Chemistry/Physics: Chemical Kinetics (Rate Laws) and Equilibrium
If the temperature of a system is changed, what does it affect? A. Keq (equilibrium) B. Q (reaction quotient) C. A and B D. None of the above
A. Keq (equilibrium) Unlike the effect of changing concentrations or pressure, the result of changing temperature is not a change in the reaction quotient, Qc or Qp, but a change in Keq. The change in temperature does not cause the concentrations or partial pressure of the reactants and products to change immediately, so Q immediately after the temperature change is the same as before the temperature change. Because Keq is now a different value, Q no longer equals Keq. The system has to move in whichever direction allows it to reach its new equilibrium state at the new temperature. -the direction is determined by the enthalpy of the reaction -if a reaction is endothermic (ΔH > 0), heat functions as a reactant -if a reaction is exothermic (ΔH < 0), heat functions as a product
Which of the following is one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants, A and B? These reactions have a constant reaction rate equal to the rate constant (rate coefficient, k). A. Zero-Order Reactions B. First-Order Reactions C. Second-Order Reactions D. Mixed-Order Reactions
A. Zero-Order Reactions A zero-order reaction is one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants, A and B. These reactions have a constant reaction rate equal to the rate constant (rate coefficient, k). That rate law for a zero-order reaction is: rate = k[A]^0[B]^0 = k where k has units of M/s It is possible to change the rate for a zero-order reaction by changing the temperature. The only other way to change the rate of a zero-order reaction is by the addition of a catalyst, which lowers the activation energy, thereby increasing the value of k. -Plotting a zero-order reaction results in a linear graph. The line shows that the rate of formation of product is independent of the concentration of reactant. The slope of such a line is the opposite of the rate constant, k.
If Q < Keq, then... A. ΔG < 0, reaction proceeds in the forward direction B. ΔG = 0, reaction is in dynamic equilibrium C. ΔG > 0, reaction proceeds in reverse direction D. Reaction does not take place
A. ΔG < 0, reaction proceeds in the forward direction Q < Keq: ΔG < 0, reaction proceeds in the forward direction - Product formation is favored
A negative ΔG indicates what? A. Exergonic reaction, energy absorbed, non-spontaneous B. Exergonic reaction, energy given off, spontaneous C. Endergonic reaction, energy absorbed, non-spontaneous D. Endergonic reaction, energy given off, spontaneous
B. Exergonic reaction, energy given off, spontaneous -ΔG = exergonic = energy given off = spontaneous In a free energy diagram, the free energy of the products will be less (lower down, farther down from the activation energy) than the free energy of the reactants.
Which of the following has a rate that is directly proportional to only one reactant, such that doubling the concentration of the reactant results in a doubling of the rate of formation of the product? A. Zero-Order Reactions B. First-Order Reactions C. Second-Order Reactions D. Mixed-Order Reactions
B. First-Order Reactions First-order reactions have a rate that is directly proportional to only one reactant, such that doubling the concentration of the reactant results in a doubling of the rate of formation of the product. The rate law for a first-order reaction is: rate = k[A]^1 or rate = k[B]^1 where k has units of s^-1 *Note that when plotting first-order reactions it can be seen that the rate of reaction is dependent on reactant concentration. When plotting ln of the first-order reaction, the slope of the line is the opposite of the rate constant, k. So if the slope is negative, k is the positive of the slope.
Based on Le Chatelier's Principle what occurs if reactants are added (or products are removed)? A. The reaction will spontaneously react in the reverse direction B. The reaction will spontaneously react in the forward direction C. Both A and B D. No change will occur
B. The reaction will spontaneously react in the forward direction if reactants are added (or products are removed), Qc < Keq, and the reaction will spontaneously react in the forward direction, increasing the value of Qc until Qc = Keq. If reactants are removed (or products are added), Qc > Keq, and the reaction will spontaneously react in the reverse direction, thereby decreasing the value of Qc, until once again Qc = Keq. *The system will always react in the direction away from the added species or toward the removed species.
