Enthalpy Flashcards
What is enthalpy?
Measure of the heat energy in a chemical system at constant pressure
What is enthalpy change?
Amount of heat energy in/out in any change in a system
reactants - products
What are features of exothermic?
From system to surroundings
Releases heat energy -ve
Temperature of surroundings increases (gain energy)
reactants above products
What are features of endothermic?
From surroundings to system
Takes in heat energy +ve
Temperature of surroundings decreases (lose energy)
products above reactants
What is the law of conservation of energy?
Energy cannot be created/destroyed, but can be converted between forms.
What is the activation energy?
Minimum energy needed for a reaction to occur.
What are standard conditions?
Standard state at standard conditions
100 kPa
298 K
solutions at 1 mol dm-3
What is the standard enthalpy change of reaction?
Occurs in a reaction in molar quantities show in chemical equation.
What is the standard enthalpy change of formation?
Occurs when 1 mole of compound is formed from its elements.
What is the enthalpy change of combustion?
Occurs when 1 mole of substance reacts completely with oxygen.
What is the enthalpy change of neutralisation?
Occurs when acid + base –> neutralisation reaction to form 1 mole of water.
Why is the enthalpy less than expected?
- Assumes the specific heat capacity of water is the same as the solution
- Some energy is released to the surroundings
-Incomplete/slow reaction - Non standard conditions
What is the average bond enthalpy?
Energy needed to break 1 mole of a specific bond type in a gaseous molecule.
Why is Hess’ law used?
If the reaction can take place via 2 routes, the total enthalpy change is the same for each route.
The enthalpy change accompanying chemical change is independent of the route of chemical change.
Why is an enthalpy different from a textbook value?
Energy is always needed to break bonds. Actual enthalpy values vary depending on chemical environment of the bond (surrounding bonds).
