Knewton's Alta Module 15

Write the expression for the equilibrium constant for each of the following reaction: 3O2(g)⇌2O3(g)

Kc=[O3]2 / [O2]3

Write the expression for the equilibrium constant for each of the following reactions: N2(g)+3H2(g)⇌2NH3(g)

Kc=[NH3]2 / [N2][H2]3

Write the expression for the equilibrium constant for each of the following reaction: 4NH3(g)+7O2(g)⇌4NO2(g)+6H2O(g)

Kc=[NO2]4[H2O]6 / [NH3]4[O2]7

At equilibrium for the reaction in aqueous medium below, the concentrations of A, B, C, and D are 0.011 M, 0.0067 M, 0.0124 M, and 0.0062 M, respectively. What is the equilibrium constant? A+2B⇌3C+D

0.024 or 2.4 X 10^-2 The equilibrium constant is found by dividing the equilibrium concentrations of the products by the equilibrium concentrations of the reactants, where each concentration is raised to the power of the coefficient of the substance in the balanced chemical equation. Thus, the equilibrium constant is Kc=[C]3[D][A][B]2=[0.0124]3[0.0062][0.011][0.0067]2=0.024.

Consider the following chemical reaction to answer the question that follows. Na2CO3+CaCl2⇌2NaCl+CaCO3 The initial concentrations of CaCl2 and Na2CO3 in a solution are both equal to 0.34 M. After the reaction reaches equilibrium, the concentration of Na2CO3 is 0.05 M. What is the expression for the change in concentration of NaCl?

2x The change in concentration is denoted as x. Since NaCl has a coefficient of 2 in the reaction equation, the coefficient of x will be 2 as well. Therefore, the change in concentration can be written as 2x.

Which of the following react relatively slowly (hours to years) rather than nearly instantaneously? (select all that apply) A) An egg spoiling in a freezer B) Calcium exposed to water C) Iron train tracks exposed to the air D) Small pieces of elemental sodium exposed to the air

A) An egg spoiling in a freezer B) Calcium exposed to water C) Iron train tracks exposed to the air The reaction of calcium with water will take several hours to observe, food rotting in a freezer may take several weeks or months to be observable, and iron reacting with oxygen in the air to form rust will take years. Elemental sodium, on the other hand, will react very rapidly with air.

What will be the value of Kp in terms of Kc for the following equilibrium: 2A(g)+B(g)→3C(g) A) Kc B) Kc(RT) C) Kc(RT) D) 1

A) Kc Kp=Kc(RT)(3−3)=Kc

What will be the equilibrium expression for the following equilibrium: C4H8→2C2H4 A) Kc=[C2H4]2 / [C4H8] B) Kc=[C2H4] / [C4H8]2 C) Kc=[C4H8]2 / [C2H4] D) Kc=[C4H8] / [C2H4]2

A) Kc=[C2H4]2 / [C4H8] The equilibrium expression will be products over reactants, with each concentration raised to the power of its stoichiometric coefficient.

Write the expression for the equilibrium constant of the following reaction: C(s)+O2(g)↽−−⇀CO2(g) A) Kc=[CO2] / [O2] B) Kc=[CO2] / [O2][C] C) Kc=[O2] / [CO2] D) Kc=[O2][C] / [CO2]

A) Kc=[CO2] / [O2]

Write the expression for the equilibrium constant for each of the following reaction: 2Fe2O3(s)+3C(s)⇌4Fe(s)+3CO2(g) A) Kc=[CO2]3 B) Kc=[Fe]4[CO2]3[Fe2O3]2[C]3 C) Kc=[Fe2O3]2[C]3[Fe]4[CO2]3 D) Kc=2[Fe2O3]3[C]4[Fe]3[CO2]

A) Kc=[CO2]3

Choose the options below that are true of chemical equilibria. (select all that apply) A) The reaction proceeds in the forward and reverse directions at the same rate. B) Both the forward and the reverse reactions are achievable. C) The relative amounts of all species must be different. D) There is no net change in concentrations of reactants and products.