If Q = Keq, then... A. ΔG < 0, reaction proceeds in the forward direction B. ΔG = 0, reaction is in dynamic equilibrium C. ΔG > 0, reaction proceeds in reverse direction D. Reaction does not take place
B. ΔG = 0, reaction is in dynamic equilibrium Q = Keq: ΔG = 0, reaction is in dynamic equilibrium
A positive ΔG indicates what? A. Exergonic reaction, energy absorbed, non-spontaneous B. Exergonic reaction, energy given off, spontaneous C. Endergonic reaction, energy absorbed, non-spontaneous D. Endergonic reaction, energy given off, spontaneous
C. Endergonic reaction, energy absorbed, non-spontaneous +ΔG = endergonic = energy absorbed = non-spontaneous In a free energy diagram, the free energy of the products will be more (higher up, closer to the activation energy) than the free energy of the reactants.
Which of the following has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant? A. Zero-Order Reactions B. First-Order Reactions C. Second-Order Reactions D. Mixed-Order Reactions
C. Second-Order Reactions A second-order reaction has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant. The following rate laws all reflect second-order reactions: rate = k[A]^1[B]^1 or rate = k[A]^2 or rate = k[B]^2 where k has units of M^-1s^-1 It is important to recognize that a second-order rate law often suggests a physical collision between two reactant molecules, especially if the rate law is first-order with respect to each of the two reactants. Plotting a second-order reaction results in a nonlinear graph. The curve shows that the rate of formation of product is dependent on the concentration of reactant. Plotting the inverse of the concentration on the y-axis reveals a linear curve; the slope of such a curve is equal to the rate constant, k.
If Q > Keq, then... A. ΔG < 0, reaction proceeds in the forward direction B. ΔG = 0, reaction is in dynamic equilibrium C. ΔG > 0, reaction proceeds in reverse direction D. Reaction does not take place
C. ΔG > 0, reaction proceeds in reverse direction Q > Keq: ΔG > 0, reaction proceeds in reverse direction - Formation of reactants is favored
Chemical Kinetics - the study of reaction rates, the effects of reaction conditions on these rates, and the mechanisms implied by such observations
Chemical Kinetics - the study of reaction rates, the effects of reaction conditions on these rates, and the mechanisms implied by such observations
Which of the following are factors that affect the reaction rate? I. Reaction Concentrations II. Temperature III. Medium IV. Catalysts A. II only B. I and II only C. I, II, and IV only D. I, II, III, and IV
D. I, II, III, and IV Factors Affecting Reaction Rate: I. Reaction Concentrations - the greater the concentrations of the reactants, the greater the number of effective collisions per unit time (this leads to an increase in the frequency factor (A) of the Arrhenius equation, which will increase the reaction rate) - the reaction rate will increase for all but zero-order reactions II. Temperature - for nearly all reactions, the reaction rate will increase as the temperature increases - increasing the temperature, increases the average kinetic energy of the molecules, which increases the reaction rate - there are optimal temperatures (range) for a reaction, and if the temperature is too high it may result in a decrease in reaction rate III. Medium - the rate at which a reaction takes place may also be affected by the medium in which it takes place - furthermore, the physical states of the medium (liquid, solid, or gas) can also have a significant effect - generally, polar solvents are preferred because their molecular dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, permitting the reaction to occur faster IV. Catalysts - are substances that increase reaction rate without themselves being consumed in the reaction - catalysts interact with the reactants and stabilize them so as to reduce the activation energy necessary for the reaction to proceed, for both the forward and reverse reactions - they will not transform a non-spontaneous reaction into a spontaneous one; they only make spontaneous reactions move more quickly toward equilibrium - all enzymes are catalysts - in homogenous catalysis, the catalyst is in the same phase (solid, liquid, gas) as the reactants - in heterogenous catalysis, the catalyst is in a distinct phase
Which of the following is incorrect regarding the properties of the law of mass? A. The concentrations of pure solids and pure liquids do not appear in the equilibrium constant expression B. Keq is temperature-dependent C. The larger the value of Keq, the farther to the right the equilibrium position D. If the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is -Keq
