Topic 5 - Chemical Energetics Flashcards
Enthalpy (H)
energy required to break a bond between gaseous atoms
also called bond energy
also called bond strength
Change in Enthalpy (ΔH)
the change in bond energy during a chemical reaction
Exothermic
reactions transfer heat energy from the reactants to their surroundings
give off heat energy so ΔH is negative
Endothermic
reactions transfer heat from the surroundings to the reactants
take in heat energy so ΔH is positive
Standard Conditions (SC)
normal room conditions:
101 kPa - pressure
298 K - temperature
1 mol/dm^3 - concentration of solutions
each substance is in its normal physical state
Standard Enthalpy Change of COMBUSTION
ΔH when one mole of a substance is burnt in excess oxygen under SC
Standard Enthalpy Change of FORMATION
ΔH when one mole of a compound is formed from its elements under SC
Standard Enthalpy Change of NEUTRALISATION
ΔH when one mole of water is formed by the reaction of an acid with an alkali under SC
Standard Enthalpy Change of REACTION
ΔH when the amounts of reactants shown in the equation react to give products under SC
Hess’ Law
total ΔH change in a reaction is independent of the route by which it takes place as long as the initial and final conditions are the same
Hess’ Law Energy Cycle
diagram to help calculate ΔH changes
they look like vector triangles
Reaction Pathway Diagram
to help calculate ΔH changes
look like 3 platforms on a graph
Activation Energy
minimum energy required for a collision to be effective
Kinetic Stability
compound is kinetically stable when the activation energy is high - reaction is difficult to start
Thermodynamic Stability
chemical change is thermodynamically stable when ΔH is (a large) positive value - the reaction is difficult to keep going
