Chapter 7 HW

Given the concentrations, calculate the equilibrium constant for this reaction: PCl3(g)+Cl2(g)⇌PCl5(g) At equilibrium, the molar concentrations for reactants and products are found to be [PCl3]=0.20 M, [Cl2]=0.25 M, and [PCl5]=1.20 M. What is the equilibrium constant (Kc) for this reaction? Express your answer using two significant figures.

ANSWER: 24 (FORMULA: 1.20M / (.20 x.25M) = 24)

What will happen to the rates of the forward and reverse reactions when a catalyst is added?

Both forward and reverse rates increase.

The equilibrium-constant expression is a mathematical equation that can be rearranged to solve for any of the variables in it. Rearrange the equilibrium-constant expression to solve for [CH3OH].

Kc[CO][H2]^2

Use the table of bond dissociation energy values given in the introduction to rank the following compounds based on the energy required to break the bond. Rank the compounds in increasing order of their bond dissociation energy. To rank items as equivalent, overlap them.

Lowest to Highest Bond dissociation energy: H2N-H , H3C-H , H-H , O=O , H2C=CH2 , HC =CH

Consider the reaction 2H2(g)+O2(g)⇌2H2O(g) How will the equilibrium shift when hydrogen gas is removed?

The system will shift toward the reactants.

Part D For Kc=420, complete the following statement. At equilibrium, this reaction favors the

products

The iron catalyst is removed.

the reaction rate decreases

When the concentration of PCl3(g) is increased to 1.2 M, the ratio of products to reactants is 4.0. The equilibrium constant for the reaction is 24. In which direction will the reaction shift to regain equilibrium?

toward the products

The enthalpy for the formation of 1 mole of NH3(aq) is -80.29 kJ/mol. What is the enthapy for the formation of 3 moles of NH3(aq)?

-240.87 kJ (3 x -80.29 = -240.87 kJ)

Equilibrium in the human body The principles of equilibrium help maintain the human body. Your overall health depends on your body's ability to react appropriately to stress in order to restore its biological balance. This process of maintaining biological balance is called homeostasis and it is similar to the way chemical equilibrium follows Le Châtelier's principle. One example of an equilibrium reaction involved in homeostasis is oxygenation of the blood. This process is driven by an equilibrium between hemoglobin (Hb), oxygen (O2), and oxyhemoglobin (HbO2) and is represented by the equation: Hb(aq)+O2(g) ⇋ HbO2(aq) Part C - Apply the method to an equilibrium reaction involved in homeostasis in biological organisms As altitude increases, atmospheric pressure decreases. For instance, at 18,000 feet you would obtain 29% less oxygen than you would at sea level, which means that less oxygen is diffused into the blood from the lungs. This can lead to altitude sickness, or hypoxia as it is called. Imagine a mountain climber, who lives at sea level, trying to climb Mount Kilimanjaro for the first time. The climber is very fit so he plans to summit the 19,341 foot peak in four days, though it usually takes first-time climbers five or six days. Use your knowledge of equilibrium to predict the climber's reaction to this speedy climb, and the reactions needed to restore equilibrium. Match the equations to the appropriate blanks in the sentences. Make certain each sentence is complete before submitting your answer.

1. equilibrium shifts to O2 release 2. equilibrium shifts to HbO2 formation 3.equilibrium shifts to HbO2 formation 4. equilibrium shifts to O2 release (NOTE: The body compensates for lower oxygen levels by producing more red blood cells, which will increase the supply of hemoglobin (Hb) in the blood. Adding more hemoglobin to the system will shift the reaction in the direction of the HbO2 product, restoring required levels in the blood and easing the symptoms of altitude sickness. Climbers learn that a slow ascent makes for a more comfortable summit.)

The energy diagram shown represents the chemical reaction between solid ammonium chloride and solid barium hydroxide octahydrate: 2NH4Cl(s)+Ba(OH)2⋅8H2O(s)→2NH3(aq)+BaCl2(aq)+10H2O(l) The ΔH (Delta H) for this reaction is 54.8 kJ . How much energy would be absorbed if 27.7 gg of NH4Cl reacts? Express your answer with the appropriate units.