D. If the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is -Keq The following are the accurate properties of the law of mass: 1. The concentrations of pure solids and pure liquids do not appear in the equilibrium constant expression 2. Keq is characteristic of a particular reaction at a given temperature; the equilibrium constant is temperature-dependent 3. The larger the value of Keq, the farther to the right the equilibrium position 4. If the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is 1/Keq
Which of the following sometimes refer to non-integer orders (fractions) and in other cases to reactions with rate orders that vary over the course of the reaction? A. Zero-Order Reactions B. First-Order Reactions C. High-Order Reactions D. Mixed-Order Reactions
D. Mixed-Order Reactions Mixed-order reactions sometimes refer to non-integer orders (fractions) and in other cases to reactions with rate orders that vary over the course of the reaction. Fractions are more specifically described as broken-order. The term mixed-order has come to refer solely to reactions that change order over time. An example of a mixed-order rate law: rate = (k₁[C][A]²) / (k₂ + k₃[A]) where A represents a single reactant and C, a catalyst. when k₃[A] >> k₂ the reaction will appear to be first-order. When [A] is low, k₂>> k₃[A], making the reaction appear second-order with respect to A. *High-order reactions refer to processes with third-order rates or higher, which are rare because it is more rare for three particles to collide simultaneously with the correct orientation and sufficient energy to undergo a reaction.
Equation: Arrhenius Equation A much more quantitatively rigorous analysis of the collision theory can be accomplished through the Arrhenius equation: k = Ae^(-Ea/RT) k = rate constant of a reaction A = frequency factor -aka attempt frequency, is the measure of how often molecules in a certain reaction collide, with the unit s^-1 Ea = activation energy of the reaction R = ideal gas constant (8.314 J/mol-K) T = temperature in kelvins *Pay attention to the relationships between the variables and the exponent rules that govern the equation - As the frequency factor (A) of the reaction increases, the rate constant of the reaction also increases in a direct relationship - If the temperature (T) of a chemical system were to increase to infinity, while all other variables are held constant, the values of the exponent would have a magnitude less than 1 ---> however, since there is a negative sign, as the magnitude of the exponent gets smaller, it actually moves from a more negative value toward zero. Therefore, the exponent thus becomes less negative (or more positive), which means that the rate constant actually increases · This should make sense, because usually the rate of the reaction increases with temperature - Lower activation energy and high temperatures make the negative exponent of the equation smaller in magnitude and thus increases the rate constant k - The frequency factor (A) can be increased by increasing the number of molecules in a vessel ---> when there are more molecules, the opportunities for collision are increased
Equation: Arrhenius Equation A much more quantitatively rigorous analysis of the collision theory can be accomplished through the Arrhenius equation: k = Ae^(-Ea/RT) k = rate constant of a reaction A = frequency factor -aka attempt frequency, is the measure of how often molecules in a certain reaction collide, with the unit s^-1 Ea = activation energy of the reaction R = ideal gas constant (8.314 J/mol-K) T = temperature in kelvins *Pay attention to the relationships between the variables and the exponent rules that govern the equation - As the frequency factor (A) of the reaction increases, the rate constant of the reaction also increases in a direct relationship - If the temperature (T) of a chemical system were to increase to infinity, while all other variables are held constant, the values of the exponent would have a magnitude less than 1 ---> however, since there is a negative sign, as the magnitude of the exponent gets smaller, it actually moves from a more negative value toward zero. Therefore, the exponent thus becomes less negative (or more positive), which means that the rate constant actually increases · This should make sense, because usually the rate of the reaction increases with temperature - Lower activation energy and high temperatures make the negative exponent of the equation smaller in magnitude and thus increases the rate constant k - The frequency factor (A) can be increased by increasing the number of molecules in a vessel ---> when there are more molecules, the opportunities for collision are increased
Equation: Definition of Rate For the general reaction aA + bB ---> cC + dD rate = -Δ[A]/aΔt = -Δ[B]/bΔt = Δ[C]/cΔt = Δ[D]/dΔt Rate is expressed in the units of moles per liter per second (mol/L·s) or molarity per second (M/s).
