Enthalpy Flashcards
What is Enthalpy?
Enthalpy is the thermal energy stored within a system.
How is Enthalpy measured?
Enthalpy is impossible to measure directly, so instead the energy lost to or gained from the surroundings is instead measured. The most common way to do this is to measure a change in thermal energy.
What is a system?
The actual chemical reaction (atoms and bonds involved).
What are the surroundings?
Everything apart from the System. This includes the aqueous substances substances may be dissolved in.
What is a Chemical System?
Reactants and Products.
What can Enthalpy change be defined as?
Heat exchanged with surroundings (at constant pressure).
Difference between enthalpy of products and reactants. (ΔH = Hproducts - Hreactants)
What do energy profile diagrams show?
These show how the enthalpies of the products and reactants change over the course of the reaction.
They also emphasise the exothermic/endothermic nature of the reaction.
What is the Activation Energy?
Minimum energy required to start a reaction by breaking the bonds in the reactants.
Why are Exothermic reactions described as self-sustaining?
The net energy output of the reaction once started provides the activation energy so the reaction self-sustains.
Describe the properties of an Exothermic reaction.
ΔH is negative.
Heat is lost to the surroundings.
Reacting Chemicals loose energy.
Heat lost gained by chemicals = heat gained by surroundings.
Describe the properties of an Endothermic reaction.
ΔH is positives.
Heat is gained from the surroundings.
Reacting Chemicals gain energy.
Heat gained gained by chemicals = heat lost by surroundings.
What are the Standard Conditions?
100KPA (1 ATM)
273K
All substances in standard state.
Enthalpy Change of Formation:
ΔrHθ - Energy change that takes place when 1 Mole of a compound is formed from its constituent elements in their standard states under standard conditions.
Fractions must be used when writing a balanced equation as there can only be 1 mole of product.
Enthalpy change of Combustion:
ΔcHθ - Energy change that takes place when 1 mole of a substance is fully combusted.
Enthalpy change of neutralisation:
ΔneutHθ - Energy change associated with the formation of 1 mole of water from a neutralisation reaction.
