Enthalpy Flashcards
Describe the changes in enthalpy in an exothermic reaction
- System loses heat energy to the surroundings
- so q is negative
Describe the change in enthalpy in an endothermic reaction
- System gains heat energy from the surroundings
- so q is positive
What is q?
The amount of heat flowing in and out of the system during a chemical reaction
When is q negative?
During an exothermic reaction
When is q positive?
Endothermic
When is ∆H negative?
Exothermic reaction
What sign does ∆H have in an endothermic reaction?
∆H is positive
What is enthalpy change?
The energy absorbed in a reaction carried out at a constant pressure
How is ∆H represented?
∆H = ΣHproducts - ΣHreactants
What is the enthalpy change of combination?
The enthalpy change when 1 mole of a substance is burnt in excess oxygen at 100 kPa pressure and 298K
What is the enthalpy change of atomisation of an element?
The enthalpy change under standard conditions of 100 kPa and 298K when 1 mole of gaseous atoms is formed from it’s element in its standard state
What is the equation for the atomisation of bromine?
1/2Br(l) –> Br(g)
What is the definition of enthalpy of reaction?
The enthalpy change under standard conditions of 100 kPa and 298K when the moles in the written equation completely react
What is Hess’ law?
The enthalpy change of a reaction is independent of the route taken
What is the problem with using enthalpy change of formation?
It can’t be measured directly in organic compounds