14.2 kJ

Photosynthesis can be represented by 6CO2(g)+6H2O(l)⇌C6H12O6(s)+6O2(g) Which of the following will be false when the photosynthesis reaction reaches equilibrium? 1.The rate of formation of O2 is equal to the rate of formation of CO2. 2.The concentration of O2O2 will begin decreasing. 3.The concentrations of CO2CO2 and O2O2 will not change. 4.The concentrations of CO2CO2 and O2O2 will be equal.

2 and 4 are false (At equilibrium, the forward and reverse rates are equal. So the rate of formation of O2 will be equal to the rate of formation of CO2 . Concentrations do not change at equilibrium because all species are being produced at the same rate as they are being consumed.)

Label the energy diagram (Figure 1) by matching each term to the appropriate letter from the graph. Drag each item to the appropriate bin.

A. energy of reactants B. activation energy C. heat of reaction D. energy of products

The heat of reaction for a chemical reaction can be calculated by finding the sum of the bond energies of the products and subtracting that from the sum of the bond energies of the reactants: Heat of reaction==Sum of the energy for the bonds broken − Sum of the energy for the bonds formedSum of reactant bond energies − Sum of product bond energiesHeat of reaction=Sum of the energy for the bonds broken − Sum of the energy for the bonds formed=Sum of reactant bond energies − Sum of product bond energies When calculating the sum of the bond energies, each bond in the reaction must be accounted for. For example, CH4CH4 is a reagent with a coefficient of 1 in the reaction. There are four C−HC−H bonds in methane and one methane molecule per reaction, for a total of four C−HC−H bonds on the reactant side. All four bonds must be accounted for when finding the sum of the bond energies for the reactants. Calculate the heat of reaction using the average bond dissociation energies given in the introduction and your answer to Part B for the reaction CH4 + 2O2 → CO2 + 2H2O Express your answer in kilojoules per mole to three significant figures.

ANSWER: -818

An intermediate step in the process that ultimately produces sulfuric acid, is the catalytic oxidation of SO2SO2 to SO3SO3. Vanadium(V) oxide is the catalyst. The three chemical species in the reaction are in equilibrium as follows: 2SO2(g)+O2(g)⇌2SO3(g) Which of the following are true statements about this reaction? 1.When SO2 is mixed with O2 in a container, the initial rate of the forward reaction (production of SO3) is faster than the initial rate of the reverse reaction (production of SO2). 2.As SO2 is used up and SO3 accumulates, the rate of the forward reaction increases and the rate of the reverse reaction decreases. 3.At equilibrium the rate of production of SO3 reaches zero. 4.At equilibrium the concentration of SO2 will no longer be changing.

ANSWER: 1 and 4 (1. When SO2 is mixed with O2 in a container, the initial rate of the forward reaction (production of SO3) is faster than the initial rate of the reverse reaction (production of SO2). 4. At equilibrium the concentration of SO2 will no longer be changing.)

When a chemical reaction is at equilibrium, the forward and reverse reactions occur at the same rate. The concentrations of the different compounds in the reactions will remain constant. The relationship between the concentrations of the reactants and products is expressed as the equilibrium constant, Kc. Balance this equation, and then enter the coefficients, in order, below. CH4(g) + Br2(g) ⇌ CBr4(g) + HBr(g) Express your answer as integers separated by commas (e.g., 1,2,3,4), where 1 indicates the lack of a coefficient.

ANSWER: 1,4,1,4 (The complete balanced equation is as follows: CH4(g)+4Br2(g)⇌CBr4(g)+4HBr(g)

As we exercise, our bodies metabolize glucose, converting it to CO2 and H2O, to supply the energy necessary for physical activity. The simplified reaction is: C6H12O6(aq)+6O2(g)→6CO2(g)+6H2O(l)+678 kcal(2840 kJ) How many grams of water would have to be evaporated as sweat to remove the heat generated by the metabolism of 1 mol of glucose? (See Chemistry in Action: Regulation of Body Temperature on p. 195 in the textbook). Express your answer to four significant figures and include the appropriate units.

