4.1 - Enthalpy Flashcards
(9 cards)
Standard enthalpy change of combustion
1 mole of a substance combusts fully in oxygen under standard conditions
Standard enthalpy change of formation
1 mole of a compound forms from the elements under standard conditions
Standard enthalpy change of neutralisation
When an acid and alkali neutralise under standard conditions, to form 1 mole of water
Standard enthalpy change of reaction
When a reaction occurs in the molar quantities shown in the chemical equation under standard conditions
What are standard conditions:
- 298K temperature
- 101kPa
- plimsoll denotes standard conditions
what are key features of this profile diagrams (activation energy, difference between exothermic and endothermic reactions and what happens when there’s less enthalpy)
- activation energy is the minimum energy needed for a reaction to occur
- in exothermic reactions, reactants have a higher energy than the products and the enthalpy points upwards
- in endothermic reactions, reactants have lower energy and the enthalpy points downwards
- when there’s less enthalpy, substances are more stable
What happens during endothermic reactions? (what happens to heat, what is the enthalpy change value and what happens to the temperature of the surroundings)
- absorb heat energy from the surroundings
- have a positive enthalpy change
- temperature of surroundings decrease
What happens during exothermic reactions? (what happens to heat, what is the enthalpy change value and what happens to the temperature of the surroundings)
- release heat energy to the surroundings
- have a negative enthalpy change value
- temperature of the surroundings increases
what happens during enthalpy change (what pressure does it occur under and what is it known as?)
- chemical bonds can break or form and energy is often released or absorbed as heat
- often occurs at constant pressure
- known as enthalpy change (kJ/mol)
