Back

Exothermic reaction

### Card 3

#### Front

Σ Bond enthalpies of the reactants > Σ Bond enthalpies of the products

### Card 4

#### Front

In their standard states have a value of zero enthalpy for their standard enthalpies of formation.

### Card 5

#### Front

The heat energy change measured under conditions of constant pressure.