A) The reaction proceeds in the forward and reverse directions at the same rate. B) Both the forward and the reverse reactions are achievable. D) There is no net change in concentrations of reactants and products.

If K is followed by a subscript p, that means it is evaluated using: A) concentrations B) molar coefficients C) partial pressures D) none of the above

A) concentrations The subscript p will stand for partial pressure, and the expression will include the partial pressures of each component in the equilibrium.

In a chemical equilibrium, the forward and reverse reactions: A) continue to occur at the same rate B) stop occurring once equilibrium is met C) start and stop to maintain constant concentrations D) depend on changes in temperature

A) continue to occur at the same rate

As a reaction proceeds in the forward direction, K will: A) increase B) decrease C) stay the same D) depends on the equilibrium

A) increase As more products are generated, the numerator in the equilibrium expression will increase, so K will increase.

The rate of a reaction usually increases as temperature _________________ and concentration ________________. A) increases, increases B) increases, decreases C) decreases, increases D) decreases, decreases

A) increases, increases

The average rate of a reaction is a good approximation of an instantaneous rate when: A) measurement of concentration occurs over a very short interval. B) measurement of concentration occurs over a very long interval. C) the reaction proceeds very quickly. D) none of the above

A) measurement of concentration occurs over a very short interval.

A catalyst lowers the activation energy of a reaction by: A) providing an alternate pathway or mechanism B) reducing the energy stored in the bonds of the reactants C) increasing the electrostatic attraction between the reactants D) none of the above

A) providing an alternate pathway or mechanism Any catalyst will somehow offer an alternate pathway or mechanism for a reaction than would otherwise occur without the catalyst. A catalyst does not change the energy in the bonds of the reactants nor change the electrostatic attraction between the reactants.

Which is an example of the chemical nature of substances affecting the rate of reaction? A) sodium oxidizes overnight whereas iron takes much longer B) large pieces of iron react slowly with acids while finely divided iron reacts more rapidly C) food spoils more quickly on the kitchen counter than in the fridge D) all of the above

A) sodium oxidizes overnight whereas iron takes much longer The difference in rate of sodium and iron oxidation is due to the difference in chemical identity between sodium and iron. The difference in rate of food spoilage is due to temperature and the difference in the example of the iron reaction in acid is due to surface area.

Which is an example of the state of subdivision of the reactants affecting the rate of reaction? A) the smaller the piece of wood, the faster it burns B) food spoils more quickly on the kitchen counter than in the fridge C) a reaction slows down as it proceeds D) calcium reacts with water moderately, sodium reacts with water explosively

A) the smaller the piece of wood, the faster it burns The smaller pieces of wood have greater surface area available for combustion and thus the rate of reaction is influenced by subdivision. The rate of reaction in the other examples are influenced by temperature (choice b), concentration (choice c), and chemical nature (choice d).

Which of the following demonstrate homogeneous equilibria? (select all that apply) A) A reaction involving both gases and liquids B) A reaction occurring in a liquid-phase solution C) A reaction involving exclusively gaseous species D) A reaction between solutes in liquid solution

B) A reaction occurring in a liquid-phase solution C) A reaction involving exclusively gaseous species D) A reaction between solutes in liquid solution

What will be the Kp for the following equilibrium: Cl2(l)→Cl2(g) A) Kp=PCl2(g) / PCl2(l) B) Kp=PCl2(g) C)Kp=1 / PCl2(g) D) Kp=PCl2(l)

B) Kp=PCl2(g) We will only include gases, not pure liquids, and chlorine gas is the product, so the Kp will be equal to the partial pressure of chlorine gas.

Which is an example of concentration affecting the rate of reaction? A) a reaction proceeds more quickly in the laboratory when placed over a Bunsen burner B) a solution containing two moles of each reactant has a faster instantaneous rate of reaction than a solution of the same volume with only one mole of each reactant C) sodium oxidizes overnight whereas iron takes much longer D) large pieces of iron react slowly with acids while finely divided iron reacts more rapidly

B) a solution containing two moles of each reactant has a faster instantaneous rate of reaction than a solution of the same volume with only one mole of each reactant Solutions containing differing concentrations of reactants will exhibit differing initial rates. Choice A describes the effect of temperature on reaction rate, choice C describes the effect of chemical nature on reaction rate, and choice D describes the effect of surface area of reactants on reaction rate.