Equation: Definition of Rate For the general reaction aA + bB ---> cC + dD rate = -Δ[A]/aΔt = -Δ[B]/bΔt = Δ[C]/cΔt = Δ[D]/dΔt Rate is expressed in the units of moles per liter per second (mol/L·s) or molarity per second (M/s).
Equation: Law of Mass Action (Equilibrium Constant) For a generic reversible reaction aA + bB ⇌ cC + dD, the law of mass action states that, if the system is at equilibrium at a constant temperature, then the following ratio is constant: Kc = Keq = ([C]^c[D]^d) / ([A]^a[B]^b) Kc and Keq = equilibrium constant and the subscript c indicates that it is in terms of concentration. * When writing the equilibrium constant expression for reactions, only include aqueous and gas species. Do not include liquids and solids because they are essentially incompressible!
Equation: Law of Mass Action (Equilibrium Constant) For a generic reversible reaction aA + bB ⇌ cC + dD, the law of mass action states that, if the system is at equilibrium at a constant temperature, then the following ratio is constant: Kc = Keq = ([C]^c[D]^d) / ([A]^a[B]^b) Kc and Keq = equilibrium constant and the subscript c indicates that it is in terms of concentration. * When writing the equilibrium constant expression for reactions, only include aqueous and gas species. Do not include liquids and solids because they are essentially incompressible!
Equation: Radioactive Decay A classic example of a first-order reaction is the process of radioactive decay. [A]ₜ = [A]₀e^-kt [A]ₜ = concentration of A (a radioactive substance) at time t [A]₀ = initial concentration of A (a radioactive substance) k = rate constant t = time It is important to recognize that a first-order rate law with a single reactant suggests that the reaction begins when the molecule undergoes a chemical change all by itself, without a chemical interaction, and usually without a physical interaction with any other molecule. *Note that when plotting first-order reactions it can be seen that the rate of reaction is dependent on reactant concentration. When plotting ln of the first-order reaction, the slope of the line is the opposite of the rate constant, k. So if the slope is negative, k is the positive of the slope.
Equation: Radioactive Decay A classic example of a first-order reaction is the process of radioactive decay. [A]ₜ = [A]₀e^-kt [A]ₜ = concentration of A (a radioactive substance) at time t [A]₀ = initial concentration of A (a radioactive substance) k = rate constant t = time It is important to recognize that a first-order rate law with a single reactant suggests that the reaction begins when the molecule undergoes a chemical change all by itself, without a chemical interaction, and usually without a physical interaction with any other molecule.
Equation: Rate Law For nearly all forward, irreversible reactions, the rate is proportional to the concentrations of the reactants, with each concentration raised to some EXPERIMENTALLY determined exponent. For the general reaction aA + bB ---> cC + dD the rate is proportional to [A]^x[B]^y. By including a proportionality constant, k, we can say that rate is determined according to the following equation: rate = k[A]^x[B]^y k = reaction rate coefficient or rate constant the x and y exponents are the orders of the reaction and MUST be determined experimentally Rate is always measured in units of concentration over time; that is, molarity per second. The exponents x and y can be used to state the order of the reaction with respect to each reactant or overall: x is the order with respect to reactant A, and y is order with respect to reactant B. The overall order of the reaction is the sum of x and y *Remember that the stoichiometric coefficients for the overall reaction are often different from those for the rate law and will, therefore, not be the same as the order of the reaction, unless the reaction occurs via a single-step mechanism.
