Exam 1 Flashcards
Endothermic
products are higher in energy than the reactants
increase in enthalpy
positive H
Exothermic
products are lower in energy than the reactants
decrease in enthalpy
negative H
When is there a negative H?
when the reaction is exothermic
When is there a positive H?
when the reaction in endothermic
bond enthalpy
change in enthalpy that is associated with breaking a particular bond in 1 mole of gaseous molecules
bond enthalpy in polyatomic molecules
hard to measure since it requires different amounts of energy to break the second bond versus the first
therefore, we use an average bond enthalpy for polyatomic molecules
What is enthalpy measured in?
kJ / mol
examples of intramolecular forces
covalent and ionic bonds
Entropy sign
S
Enthalpy sign
H
Entropy
a measure of how spread out or dispersed a systems energy is
Formula for the # of energetically favorable ways molecules can be arranged (W)
W = x^n
x= # of cells that can be occupied n= # of molecules
*relates to entropy
Protein folding and energy
decrease in entropy in the system because there are less possible arrangements
reaction is exothermic though so the surroundings experience an increase in entropy
this makes the universal entropy increase
entropy of a perfect crystalline
0 since molecules are set in place
allows us to determine the absolute entropy of crystalline solids
free energy
energy available to do work
G
gibb’s free energy
G < 0
spontaneous
G > 0
nonspontaneous
G = 0
at equilibrium
When does temperature come into play when determining spontaneity?
when both H (enthalpy) and S (entropy) are either both positive or both negative
in this case one must consider temperature
athermic
not producing or absorbing heat
What information do you use to calculate delta H?
bond energies
heat of a reaction
the difference in energy between the products and reactants
will the heat of a reaction be positive or negative for exothermic reactions? why?
negative since the products are lower in energy than the reactants
unit for entropy
kJ / K
- according to ALEKS
- may want to update with notes
unit for enthalpy
kJ / mol
Why is equilibrium dynamic?
there are always forward and backward reactions taking place
equilibrium just occurs when these reactions do not change the overall numbers of reactants and products
Does decreasing volume increase the concentration of products?
it depends on if the number of moles of products is less than the number of moles of reactants
system wants to decrease the number of moles which could go in either directions
Does increasing temperature increase the concentration of products in an exothermic reaction?
no
adding heat will make the endothermic reaction occur, which is the reverse in an exothermic reaction
Does increasing temperature increase the concentration of products in an endothermic reaction?
yes
adding heat will make the endothermic reaction occur, which is the forward in this reaction
When a chemical equation is in dynamic equilibrium what is true?
- rate of forward reaction = rate of backward reaction
2. concentrations are not changing
relationship between free energy and equilibrium constant
(delta)G = -RTln(K)
R = gas constant
what is the gas constant?
8.314 J*k/mol
what is the symbol for the equilibrium constant?
K
Le Chatelier’s Principle
when a stress is applied to a system at equilibrium, the system will respond in a direction that minimizes the effect of the stress
the system will shift
shift to the right
forward reaction will make more products
shift to the left
reverse reaction will make more reactants
When Q < K …
we know we have more reactants, so shifts to the right to make more products
When Q > K …
we know we have more products, so shifts to the left to make more reactants
What happens to K when an endothermic system is heated? Why?
increases
now have more products
*check that not Q
What happens to K when an endothermic system is cooled? Why?
decreases
now have more reactants
*check that not Q
What does the saying “the solvent dissolves in water” indicate?
a spontaneous reaction
Does an increase in temperature lower the activation energy?
no
Does an increase in temperature increase the rate of a reaction?
yes
an increase in temp, increases the # of collisions, so the reaction speeds up
Why do we label the x-axis reaction progress instead of time?
a reaction can go backwards
rate of a chemical reaction
how fast a reactant is used up OR how fast a product is formed
units of rate of reaction
M / s
Ways to increase the rate of reaction?
