Orgo Ch 4.6-4.8
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In a calorimeter, the __ is used to calculate the precise value of the __.
- temerature rise
- heat of reaction
When heat is evolved, the reaction is __ and the heat of reaction is __
How can we predict whether a reaction is endo or exothermic without actually measuring the heat?
What must we know?
- calculate an approx. heat of reaction by adding and subtracting the energies involved in the breaking and forming of bonds
- the energies of the affected bonds
The __ is the amount of enthalpy required to break a particular bond __-- that is, in such a way that each bonded atom retains one of the bon's two electrons. In contrast, when a bond is broken __, one of the atoms retains both electrons.
- bond-dissociation enthalpy (BDE, or bond-dissociation energy)
__ forms __, while __ forms __.
- homolytic cleavage (radical cleavage)
- free radicals
- heterolytic cleavage (ionic cleavage)
Enthalpies for __ depend strongly on the solvent's ability to __ that result.
- heterolytic (ionic) cleavage
- solvate the ions
__ is used to define __ because the values do not vary so much with different solvents or with no solvent.
- homolytic cleavage
- bond-dissociation enthalpies
__ is used to show the movement of the electron pair in an ionic cleavage, and __ are used to show the separatio of individual electrons in a homolytic cleavage.
- a curved arrow
- curved half-arrows
Energy is released when bonds are __, and energy is consumed to __. Therefore, __ are always __ (__).
- break bonds
The overal enthalpy change for a reaction is the __ .
sum of the dissociation enthalpies of the bonds broken minus the sum of the dissociation enthalpies of the bonds formed
WHat can we use BDE values for?
to predict the heat of reactiosn that involves breaking and forming of bonds
The bond-dissociation enthalpies also __.
provide the heat of reaction for each indivdiual step
the sum of the values of ΔHo for the individual __ gives the __ for the reaction.
- propagation steps
- overall enthalpy change
True or False:
There are alternative mechanisms that will vary in energies.
study of reaction rates
How fast a reaction goes is just as important as the __
position of its equilibrium
Just because __ favors a reaction (__) does not mean that it will actually occur.
- negative ΔGo
The __ is a measure of how fast the products appear and the reactants disappear. This can be determined by measuring the increase in the __ or __.
- rate of a reaction
- increase in the concentrations of products with time
- decrease in the concentrations of the reactants with time
Reaction rates depend on __. How?
- concentration of the reactants
- the greater the concentrations, the more often the reactants collide and the greater the chance of reaction
A __ is the relationship between the concentration of the reactants and the observed reaction rate. Each reaction has its own __, determined __ by changing the __ and measuring the change in the rate.
- rate equation (rate law)
- rate equation
- changing the concentrations of the reactants
The reaction rate is usually proportional to what?
- the concentrations of the reactants ([A] and [B]) raised to some powers, a and b
- rate law= kr[A]a[B]b
- kr is the rate constant
- the powers must be determined experimentally
The rate equation depends on what?
the mechanism of the reaction and on the rates of the individual steps
In the general rate equation, the power a is called the __ of the reaction with respect to reactant A, and b is the __ of the reaction with respect to B. the sum of these powers, (a+b), is called the __ of the reaction.
the reagent raised to the first power
Two first order reactions result in __.
second order overall because the sum of the powers of the concentrations in the rate equation is 2 (first order +first order)
Reactions of the same overall type do not necessarily have the same same form of __.
The most important fact to remember is that the __.
rate equation must be determined experimentally
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