3.1.4 Energetics Flashcards
ΔH =
ΔH= energy to break bonds (+ve) + energy to make bonds (-ve)
Enthalpy change
The change in heat energy of a substance at constant pressure.
Endothermic
Describes a reaction in which heat is taken in as the reactants change to products; the temperature of the surroundings thus drops
Exothermic
Describes a reaction in which heat is given out as the reactants change to products; the temperature of the surroundings rises
Describe the enthalpy diagram for an endothermic reaction
Reactants are at a lower enthalpy (y axis) than the products, which are at a higher enthalpy. The difference is a +ve ΔΗ
Describe the enthalpy diagram for an exothermic reaction
The enthalpy of the reactants is higher than that of the products. The difference is -ve ΔΗ
Standard conditions
100kPa
298K
Enthalpy of Formation
The standard molar enthalpy of formation, ΔfH, is the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states
Enthalpy of combustion
The standard molar enthalpy of combustion ΔcH, is the enthalpy change when one mole of substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states
q=
q=mcΔT
Calorimeter
An instrument for measuring the heat changes that accompany chemical reactions
What does q=mcΔT stand for and what are the units
q : Energy change (J)
m : mass (g)
ΔΤ : change in temperature (K)
Equation to calculate enthalpy change per mole of a substance
ΔH = q/mol
Give the units of ΔH = q/mol
ΔΗ (kJ/mol)
q (kJ)
moles
Specific heat capacity
Is the amount of energy required to raise 1g of a substance by 1K without a change in states. (J g-1 K-1)
Thermochemical cycle
A sequence of chemical reactions (with the enthalpy changes) that convert a reactant into a product.
The total enthalpy change of the sequence of the reactions will be the same as that for the conversion of the reactant to the product directly (or by any other route).
Hess’s Law
Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Mean bond Enthalpy
The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds.
Bond disassociation enthalpy
The enthalpy change required to break a covalent bond with all species in standard states