2.1 Flashcards
What is enthalpy, H
Heat content of a system at constant pressure
What is enthalpy change
Head added to a system at constant pressure
Equation to measure enthalpy change
Enthalpy of products - enthalpy of reactants
Exothermic reaction
- releases energy to surroundings
- temperature rise
- enthalpy change is negative
Endothermic reactions
- absorb energy from surroundings
- temperature drop
- enthalpy change is positive
What is the principle of conservation of energy
energy cannot be created or destroyed only changed from one form to another.
Conditions for standard enthalpy change
- all substances in their standard states
- 298 K (25 c)
- 1 atm (101000 Pa)
What is the standard enthalpy change of formation
the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions.
What is the standard enthalpy change of formation of all elements in their standard state
0 kJ mol-1
Standard enthalpy change of formation of water
H2 (g) + 1/2 O2 (g) —–> H2O (l) /\fH = -286 kJ mol-1
What is the standard enthalpy change of combustion
enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.
What is the standard enthalpy change of combustion of methane
CH4 (g) + 2O2 (g) —-> CO2 (g) + 2H2O (l) /\cH = -891 kJ mol-1
Equation for the standard enthalpy of reaction
/\rH = sum/\fH(products) - sum/\fH(reactants)
What is Hess’s law
the total enthalpy change for a reaction is independent of the route taken from the reactants to the products
What is bond enthalpy
the enthalpy required to break a covalent X-Y bond into X atoms and Y atoms, all in the gas phase