3.1.4 energetics Flashcards
enthalpy change
the heat energy change measured under conditions of standard pressure
enthalpy change
the heat energy change measured under conditions of standard pressure
standard enthalpy change
refers to standard conditions:
- 100kPa
- 298K (25°C)
standard enthalpy of combustion
the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states
standard enthalpy of formation
the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions all reactants and products being in their standard states
define a exothermic reaction
in a exothermic reaction energy is transferred from the system to the surroundings.
The products have less energy than the reactants.
show a exothermic reaction profile and give examples of exothermic processes
Common oxidation exothermic processes are the
combustion of fuels and the oxidation of
carbohydrates such as glucose in respiration.
define a endothermic reaction
in an endothermic reaction , energy is transferred
from the surroundings to the system.
The products have more energy than the reactants.
They require an input of heat energy e.g. thermal
decomposition of calcium carbonate.
.
heat change equation
q = m x c x ∆T
heat change (J) = mass of water (g) x specific heat capacity (J/kg°C) x temperature change (°C)
hess’s law
the total enthalpy change for a reaction is independent of the route by which the chemical change takes place
mean bond enthalpy
the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
equation for mean bond enthalpy
∆H = Σ bond energies broken - Σ bond energies made
explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law
Mean bond enthalpies are mean values calculated across a range of compounds containing that bond where as Hess’s law find the actual values of the bond enthalpies in that reaction
bond dissociation energy (bond enthalpy)
the amount of energy required to break one mole of a specific covalent bond in the gas phase
why are mean bond enthalpy values positive
These values are positive because energy is required to break a bond.
