Enthalpy
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What is the law of the conservation of energy?
Energy cannot be created or destroyed, just converted from one form to another.
Define enthalpy
The heat content that is stored in a chemical system.
Define enthalpy change
The heat exchanged with the surroundings during a chemical reaction. The difference between the enthalpy of the products and the reactants.
Define exothermic
A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.
Define endothermic
A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat beaing taken out of the surroundings.
Define activation energy
The minimum energy required for a reaction to take place.
What are the standard conditions?
Standard pressure - 101kPa Standard temperature - 298K Standard concentration - 1mol/dm^3 Standard state - The physical state of a substance under standard conditions
Define standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of formation
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of combustion
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
What is the equation used to measure an energy change?
Q=mcΔT Q - energy change with surroundings (J) m - mass (g) c - specific heat capacity (J/g/K) ΔT - change in temperature (K)
How is enthalpy change worked out from the energy change?
ΔH = -Q/n ΔH - enthalpy change (J/mol) Q - energy change with surroundings (J) n - number of moles (mol)
Define standard enthalpy change of neutralisation
The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O, under standard conditions, with all reactants and products in their standard states.
What are the causes for less energy being transferred than expected when working out ΔH(c)?
Heat loss to the surroundings Incomplete combustion Evaporation Non-standard conditions
How can heat loss be accounted for using a graph of temperature against time?
Extrapolate the cooling curve back to when it was added.
Enthalpy change of neutralisation always has the same value, what is it?
-57.5 kJ/mol
Define average bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule.
What are some properties of bond enthalpies?
Energy is always required to break bonds Bond enthalpies are always endothermic Bond enthalpies always have a positive enthalpy value
What kind of energy and enthalpy changes are bond formation?
Exothermic and releases energy
What is Hess’ law?
If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.
What is the equation for the enthalpy change from formation enthalpies?
ΔH = ΣΔHf°(Products) - ΣΔHf°(Reactants)
What is the equation for the enthalpy change from combustion enthalpies?
ΔH = ΣΔHc°(Reactants) - ΣΔHc°(Products)
How to calculate enthalpy change from bond enthalpies?
ΔH = Σ(Reactant Bond Enthalpies) - Σ(Product Bond Enthalpies)