What is enthalpy?
Heat content in a substance
What is enthalpy change?
the heat energy change at constant pressure
What is the difference between a closed system and an open system?
closed system = only exchanges energy not matter
open system = exchanges energy and matter
How do you calculate enthalpy change?
heat content of products - heat contents of reactants
or
heat energy change/number of moles of reactant
What does enthalpy change depend on?
pressure and temperature
What are standard states and conditions?
- elements or compounds in their normal physical state e.g: chlorine gas or water liquid
- pressure of 100kPa / 1 atm
- temperature of 25°C (298K)
What is standard enthalpy change of formation?
the enthalpy change when one mole of the compound in its standard state is formed from its constituent elements under standard conditions. e.g: H2(g) + 1/2O2 -> H2O
What is standard enthalpy change of combustion?
the enthalpy change when one mole of the substance burns completely under standard conditions e.g: C3H8(g) + 5O2 -> 3CO2 + 4H2O
Why is the standard enthalpy change of formation of an element always 0?
By definiton
If a reaction is endothermic, what will the energy level diagram look like?
Products are higher than reactants
If a reaction is exothermic, what will the energy level diagram look like?
Products are lower than reactants
If a reaction is exothermic what will the enthalpy change be?
negative
If a reaction is endothermic what will the enthalpy change be?
positive
What is enthalpy change measured in?
kJmol-1
Define standard enthalpy change of a reaction
The heat energy change measured under standard conditions
What is the equation used to calculate heat energy change from experimental data?
q=mc△T
What is average bond enthalpy?
when one mole of a covalent bond is broken in gaseous conditions
