DF.4 Flashcards
Bond enthalpies
What is required in order to break a bond?
Energy to overcome the forces of attraction between the atoms or ions.
What is the quantity of energy needed to break a particular bond in a molecule called?
Bond dissociation enthalpy or just bond enthalpy.
What sign does bond enthalpies always have?
+ positive, because breaking a bond always involve an input of energy, it’s always an endothermic process.
What are bond enthalpy values able to indicate about a specific bond?
It’s relative strength, the bigger the bond enthalpy, the more energy is required to break it, therefore the stronger the bond.
What is the molecular orbital?
The region between atoms where shared electrons could exist.
When atoms come together to form a bond, as well as there being attractive forces between the nuclei and electrons, what other force is also there?
Repulsive forces between the atoms’ nuclei.
What is the equilibrium bond length?
The most stable and optimized distance between the atoms, where the repulsive forces are minimized and the attractive forces maximized.
What is the relationship between the bond length and bond strength?
The shorter the bond length, the stronger the bond, the bigger the bond enthalpy.
Why are bond enthalpies normally averages?
Because the exact value of bond enthalpy also depends on the particular compound the bond is in.
How are bond enthalpies normally measured?
Indirectly using enthalpy cycles.
How does the bond breaking energy compare to the bond making energy in an exothermic reaction?
The energy taken in to break bonds is less than the energy given out when bonds are made, therefore the reaction gives out energy overall and is exothermic.
How does the bond breaking energy compare to the bond making energy in an endothermic reaction?
The energy taken in to break bonds is more than the energy given out when bonds are made, therefore the reaction takes in energy overall and is endothermic.
Why does most reactions require some heat input to get them started?
Because reactions need some energy to stretch and break bonds initially.
What sign is put in front of the enthalpy change when bonds are made?
- negative, because making bonds is an exothermic process.
Why is the value of enthalpy change calculated from bond enthalpies different from the standard measured values sometimes?
- Bond enthalpies given are only averages and are not specific to the molecule.
- Bond enthalpies are quoted in gaseous states, and sometimes it doesn’t fit with the standard conditions of enthalpy changes.