Equation: Rate Law For nearly all forward, irreversible reactions, the rate is proportional to the concentrations of the reactants, with each concentration raised to some EXPERIMENTALLY determined exponent. For the general reaction aA + bB ---> cC + dD the rate is proportional to [A]^x[B]^y. By including a proportionality constant, k, we can say that rate is determined according to the following equation: rate = k[A]^x[B]^y k = reaction rate coefficient or rate constant the x and y exponents are the orders of the reaction and MUST be determined experimentally Rate is always measured in units of concentration over time; that is, molarity per second. The exponents x and y can be used to state the order of the reaction with respect to each reactant or overall: x is the order with respect to reactant A, and y is order with respect to reactant B. The overall order of the reaction is the sum of x and y *Remember that the stoichiometric coefficients for the overall reaction are often different from those for the rate law and will, therefore, not be the same as the order of the reaction, unless the reaction occurs via a single-step mechanism.
Equation: Rate of Reaction (from the collision theory of chemical kinetics) rate = Z x f Z = total number of collisions occurring per second f = the fraction of collisions that are effective The collision theory of chemical kinetics states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules. However, an effective collision (one that leads to the formation of products) occurs only if: 1. the molecules collide with each other in the correct orientation 2. collide with sufficient energy to break their existing bonds and form new ones -the minimum energy of collision necessary for a reaction to take place is called the activation energy, Ea, or the energy barrier
Equation: Rate of Reaction (from the collision theory of chemical kinetics) rate = Z x f Z = total number of collisions occurring per second f = the fraction of collisions that are effective The collision theory of chemical kinetics states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules. However, an effective collision (one that leads to the formation of products) occurs only if: 1. the molecules collide with each other in the correct orientation 2. collide with sufficient energy to break their existing bonds and form new ones -the minimum energy of collision necessary for a reaction to take place is called the activation energy, Ea, or the energy barrier
Equation: Reaction Quotient For a generic reversible reaction aA + bB ⇌ cC + dD Qc = ([C]^c[D]^d) / ([A]^a[B]^b) Q = reaction quotient -serves as a timer to indicate how far the reaction has proceeded toward equilibrium -this can be done at any point during a reaction in order to measure the concentrations of all the reactants and producs *the equation looks identical to the equation for Keq; however, while the concentrations used for the law of mass action are equilibrium (constant) concentrations, the concentrations of the reactants and product are not constant when calculating a value for Q of a reaction. -the utility of Q is not the value itself but rather the comparison that can be made between Q at any given moment in the reaction to the known Keq for the reaction at a particular temperature. For any reaction, if: Q < Keq, then the forward reaction has not yet reached equilibrium -there is a greater concentration of reactants (and smaller concentration of products) than at equilibrium -the forward rate of reaction is increased to restore equilibrium Q = Keq, then the reaction is in dynamic equilibrium -the reactants and products are present in equilibrium proportions -the forward and reverse rates of reaction are equal Q > Keq, then the forward reaction has exceeded equilibrium -there is a greater concentration of products (and smaller concentration of reactants) than at equilibrium -the reverse rate of reaction is increased to restore equilibrium
Equation: Reaction Quotient For a generic reversible reaction aA + bB ⇌ cC + dD Qc = ([C]^c[D]^d) / ([A]^a[B]^b) Q = reaction quotient -serves as a timer to indicate how far the reaction has proceeded toward equilibrium -this can be done at any point during a reaction in order to measure the concentrations of all the reactants and producs *the equation looks identical to the equation for Keq; however, while the concentrations used for the law of mass action are equilibrium (constant) concentrations, the concentrations of the reactants and product are not constant when calculating a value for Q of a reaction. -the utility of Q is not the value itself but rather the comparison that can be made between Q at any given moment in the reaction to the known Keq for the reaction at a particular temperature. For any reaction, if: Q < Keq, then the forward reaction has not yet reached equilibrium -there is a greater concentration of reactants (and smaller concentration of products) than at equilibrium -the forward rate of reaction is increased to restore equilibrium Q = Keq, then the reaction is in dynamic equilibrium -the reactants and products are present in equilibrium proportions -the forward and reverse rates of reaction are equal Q > Keq, then the forward reaction has exceeded equilibrium -there is a greater concentration of products (and smaller concentration of reactants) than at equilibrium -the reverse rate of reaction is increased to restore equilibrium
Equilibrium
Equilibrium
Experimental Determination of Rate Law 1. Look for two trials in which the concentration of all but one of the substances are held constant -Pay attention to the table and do not get mix up the rows 2. Determine how much the rate increases and how much the concentration of the substance increases and figure out the exponent: rate increase = substance increase^x -For example: if the concentration A is doubled and the rate increase is also doubled, then: 2 = 2^x ---> x = 1, so the rate is 1 and [A]^1 3. Repeat for each reactant 4. To calculate the overall reaction order, add the orders of the reactants. 5. To calculate k, substitute the values from any of the trails into the rate law: rate = k[A]^x[B]^y *refer to page 161 from the Kaplan General Chemistry review book if still confused.