In order for a reaction to occur, molecules must collide with kinetic energy that is greater than the: A) bond enthalpies of the reactants B) activation energy of the reaction C) change in enthalpy of the reaction D) enthalpy of combustion for each reactant

B) activation energy of the reaction

The rate at any instant is equal to _______________ of a line tangential to this curve at that time. A) the inverse of slope B) the slope C) the initial rate D) the average

B) the slope

Of the following, which is a false statement regarding chemical equilibrium? A) All reactions are reversible in theory, but it is not always feasible to produce conditions that lead to any significant formation of reactants from the products. B) Some reactions favor the forward direction so much over the reverse direction that they proceed until consuming nearly all of the reactants. C) A reversible reaction means a reaction that is at equilibrium between reactants and products. D) A reversible reaction that is in a state of equilibrium between reactants and products is running in both the forward and reverse directions at the same rate.

C) A reversible reaction means a reaction that is at equilibrium between reactants and products. Reversible reactions are those that can proceed in both forward and reverse directions simultaneously under a certain set of conditions. All reactions are reversible, but under normal conditions, a reaction will often only proceed in one direction to any significant extent. Reactions at equilibrium do not cease reacting; they react at equal rates in the forward and reverse directions so that quantities of reactants and products remain the same.

What is the equilibrium constant for the reversible reaction shown below? P4(s)+5O2(g)⇌P4O10(s) A) Kc=[P4][O2]5 / [P4O10] B) Kc=[P4O10] / [P4][O2] C) Kc=1 / [O2]5 D) Kc=[P4O10] / [P4][O2]5

C) Kc=1 / [O2]5

Find the equilibrium expression for the reversible reaction below. 2HI(g)⇌H2(g)+I2(g) A) Kc=[HI]2 / [H2][I2] B) Kc=[H2][I2] / [HI] C) Kc=[H2][I2] / [HI]2 D) Kc=[HI] / [H2][I2]

C) Kc=[H2][I2] / [HI]2 The equilibrium expression is found by dividing the product of the concentrations of the products by the product of the concentrations of the reactants, where each concentration is raised to the power of the coefficient of the substance in the balanced chemical equation. Because each species in this reaction is a gas, we include each in the equilibrium expression.

Which of the following is an example of reactant concentration affecting the rate of a reaction? A) A wooden table burns more quickly in a fire than an iron headboard. B) Five 4-inch diameter saplings burn more rapidly than a 20-inch tree of the same height. C) Phosphorus burns much more rapidly in an atmosphere of pure oxygen than in air, which is about 21% oxygen. D) Food spoils more quickly on a kitchen counter than in a refrigerator.

C) Phosphorus burns much more rapidly in an atmosphere of pure oxygen than in air, which is about 21% oxygen. The rate of a reaction usually increases when the concentration of one or more of the reactants increases, therefore phosphorus burns more rapidly in 100% oxygen compared to 21% oxygen. The remaining choices address scenarios where the rate of reaction is influenced by chemical composition, surface area, and temperature.

Heterogeneous equilibria can include: A) chemical changes only B) physical changes only C) both chemical and physical changes D) neither chemical nor physical changes

C) both chemical and physical changes There are examples of heterogeneous equilibria that include physical changes like any phase change, as well as chemical changes, like the dissociation of ionic solids, or the oxidation of metals.

Equilibrium is a ______________ process. A) discrete B) rare C) dynamic D) static

C) dynamic Chemical equilibrium is a dynamic process. The forward and reverse reactions occur at equal rates, and the concentrations of products and reactants remain constant.

Angela has a solid piece of chalk that she is preparing to react in an experiment with a liquid solution in a beaker. What should she do to the chalk so that she gets the most rapid reaction? A) break the chalk in half B) crowd the beaker with large pieces of solid chalk C) grind the chalk until it is a fine powder D) use a piece of pink chalk instead

C) grind the chalk until it is a fine powder The rate of a reaction between two reactants in different phases depends to a great extent on the surface contact between them. A finely divided solid has more surface area available for reaction than does one large piece of the same substance. Thus a liquid will react more rapidly with a finely divided solid than with a large piece of the same solid. If Angela grinds her chalk down, she increases the surface area of the chalk that is available for the reaction and thus increases the reaction rate.