- temperature
- amount of reactant
* comes down to the # of collisions occuring
Is the reaction quotient at equilibrium?
no
Q = K
equilibrium
sublimation
phase change from a solid to gas
endothermic because need to overcome dispersion forces in solid state (+)
entropy also increases from solid to gas (+)
so, if the reaction is spontaneous comes down to temperature
Which thermodynamic situation is entropy driven?
when enthalpy is positive and entropy is positive, but reaction is spontaneous at high temperatures
Which thermodynamic situation is enthalpy driven?
when enthalpy is negative and entropy is negative, but reaction is spontaneous at low temperatures
example of a MACROSTATE
a phase
solid, liquid, or gas
Why do proteins fold spontaneously?
their is no change in enthalpy because the hydrogen bonds made between itself are balanced by the hydrogen bonds lost with between the protein and water
entropy is positive because of the water
before folded, the water has to organize itself around the amino acid chain in a very structured way
after folded, the water is more disorganized, which allows entropy to drive protein folding and make it spontaneous
kinetics defintion
path dependent mechanisms
thermodynamics defintion
dependent on energy difference between reactants and products
path does not matter
state function
state function defintion
defined as a property that only depends on the initial and final state of a system
How can a reaction occur even is the energy level is below the activation energy?
energy is an average and individual atoms can have enough energy to make the reaction occur
What two factors are important for a reaction to occur?
amount of energy and orientation
How to calculate K for the reverse reaction, given K of the forward reaction?
1/K
How to calculate K for an equation multiplied by a number?
(K)^n
Why did the box freeze to the beaker in class example?
Because endothermic reactions take energy/heat from their surroundings
Why is there a needed amount of activation energy?
Energy is needed to break the bonds in the reactants before you can form new bonds in the products
Best possible orientation for collisions to happen
When particles are moving directly towards each other they will have greater impact and provide more energy to the reaction
Why does the rate of the reaction increase with temperature?
when temperature increases a greater number of particles have energy in excess to the activation energy so there is a greater probability of the reaction occurring
Misconception about temperature and rate of reaction
Particles move faster which makes collisions more likely
This is true, but not the main reason why
What happens when you double the concentration of a reactant?
The rate of reaction is doubled
Enthalpy
Sum of internal energy plus pressure and volume of the system
Gibb’s Free Energy
The energy that is available to do work
Enthalpy of phase change
All phase changes are accompanied by changes in enthalpy, since there is a change in energy levels
For example, melting is an endothermic process and freezing is an exothermic process
The hydrophobic effect
When water arranges itself around two separate non-polar molecules, water is highly structured
When water arranges itself around one combine non-polar molecules, water is not as structured and disorder increases
Entropy driven change
Energy changes between physical and chemical changes
Physical changes include phase changes, so there is a change in enthalpy and energy levels. Breaking intermolecular forces
Chemical changes involve the complete breaking of intramolecular bonds, so there are larger changes in energy levels
List types of bonds in order of decreasing strength
Ionic
Covalent
Dipole-dipole
Temperature
a property of matter that describes the energy of motion of component particles
comparatively measures hotness and coldness based on an absolute zero
Law of Mass Action
the principle that the RATE of a chemical reaction is proportional to the masses of the reacting substances
What two things do changes in temperature alter?
The rate of the reaction
The equilibrium coefficient (k)
How does a catalyst affect a reaction?
it increases the rate of the reaction by lowering the amount of activation energy needed
How many kJ per kcal?
4.184 kJ / kcal
Why is combustion an exothermic reaction?
Because heat is released
What happens when volume is decreased?
there is an increase in the partial pressure of all the substances
so, system shifts to least number of moles to compensate
what happens when there is an increase in volume?
there is a decrease in partial pressures of all substances
system is free to expand to greater number of moles
If nonspontaneous what must one do?
Continue supplying energy to the system in order for the reaction to occur
Why does the law of mass action exclude pure liquids and solids?
Their concentrations do not change
If I have a ton of water or a drop of water it still has the same molarity
What happens when heat is added to a reaction?
The rate of both the forward and reverse reactions is increased due to more kinetic energy and more collisions
However, the increase in kinetic energy will effect the larger activation energy more
That is why when exothermic reactions are heated they go the opposite direction because the products have lower energy than the reactants
