reactivity 1.2 Flashcards
define chemical bond
a force of attraction between two atoms
breaking bonds are what type of process?
endothermic because breaking the bond requires the input of energy
when bonds are broken or made, ______ changes take place
enthalpy
what does the energy change required to break the bond depend on?
the atoms that form the bond
define bond dissociation enthalpy
the energy required to break a particular bond
other terms for bond dissociation enthalpy x2
bond enthalpy OR bond energy
bond formation is an _______ process
exothermic
if more energy is released when new bonds are formed than energy is required to break bonds, the reaction is ________. which is more stable (products v reactants)?
exothermic, products are more stable than reactants
if more energy is required to break bonds than energy is released when new bonds are formed, the reaction is ________. which is more stable (products v reactants)?
endothermic, products are less stable than reactants
bond energies are affected by ____ ____ in the molecule (the environment)
other atoms
define average bond energy
the energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds
how to find the average bond enthalpy for C-H
take the bond dissociation enthalpy for the whole molecule and divide it by the number of C-H bonds
how to find standard enthalpy of reaction when experimentation is not an option? equation?
by using bond energies
ΔH^ꝋᵣ = ΔH for bonds broken + ΔH for bonds formed
define hess’s law
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 (whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same)
the enthalpy change from _____ to _____ (direct route) is equal to the enthalpy change of _____ forming ____ and then ___ (indirect route)
elements to products, elements forming reactants and then products
what is ΔH₂ (hess’s law energy cycles)
the products can be directly formed from the elements
what is ΔH₁ and ΔHᵣ (hess’s law energy cycles)
the products can be indirectly formed from the elements
equation for direct route between elements → products (hess’s law energy cycles)
ΔH₂ = ΔH₁ + ΔHᵣ
when is hess’s law used?
to calculate enthalpy changes which can’t be found experimentally using calorimetry
two methods to solve hess’s law problems
using cycles AND using equations
what can hess’s law be used to calculate?
the standard enthalpy change of a reaction from known standard enthalpy changes
two key rules for hess’s law
if you follow the direction of the arrow you add the quantity AND if you go against the arrow you subtract the quantity