The slope of the tangent line at a particular point on a graphical plot of time vs. concentration during a reaction gives the: A) average rate of reaction B) initial rate of reaction C) instantaneous rate of reaction D) rate expression

C) instantaneous rate of reaction

Homogeneous equilibria will typically not exist in the: A) gas phase B) liquid phase C) solid phase C) none of the above

C) solid phase Gas phase and liquid phase homogeneous equilibria are common, but solids do not allow for relative motion of particles, so solid phase homogeneous equilibria are not common.

Kc expressions will exclude: A) gases B) aqueous substances C) solids D) none of the above

C) solids Pure solids and pure liquids have activities (measurements of of effective concentrations) of 1, which means that they have no effect the value of Kc expressions and can be omitted.

The structure in a reaction that has the highest potential energy is the: A) reactant B) product C) transition state D) depends on the reaction

C) transition state

When is the average rate of a reaction equal to its instantaneous rate at a given second? A) when the average rate is measured with exceptional precision B) when the reaction has reached its maximum rate C) when the reaction proceeds at a constant rate D) when t = 0

C) when the reaction proceeds at a constant rate

Consider the following chemical reaction to answer the question that follows. PCl5⇌PCl3+Cl2 The initial concentrations of PCl3 and Cl2 in a solution are both equal to 2.50×10−3 M. After the reaction reaches equilibrium, the concentration of PCl3 is 5.25×10−4 M. What is the value of the equilibrium constant for the reaction? A) 0.26×10−5 B) 1.24×10−5 C) 0.78×10−4 D) 1.40×10−4

D) 1.40×10−4

Which is an example of concentration affecting the rate of reaction? A) Calcium reacts with water moderately while sodium reacts with water explosively. B) Food spoils more quickly on the kitchen counter than in the fridge. C) The smaller the piece of wood, the faster it burns. D) A reaction slows down as it proceeds.

D) A reaction slows down as it proceeds. As a reaction proceeds, the concentrations change, and as the concentration of reactants is reduced, the reaction will go more slowly. The remaining choices address the effects of chemical nature, temperature, and surface area on reaction rate.

Which description best fits the definition of activated complex? A) An activated complex is a catalyst present in a different phase from the reactants. B) An activated complex is a catalyst present in the same phase as the reactants. C) An activated complex is a molecule or ion produced in one step of a reaction mechanism and consumed in another. D) An activated complex is an unstable combination of reactant species representing the highest energy state of a reaction system.

D) An activated complex is an unstable combination of reactant species representing the highest energy state of a reaction system

Which is NOT a postulate of collision theory? A) The rate of a reaction is proportional to the rate of reactant collisions. B) The reacting species must collide in an appropriate orientation. C) The collision between particles must occur with adequate energy. D) Every reaction must involve the collision of two molecules.

D) Every reaction must involve the collision of two molecules.

For the following equation, what is the equation for the equilibrium constant? PCl3(l)+Cl2(g)⇌PCl5(s) A) Kc=1 / [PCl3] B) Kc=[PCl5] / [PCl3][Cl2] C) Kc=1 / [PCl3][Cl2] D) Kc=1 / [Cl2]

D) Kc=1 / [Cl2]

Which term is used to evaluate the equilibrium constant (Kc)? A) pressure B) mass C) volume D) concentration

D) concentration When the reaction quotient is evaluated in terms of concentrations and is noted as Kc. The magnitude of an equilibrium constant is a measure of the yield of a reaction when it reaches equilibrium.

Which of the following is a postulate of Collision Theory? A) The rate of a reaction is equal to the rate of collisions. B) The reactants can collide in any orientation with no effect on the likelihood of the reaction. C) Collisions must occur below a maximum energy in order to lead to reactions. D) none of the above

D) none of the above