3.1.4 Energetics - Physical Flashcards
1
Q
3.1.4 Energetics
A
2
Q
Enthalpy change
A
- Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
- Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature (eg ∆H298Ɵ).
3
Q
Endothermic/ exothermic
A
- reactions can give out heat energy to surroundings or take in
- Endo - absorb energy from surroundings ( products higher in energy then reactants ) + , e.g. thermal decomposition of caco3
- exo - release energy to the surrounding - e.g. combustion of ethene
- bonds broken - Endo - + h - more energy to break bonds then energy given out
- bonds made- Endo - - h - more energy released when bonds formed then made
4
Q
Mean bond enthalpy
A
The Mean bond enthalpy is the enthalpy change needed to break the covalent bond into gaseous atoms,
- enthalpy change - total energy to break bonds - total energy released forming Bonds
- Reactants- products
- bonds of same type don’t all have same amount of energy
5
Q
Hess’s law
A
- Germanic Hess - law work out enthalpy changes
-Hess’s law - the total enthalpy change of a reaction is independent to the route taken - multiply by number of moles in equation
- formation - route - A (left side ) opposite , B right side ( same )
- combustion - arrows point downwards
6
Q
define standard enthalpy of combustion (∆cHƟ)
A
the enthalpy change when one mole of a compound is completely burnt in oxygen with all the reactants and products in their standard state under standard conditions
7
Q
define standard enthalpy of formation (∆fHƟ).
A
the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions
