3.2.1 - Enthalpy Changes Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What happens in exothermic reactions?
Energy is given out, ΔH is negative.
Equation for exothermic reaction.
C6H12O6 + 6O2 -> 6CO2 + 6H20
What happens in endothermic reactions?
Energy is absorbed, ΔH is positive.
Equation for an endothermic reaction.
6CO2 + 6H20 -> 6O2 + C6H12O6
In an endothermic reaction what is more stable?
Reactants
In an exothermic reaction what is more stable?
Products.
On an enthalpy profile diagram what are the names of the axis?
Reaction pathway and enthalpy / kJmol-1
What are the standard conditions?
298K, 1atm, 1moldm-3
Standard enthalpy change of formation?
1 mole of product must be formed.
Standard enthalpy change of combustion?
1 mole of reactant must be combusted or burnt.
Standard enthalpy change of neutralisation?
1 mole of water must be formed.
Bond breaking is?
Endothermic
Bond making is?
Exothermic
q=
mcΔT
What does q stand for?
Enthalpy change / heat lost or gained.
Average bond enthalpy?
Breaking 1 mole of bonds in gaseous molecules.
Why isn’t it always possible to measure the enthalpy change of a reaction in a lab?
- the reaction may have a high activation energy.
- the rate of reaction may be very low.
- more than one reaction could be taking place (side reaction)
In enthalpy of combustion which way do the arrows point?
Downwards.
What are the products of enthalpy of combustion?
CO2 and H2O.
What cannot be combusted?
Water, CO2 and oxygen.
Which way do the arrows point in enthalpy of formation?
Upwards.
What doesn’t need to be considered during an enthalpy of formation?
Elements.
What can be reacted during an unfamiliar enthalpy cycle?
HCl