ANSWER: 1257 grams (NOTES: Metabolism of glucose to give carbon dioxide and water is an exothermic process and liberates energy of 2840kJ/ mol of glucose. C6H12O6(aq) + 6O2(g)→6CO2(g) + 6H2O(l) + 678 kcal(2840 kJ) Water has a heat of vaporization value of 40.65 kJ/mol. That is 40.65 kJ of energy is required to evaporate 1 mole of water. So 2840 kJ can vaporize 2840 / 40.65 = 69.8646 moles of sweat (water). Mass of water = number of moles * molar mass = 69.8646 * 18 = 1257.5628 g On rounding off to four significant figures we get 1257 grams = 1.257 kg)

The energy diagram shown represents the chemical reaction between solid ammonium chloride and solid barium hydroxide octahydrate: 2NH4Cl(s)+Ba(OH)2⋅8H2O(s)→2NH3(aq)+BaCl2(aq)+10H2O(l) The ΔH for this reaction is 54.8 kJ . How much energy would be absorbed if 25.5 g of NH4Cl reacts? Express your answer with the appropriate units.

ANSWER: 13.1 kJ (NOTE: 25.5 G NH4Cl x 54.8 kJ/ (2 x 53.5 g NH4Cl) = 13.05981308 ==> 13.1 kJ The reaction between Ba(OH)2⋅8H2O and ammonium salts absorbs so much energy from its surroundings that water coming in contact with the container will freeze.)

Alexandra decides to climb Mt. Krumpett, which is 5000 m high. She determines that this will require a total of 1350 kcal of energy for the trip. For her food supply, she decides to take nutrition bars. The label states that each bar contains 50 g of carbohydrates, 10 g of fat, and 40 g of protein. Part A How many nutrition bars should Alexandra pack? Express the number of bars numerically.

ANSWER: 3 bars (FORMULA: (50g x 4 kcal/g) + (40g x 4kcal/g) + (10g x 9 kcal/g)= 450 kcal 1350 kcal / 450 kcal /bar= 3 bars.

The concentration of PCl3(g) is increased to 1.2 M, disrupting equilibrium. Calculate the new ratio of products to reactants with this higher concentration of phosphorus trichloride. Assume that the reaction has not yet regained equilibrium. Express your answer using two significant figures.

ANSWER: 4 (FORMULA: [PCl3]=1.20 M [Cl2]=0.25 M [PCl5]=1.20 M Q = [PCl5] / ([Cl2] [PCl3]) = 1.2 / (0.25 * 1.2) = 4)

The molar concentrations for the reactants and products at equilibrium are found to be [CH4]=3.0 M, [Br2]=0.4 M, [CBr4]=2.0 M, and [HBr]=2.0 M. What is the value of the equilibrium constant for this reaction? CH4(g)+4Br2(g)⇌CBr4(g)+4HBr(g) Express your answer numerically using two significant figures.

ANSWER: 420 OR 416 (either answer is correct.) ((2.0*2.0^4) / (3.0 x 0.4^4)= 416 Kc=416.67)

Alexandra decides to climb Mt. Krumpett, which is 5000 m high. She determines that this will require a total of 2250 kcal of energy for the trip. For her food supply, she decides to take nutrition bars. The label states that each bar contains 50 g of carbohydrates, 10 g of fat, and 40 g of protein. The following table shows the caloric values of three types of food in kilocalories per gram of food: Food type Carbohydrates Fat Protein Caloric value 4 kcal/1 g, 9 kcal/1 g, 4 kcal/1 g

ANSWER: 5 bars (FORMULA: (50g x 4 kcal/g) + (10g x 9kcal/g) + (40g x 4 kcal/g)= 450 kcal 2250 kcal / 450 kcal /bar= 5 bars.

Consider this combination reaction: 2Mg(s)+O2(g)→2MgO(s) ΔH=−1204 kJ What is the enthalpy for the decomposition of 1 mole of MgO(s) into Mg(s) and O2(g)?

ANSWER: 602 kJ/mol (2 mole Mgo = 1204 kj 1204/2= +602 kJ/mol)

Consider the equilibrium reaction CO(g)+H2O(g)⇋CO2(g)+H2(g) Determine how each change in the left-hand column will stress the system and in which direction the equilibrium reaction will shift in response. Drag the appropriate labels to their respective targets.

Add CO(g): increase reactant, forward reaction rate temporarily increases Remove H2O (g): Decrease reactant, Forward reaction rate temporarily decreases Add CO2 (g): Increase product, Reverse reaction rate temporarily increases Remove H2 (g): Decrease product, Reverse reaction rate temporarily decreases

Part B Natural gas is used in homes for central heating. The combustion of natural gas is represented by the equation. CH4 + 2O2 → CO2 + 2H2O Complete the table that identifies the bonds and the number of times each bond occurs for the given balanced chemical equation. Drag the appropriate labels to their respective targets.

