R1.2: Energy Cycles In Reactions Flashcards
(7 cards)
What is the bond dissociation enthalpy?
Usually referred to as bond enthalpy
The energy required to break a particular bond
- will change based on atoms involved in the bond
The energy released when a bond is formed will always be the SAME MAGNITUDE as the energy taken in when a bond is broken (opposite signs)
How can you calculate overall enthalpy changes?
Starting enthalpy - final enthalpy
If energy released when new bonds formed > energy required to break bonds -> exothermic
- products more stable than reactants
If energy required to break bonds > energy released when new bonds formed -> endothermic
- products are less stable than reactants
Use calorimetry
q = mcΔt
What are average bond enthalpies?
The average energy required to break one mole of the bond in similar compounds all in the gaseous state.
Single covalent bond < double covalent bond < triple covalent bond
Some bonds (ex: H-H or O=O or H-Cl) can only exist in one compound -> consistent bond enthalpy value
Other bonds (C-H or N-H or O-H or C-C) bond enthalpies will influenced by the nature of the other atoms attached to
-> average taken
Ex: methane
Breaking first C-H wool be easier than second -> rest of H get closer to C
-> but its impossible to measure the energy of each C-H bond
Solution: take the bond enthalpy of the whole molecule, divide by 4 (4 x C-H bonds)
How are bond enthalpies calculated?
Everything must be in a gaseous state
Difference between the total of all the bond enthalpies in the reactants and products:
Total bond enthalpies greater in product -> exothermic
Total bond enthalpies greater in reactant -> endothermic
Bond energies -> used to find the ΔHꝋr (enthalpy of reaction) when it cannot be experimentally found
- summation of bond enthalpies of all the molecule present
What is Hess’ Law (of constant heat summation)?
Basis of the law of conservation of energy
(Energy cannot be destroyed -> only change in form)
Hess’ law states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same
TLDR: as long as same end and start you good
Used to calculate the enthalpy of reactions from known standard enthalpy changes
- cannot be found experimentally
What is standard enthalpy of formation?
ΔHf⦵
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
What is standard enthalpy change of combustion?
ΔHc⦵
The enthalpy change that occurs when one mole of the substance burns completely under standard conditions
