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MCAT chemistry > chapter 4 > Flashcards

chapter 4 Flashcards

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1
Q

many reactions are?

A
  • reversible, eventually a state will be reached in which the forward and reverse reactions are proceeding at the same rate (equilibrium) so no net change in concentrations of reactants or products (dynamic equilibrium)
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2
Q

what is the equilibrium constant?

A

keq which is equal to the ceoncentration(s) of the products raised to the power of its respective coefficient divided by the concentration(s) of the reactants, which are also raised to the power of its coefficient

  • a large Keq favours the products
  • a small keq favours the reactants
    • concentrations must be equilibrium concentrations and solids/liquids should not be included in the calculation
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3
Q

what is the keq for the reverse reaction?

A
  • its the reciprocal of the keq for the forward process 1/keq
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4
Q

keq is dependent on?

A
  • temperature
    • assume temperature is at 25ºC
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5
Q

If the reaction is not in equilibrium, we use?

A

Q, the reaction quotient

  • the same equation except with Q= [products]/[reactants] with no solids or liquids and raised to their respective coefficients
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6
Q

Q vs keq says?

A
  • when Q < keq, the ratio of products to reactants is lower than it would be at equilibrium, so the reaction will proceed in the forward direction to increase the product concentrations and thus move towards equilibrium
  • Q > keq, the ratio of products to reactants is greater than it would be at equilibrium, so the reaction will proceed in the reverse direction to increase the reactant concentrations
  • Q = keq we are already at equilibrium
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7
Q

what is Le Chateliers principle?

A
  • if an equilibrium mixture is disrupted, it will shift to favour the direction of the reaction that best facilitates a return to equilibrium
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8
Q

if we add more concentration to one side?

A
  • the reaction will shift to the opposite side to balance out that excess of added concentration of one substance
    • even if it is paired with another thing, that other thing will decrease but can still reach equilibrium
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9
Q

what happens if we remove something from either side of a reaction?

A
  • the reaction will shift to the side where that substance was to try and restore it
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10
Q

Le chateliers principle can be used to?

A
  • maximize the yield of a reaction procedure
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11
Q

pressure and volume are?

A
  • inversely proportional
    • if pressure increases, volume decreases vice versa
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12
Q

Le Chateliers principle with pressure and volume?

A
  • an increase in volume means that our gas molecules will spread out, and less pressure will be exerted on the walls of the container so the system will shift in such a way as to regenerate our lost pressure so the system will shift to the side of the reaction that contains more moles of gas
  • if we reduce the volume in the container, packing more gas molecules into a smaller area and increasing pressure. to relieve this stress, the system will shift toward the side containing fewer moles of gas
    • ONLY COUNT MOLES OF GAS
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13
Q

what happens if temperature changes according to Le Chateliers principle?

A
  • we must know if a reaction is endothermic or exothermic so look for delta H values or reaction types (combustion for example is exothermic)
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14
Q

what can we add to a reaction that won’t cause a shift?

A
  • addition of a catalyst- increases the rate of a reaction, causing it to rech equilibrium more quickly. if a reaction is already at equilibrium, no disruption takes place
  • addition of an inert (unreactive) gas at constant volume
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15
Q

what is temperature?

A
  • is a property of the average kinetic energy of the particles under study and is directly proportional to average kinetic ebergy
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16
Q

what is the conversion from K to degrees Celsius?

A
  • K = ºC + 273
    • lowest temperature possible is 0K
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17
Q

what does heat refer to?

A
  • the transfer of energy, specifically thermal energy, between objects of different temperatures
18
Q

what is the zeroth law of thermodynamics?

A
  • the zeroth low- describes the thermal relationship between systems. specifically, it states that if one system (system A) is in thermal equilibrium with another (system B), and system B is in thermal equilibrium with system C, then system A must also be in thermal equilibrium with system C “thermal equilibrium” means they have the same temperature
    • if one system has a different temperature, then heat transfer will take place
19
Q

what is an open system?

A
  • those which can exchange both matter and energy with their surroundings
20
Q

what is a closed system?

A
  • energy can be exchanged with the surroundings, but matter cannot
21
Q

what is an isolated system?

A
  • does not exchange matter or energy with their surroundings and are thus perfectly insulated
22
Q

what is the first law of thermodynamics?

A
  • the first law is most relevant for closed systems
    • in such a system, the change in the internal energy of the system (deltaU) is equal to the heat transfer into the system (Q) minue the work performed by the system on its surroundings (W)
    • deltaU = Q - W
      • Q heated = positive, heat lost = negative
      • work done by the system on its surroundings = positive work, worl done on the system should be negative
23
Q

what is a technique used to measure heat?