Ch4: C-H, 4 2O2: O=O, 2 CO2: C=O, 2 2H2O: H-O, 4

Learning Goal: To use the equilibrium constant and Le Châtelier's principle to determine how a reaction will respond to external factors. A reaction is at equilibrium when the concentrations of the reactants and products no longer change over time. This does not mean the reaction is over; rather, two competing reactions continue to occur simultaneously at equal rates. The two competing reactions are the forward reaction (reactants → products) and the reverse reaction (products → reactants). If a reaction at equilibrium is subjected to a stress, the concentrations of reactants and products adjust to reestablish equilibrium. This is called Le Châtelier's principle. A stress might be a change in the concentration of reactants or products, a change in the volume of the reaction container, a change in temperature, or the addition of a catalyst. Part A Given the concentrations, calculate the equilibrium constant for this reaction: 2SO2(g) +O2(g)⇌2SO3(g) At equilibrium, the molar concentrations for reactants and products are found to be [SO2]=0.48 M, [O2]=0.40 M, and [SO3]=1.12 M. What is the equilibrium constant (Kc) for this reaction? Express your answer using two significant figures.

Kc= 14 (Kc=1.12^2 / (0.48^2 x .40) = 13.6111 ===> 14)

The synthesis of methanol from carbon monoxide and hydrogen gas is described by the following chemical equation: CO(g)+2H2(g)⇌CH3OH(g) The equilibrium constant for this reaction at 25 ∘C is Kc=2.3×104Kc=2.3×104. In this tutorial, you will use the equilibrium-constant expression to find the concentration of methanol at equilibrium, given the concentration of the reactants. Part A Determine the expression for the equilibrium constant, K_c, for the reaction by identifying which terms will be in the numerator and denominator: Kc=numerator/denominator=?? Place the terms into the appropriate bin.

NUMERATOR: [CH3OH] DENOMINATOR: [CO] , [H2]^2 NOT USED: [CH3OH]^2, [CO]^2, [H2]

The balanced equation The complete balanced equation is as follows: CH4(g)+4Br2(g)⇌CBr4(g)+4HBr(g) Determine the Kc expression for the reaction above by identifying what belongs in the numerator and what belongs in the denominator: Kc=numerator/denominator=?? Drag each item to the appropriate bin.

Numerator: [CBr4], [HBr]^4 Denominator: [CH4], [Br2]^4 Not used: [Br2], [CH4]^4, [HBr], [CBr4]^4

Le Châtelier's principle In the first part of this tutorial you analyzed why stressing a reaction by adding reactant or product would shift the equilibrium to create more reactant or more product. Adding more reactant increases the concentration of reactant, which increases the forward rate of reaction as equilibrium is re-established. Le Châtelier's principle is used to analyze this relationship. Le Châtelier's principle applies to reversible reactions that are under some form of stress. Think of the equilibrium like a pile of laundry. If you add more laundry on top of the pile, the additional pieces will roll down the pile and away from the source of the addition. This is similar to the equilibrium reaction that shifts away from a component addition. If you remove a piece of laundry from the bottom of the pile, however, the remaining laundry will fall in the direction of the removed piece. Therefore, the equilibrium shifts in the direction of the component that is removed. Part B - Identify the impact on concentration For the following equilibrium reaction, predict the direction of the equilibrium shift by specifying if each stress will cause the concentration of reactants or products to increase NH3(g)+H2O(l) ⇋ NH4+(aq)+OH−(aq) Drag the appropriate stresses to their respective bins.

Shift in the direction of the reactants: Adding OH−(aq) Adding NH4+(aq) Shift in the direction of the products: Removing OH−(aq) Removing NH4+(aq) Adding NH3(g)

The following reaction is at equilibrium in a 2.0-L vessel: 2SO2(g) +O2(g)⇌2SO3(g), Kc=1.4×101 How do the following actions affect the equilibrium of the reaction? Drag the appropriate items to their respective bins.

Shift to reactants: increase volume remove oxygen gas Shift to products: decrease volume add sulfur dioxide gas No shift: add catalyst

The following reaction is at equilibrium in a 2.0-L vessel: PCl3(g) +Cl2(g)⇌PCl5(g), Kc=2.4×101 How do the following actions affect the equilibrium of the reaction? Drag the appropriate items to their respective bins.

