2.1- Thermochemistry Flashcards
(a)
Enthalpy change
The heat energy change measured at constant pressure.
(a)
Standard conditions
A pressure of 1 atm
A stated temperature of 298K
A standard concentration of 1 mol dm-3 for solution.
(a)
Exothermic reaction
A reaction in which energy is released to the surroundings.
(a)
Endothermic reactions
A reaction in which energy is taken in from the surroundings.
(a)
Definition of the standard enthalpy change of reaction
The enthalpy change that occurs when the molar quantities react under standard conditions, with all reactants and products in their standard states.
(a)
Definition of the standard enthalpy change of formation
The enthalpy change when one mole of product is formed from its elements with all reactants and products in their standard states, under standard conditions.
(a)
Definition of the standard enthalpy change of combustion
The enthalpy change that occurs when one mole of compound is completely reacted with excess oxygen under standard conditions with all reactants and products in their standard states.
(a)
Definition of the standard enthalpy change of neutralisation
The enthalpy change which occurs when an acid and alkali react under standard conditions to produce 1 mole of water.
(b)
Hess’s Law
Hess’s law states that the total enthalpy change for a reaction is independent of the route taken from the reactants to the products.
(b)
Hess’s Law equation
Route 1 = Route 2
i.e. ΔH1 = ΔH2 + ΔH3
(b)
Hess’s Law Combustion
If ΔcH (enthalpy changes of combustion) is given, the direction of the arrows is from the reactants and products to the common combustion products.
(b)
Hess’s Law Formation
If ΔfH is given, the direction of the arrows is from the common elements to the reactants and products.
(b)
Hess’s Law Formation equation
ΔH = ΔfH(products) – ΔfH(reactants)
(b)
Hess’s Law Combustion equation
ΔH = ΔcH(reactants) – ΔcH(products)
(c)
Bond enthalpy
Bond enthalpy is the enthalpy needed to break 1 mol of the bond to give separated atoms with everything being in the gaseous state.
(c)
Average bond enthalpy
Average bond enthalpy is the average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species.
(d)
Calculating enthalpy changes
ΔH = Σ(bonds broken) – Σ(bonds formed)
(d)
Equation used to work out the energy transferred in a reaction
q = mc𝚫T
q = energy released/absorbed (J)
m = mass (g)
c = specific heat capacity (J g-1 °C-1) 𝚫T = temperature change (°C)