A
  • calorimetry
24
Q

for MCAT chemistry, the most common example of work done by a system is the expansion of gas against a flexible container or piston

the most common case of work done on a system is?

A
  • gas compression
25
Q

the equation most often used for calorimetry-related calculations is?

A
  • Q = m c deltaT
26
Q

what is enthalpy?

A
  • thermodynamic measurement of heat, with typical units of J or kJ, same units as energy
    • describes changes to a system, particularly changes that involve gain or loss of thermal energy
    • H = deltaU + P deltaV so deltaH = Q
    • endothermic processes require heat input and have positive delta H values
    • exothermic processes release heat are associated with negative delta H values
27
Q

if delta H° has the degree what does it mean?

A
  • menas that the reaction is under standard conditions, which are 1 atm and 25°C
28
Q

what is the standard enthalpy of formation (deltaHºf)?

A
  • the change in enthalpy associated with the formation of one mole of the compound from its component elements under standard conditions
    • these elements must be in their standard states: that is the phase they exist in at 1 atm and 25º
    • interchangeable with “standard heat of formation”
29
Q

how to calculate the standard enthalpy change?

A
  • deltaH°rxn = deltaH°products - deltaH°reactants
    • ​pure elements value is zero
    • divide the number by the coefficient that is in front of it if there is one for final answer
30
Q

what is the concept of bond dissociation energy?

A
  • the bond dissociation energy is the enthalpy change associated with the breaking (technically homolysis, or splitting where electrons are divided equally) of a particualr bond
    • bond enthalpies can be used to calculate deltaH°rxn, but in this case the relevant equation is
      • deltaH°rxn = deltaH (bonds broken/reactants) - deltaH (bonds formed/products)
        • negative deltaH°rxn if exothermic, positive if endothermic
31
Q

what is Hess’s law used for?

A
  • calculate the deltaH for the overall process
    • states that the enthalpy change of the individual steps of a chemical reaction can be added together to equal the overall enthalpy change of the reaction
32
Q

what is entropy?

A
  • related to the number of configurations a system can possibly have, with more potential configurations corresponding to greater entropy
    • solids < liquids < gases
33
Q

what is the second law of thermodynamics?

A
  • the entropy of an isolated system will never decrease, and the entropy of the universe is always increasing
    • a reaction may decrease in entropy provided it is paired with processes that increase the entropy so that the overall entropy of the system increases
34
Q

entropy is associated with?

A
  • deltaS
    • an entropy increase (+ deltaS) is associated with favourable reactions
    • an entropy decreases (- deltaS) is associated with unfavourable reactions
      • units for entropy are J/Kelvin or J/Kelvin•mol
      • standard entropy is described as deltaS°
      • the entropy of an overall reaction is termed deltaS°rxn and deltaS°rxn = deltaS°products = deltaS°reactants
35
Q

what is Gibbs free energy?

A
  • the maximum amount of work that can be performed by a system consistinf of one or more reversible chemical reactions
    • quantitiy that tells us whether a chemical reaction is spontaneous
    • deltaG = deltaH - TdeltaS
36
Q

what are exergonic vs endergonic reactions?

A
  • negative delta G = exergonic / spontaneous
  • positive delta G = endergonic / non spontaneous
37
Q

thermodynamics has no affect on?

A
  • reaction rate/kinetics
38
Q

Delta G table summarized:

A
39
Q

equation for deltaG°rxn = deltaG°products - deltaG°reactants

A
40
Q

the equation relating deltaG°rxn to keq and Q?

A

deltaG°rxn = -RTlnkeq

deltaGrxn = deltaG°rxn + RTlnQ

  • if keq = 1 then deltaG°rxn is 0, impying the reaction is already at equilibrium
  • if keq is greater than 1 means products are favoured and deltaG°rxn is negative
  • if keq is less than 1 means reactants are favoured and deltaG°rxn is positive
  • if Q is sufficiently small, the RTlnQ will result in a negative deltaG°rxn and a spontaneous rxn and this means Q < keq
  • if Q is large, the RTlnQ will result in a positive deltaG°rxn and a nonspontaneous reaction and this means Q > keq
  • when Q = keq, the reaction is already at equilibrium and deltaG°rxn = 0
41
Q

when to know if a reaction will follow the kinetic or thermodynamic pathway?

A
  • kinetic product forms more quickly, less thermodynamically favourable, lower activation energy, favoured at low temperatures
  • thermodynamic product forms more slowly, more thermodynamically favourable, higher activation energy, and favoured at higher temperatures

MCAT chemistry (12 decks)

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