Experimental Determination of Rate Law 1. Look for two trials in which the concentration of all but one of the substances are held constant -Pay attention to the table and do not get mix up the rows 2. Determine how much the rate increases and how much the concentration of the substance increases and figure out the exponent: rate increase = substance increase^x -For example: if the concentration A is doubled and the rate increase is also doubled, then: 2 = 2^x ---> x = 1, so the rate is 1 and [A]^1 3. Repeat for each reactant 4. To calculate the overall reaction order, add the orders of the reactants. 5. To calculate k, substitute the values from any of the trails into the rate law : rate = k[A]^x[B]^y *refer to page 161 from the Kaplan General Chemistry review book if still confused.
True or False: Kinetic products are favored at high temperatures with high heat transfer. Thermodynamic products are favored at low temperatures with low heat transfer.
False. Kinetic products are favored at low temperatures with low heat transfer. Thermodynamic products are favored at high temperatures with high heat transfer.
True or False: The transition state has less energy than both the reactants and the products.
False. The transition state, also called the activated complex, has greater energy than both the reactants and the products and is denoted by the symbol ‡. The energy required to reach this transition state is the activation energy. Relative to reactants and products, transition states have the highest energy. They are only theoretical structures and cannot be isolated (unlike reaction intermediates which can be isolated).
True or False: The reaction rate is usually measured at the end of the reaction.
False. While the reaction rate can be measured at any time during the reaction, it is usually measured at or near the beginning of the reaction to minimize the effects of the reverse reaction.
Free Energy Diagram - is a plot with free energy (ΔG) or enthalpy (ΔH) on the y-axis and reaction coordinate on the x-axis - illustrates the relationship between the activation energy, the free energy of the reaction, and the free energy of the system - the free energy change of the reaction (ΔGrxn) is the difference between the free energy of the products and the free energy of the reactants - the transition state exists at the peak of the energy diagram - the activation energy (Ea) of the forward reaction is the difference in free energy between the transition state and the reactants - the activation energy (Ea) of the reverse reaction is the difference in free energy between the transition state and the products
Free Energy Diagram - is a plot with free energy (ΔG) or enthalpy (ΔH) on the y-axis and reaction coordinate on the x-axis - illustrates the relationship between the activation energy, the free energy of the reaction, and the free energy of the system - the free energy change of the reaction (ΔGrxn) is the difference between the free energy of the products and the free energy of the reactants - the transition state exists at the peak of the energy diagram - the activation energy (Ea) of the forward reaction is the difference in free energy between the transition state and the reactants - the activation energy (Ea) of the reverse reaction is the difference in free energy between the transition state and the products
Le Chatelier's Principle regarding changes in pressure (and volume): Consider the following reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃ The left side of the reaction has a total of 4 moles of gas molecules, while the right side only has two moles. When the pressure of this system is increased (volume reduced), the system will react in the direction that produces fewer moles of gas. In this case, that direction is to the right, and more ammonia will form. If the pressure is decreased (volume increased), the system will react in the direction that produces more moles of gas; thus, the reverse reaction will be favored, and more nitrogen and hydrogen gas will reform.