Shift to reactants: increase volume remove phosphorus trichloride gas Shift to products: decrease volume add chlorine gas No shift: add catalyst

Consider the exothermic reaction Fe2O3(s)+2Al(s)⇌Al2O3(s)+2Fe(s) In what direction will the equilibrium shift when heat is removed?

The equilibrium will shift toward the products

Now, consider a situation in which the concentrations of CO, H2, and CH3OH are all 2.4 M Which statement best describes what will occur?

The forward reaction will be favored until equilibrium is reached.

When HCl(aq) and NaOH(aq) are mixed in a beaker, the beaker feels warm to the touch. What is known about the enthalpy of this reaction?

The reaction is exothermic.

If the reaction N2(g)+3H2(g)⇌2NH3(g) is at equilibrium, what direction will the reaction shift if NH3 gas is added?

The reaction will shift to produce reactants.

If an endothermic reaction is at equilibrium, how will the equilibrium shift when heat is added to the reaction?

The reaction will shift toward the products.

Suppose that the molar concentrations for CO and H2 at equilibrium are [CO] = 0.05 M and [H2] = 0.08 M. Use the formula you found in Part B to calculate the concentration of CH3OH. Express your answer to one decimal place and include the appropriate units. View Available Hint(s)

[CH3OH]= 7.4 M ([CH3OH] = 2.3 X 10^4 X 0.05 X (0.08^2) = 7.3600 --> 7.4 M )

What would happen to the rate of the forward reaction if the concentration of nitrogen were decreased?

decrease

Ammonia, NH3NH3, is used in numerous industrial processes, including the production of pharmaceuticals such as sulfonamide and antimalarials and vitamins such as the B vitamins. The equilibrium equation for the synthesis of ammonia (sometimes known as the Haber process) is N2(g)+3H2(g)⇌2NH3(g) Part A The Haber process is typically carried out at a temperature of approximately 500 ∘C. What would happen to the rate of the forward reaction if the temperature were lowered to 100 ∘C?

decrease (The reaction rate would decrease. Most reactions double in rate for each 10 ∘C increase in temperature.)

If a reaction is exothermic and produces large quantities of heat reaching equilibrium, its equilibrium constant will be

greater than zero

Which of the following would increase the rate of the reverse reaction?

increasing the concentration of ammonia (The concentration of NH3NH3 affects how quickly N2N2 and H2H2 can be made)

As we exercise, our bodies metabolize glucose, converting it to CO2 and H2O, to supply the energy necessary for physical activity. The simplified reaction is: C6H12O6(aq)+6O2(g)→ 6CO2(g)+6H2O(l)+678 kcal(2840 kJ) How many grams of water would have to be evaporated as sweat to remove the heat generated by the metabolism of 2 mol of glucose? (See Chemistry in Action: Regulation of Body Temperature on p. 195 in the textbook). Express your answer to four significant figures and include the appropriate units.

m=2511g

A reaction that has a large equilibrium constant has

more products then reactants at equilibrium

Consider the reaction N2O4(g)→2NO2(g) where N2O4 is colorless and NO2 is brown. At a temperature of 5∘C, the reaction is colorless at equilibrium. This indicates that the equilibrium constant for this reaction at this temperature is

small (less than 1). (Keq = [NO2]2 / [N2O4] At equilibrium concentration of NO2 is very less and concentration of N2O4 will be more. hence equilibrium constant will be less than one)

A chemical reaction is at equilibrium when

the rate of the forward reaction equals the rate of the reverse reaction.

Ammonia is synthesized industrially by reaction of nitrogen and hydrogen in the presence of an iron catalyst according to the equation 3H2(g)+N2(g)→2NH3(g). What effect will the following changes have on the reaction rate? The temperature is lowered from 700 K to 600 K.

the reaction rate decreases

The concentration of H2 gas is halved.

the reaction rate decreases

Ammonia is synthesized industrially by reaction of nitrogen and hydrogen in the presence of an iron catalyst according to the equation 3H2(g)+N2(g)→2NH3(g). What effect will the following changes have on the reaction rate? The temperature is raised from 600 K to 700 K.

the reaction rate increases