Topic 8 + 10 - energetics and equilibria Flashcards
Define enthalpy change
The heat change measured under conditions of constant pressure
What are standard conditions?
100 kPa
Stated temp which is usually 298K
Define standard enthalpy of combustion
The enthalpy change where one mole of a substance is burned completely in oxygen under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy of formation
The enthalpy change where one mole of a substance is formed from its elements under standard conditions, with all reactants and products being in their standard states.
Define Hess’ law
The enthalpy change of a reactions depends only on the initial and final states of the reaction and is independent of the route by which the reaction occurs.
Define mean bond enthalpy
The enthalpy change when one mole of a covalent bond is broken, with all species in the gaseous state, averaged over a range of different compounds.
Breaking bonds is?
Endothermic
Energy is required
Enthalpy change is positive
Making bonds is?
Exothermic
Energy is released
Enthalpy change is negative
Explain why values calculated from mean bond enthalpies often differ from those determined by using Hess’ law?
Mean bond enthalpies is an average while Hess’ law uses specific enthalpies for particular reactions.
Mean bond enthalpies do not include intermolecular forces while Hess’ law does include these effects through standard enthalpy changes.
What does it mean if a reaction is dynamic/
Occurs in both the forwards and backwards directions simultaneously.
What happens during a reversible reaction at equilibrium?
Forward and backward reactions occur at the same rate.
Concentrations of reactants and products remain constant.
Define Le Chatlier’s Principle?
a system at equilibria will shift or move to oppose any change imposed upon it