Le Chatelier's Principle regarding changes in pressure (and volume): Consider the following reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃ The left side of the reaction has a total of 4 moles of gas molecules, while the right side only has two moles. When the pressure of this system is increased (volume reduced), the system will react in the direction that produces fewer moles of gas. In this case, that direction is to the right, and more ammonia will form. If the pressure is decreased (volume increased), the system will react in the direction that produces more moles of gas; thus, the reverse reaction will be favored, and more nitrogen and hydrogen gas will reform.
Le Chatelier's Principle regarding changes in temperature: Consider the following endothermic reaction: N₂O₄(g) (with Δ) ⇌ 2NO₂(g) Since this is an endothermic reaction, heat acts as a reactant. The equilibrium position can be shifted by changing the temperature. -When heat is added and the temperature increases, the reaction shifts to the right, and the flask turns a reddish-brown due to an increase in [NO₂] -When heat is removed and the temperature decreases, the reaction shifts to the left, and the flask turns more transparent due to the increase in [N₂O₄]
Le Chatelier's Principle regarding changes in temperature: Consider the following endothermic reaction: N₂O₄(g) ⇌(with Δ) 2NO₂(g) Since this is an endothermic reaction, heat acts as a reactant. The equilibrium position can be shifted by changing the temperature. -When heat is added and the temperature increases, the reaction shifts to the right, and the flask turns a reddish-brown due to an increase in [NO₂] -When heat is removed and the temperature decreases, the reaction shifts to the left, and the flask turns more transparent due to the increase in [N₂O₄]
Reaction Orders We classify chemical reactions as zero-order, first order, second-order, higher order, or mixed-order on the basis of kinetics. Consider the generic reaction aA + bB ---> cC + dD for the following note cards.
Reaction Orders We classify chemical reactions as zero-order, first order, second-order, higher order, or mixed-order on the basis of kinetics. Consider the generic reaction aA + bB ---> cC + dD for the following note cards.
Reaction Rates
Reaction Rates
True or False: At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, entropy is at a maximum, and Gibbs free energy is at a minimum.
True. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, entropy is at a maximum, and Gibbs free energy is at a minimum. - This links the concepts of thermodynamics and kinetics -entropy, in this case is referring to the measure of the distribution of energy throughout a system or between a system and its environment
True or False: In a reaction, the free energy needed for the kinetic pathway is lower than that of the thermodynamic pathway.
True. In a reaction, the free energy needed for the kinetic pathway is lower than that of the thermodynamic pathway. Therefore, the kinetic products often form faster than the thermodynamic products and are sometimes called "fast" products. However, the free energy of the thermodynamic product is significantly lower that that of the kinetic product. Thermodynamic products are therefore associated with greater stability, and with a more negative ΔG than kinetic products. -Despite proceeding more slowly than the kinetic pathway, the thermodynamic pathway is more spontaneous (more negative ΔG).
True or False: Le Chatelier's principle states that if a stress is applied to a system, the system shifts to relieve that applied stress.
True. Le Chatelier's principle states that if a stress is applied to a system, the system shifts to relieve that applied stress. Regardless of the form the stress takes, the reaction is temporarily moved out of its equilibrium states. This is either because the concentrations or partial pressures of the system are no longer in the equilibrium ratio or because the equilibrium ratio itself has changed as a result of a change in the temperature of the system. The reaction then responds by reacting in whichever direction - either forward or reverse - will result in a reestablishment of the equilibrium state.
True or False: The rate of the whole reaction is only as fast as the rate-determining step.
True. The rate of the whole reaction is only as fast as the rate-determining step. The slowest step in any proposed mechanism is called the rate-determining step because it acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step.