Difficult Chemistry Questions And Answers

1. A coffee-cup calorimeter contains 10.0 g of water at 59.00⁰C. If 3.00 g gold at 15.20 ⁰C is placed in the calorimeter, what is the final temperature of the water in the calorimeter? The specific heat of water is 4.18 J/g ·⁰C; the specific heat of gold is 0.128 J/g ·⁰C.

55.37 C

58.60 C

59.40 C

60.80 C

64.19 C

Solving: qreaction = -qcarlorimeter
3.00 g ×0.128 J/g0C × (Tf – 15.2 0C) = - 10.0 g × 4.18 J/g0C × (Tf –59.000C)
0.384 Tf – 5.84 = -41.8 Tf +2466.2 42.184 Tf = 2472.04 Tf = 58.6 0C

Explanation

Solving: qreaction = -qcarlorimeter
3.00 g ×0.128 J/g0C × (Tf – 15.2 0C) = - 10.0 g × 4.18 J/g0C × (Tf –59.000C)
0.384 Tf – 5.84 = -41.8 Tf +2466.2 42.184 Tf = 2472.04 Tf = 58.6 0C

2. Which of the following has the highest electronegativity?

Sodium (Na)

Fluorine (F)

Oxygen (O)

Chlorine (Cl)

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. On the periodic table:

Electronegativity increases across a period (left to right)

Electronegativity decreases down a group (top to bottom)

Fluorine is in Group 17 (halogens) and Period 2 — it is the most electronegative element in the entire periodic table, with a value of 3.98 on the Pauling scale. It strongly attracts electrons in bonds, more than oxygen or chlorine. Sodium, a metal, has very low electronegativity.

Explanation

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. On the periodic table:

Electronegativity increases across a period (left to right)

Electronegativity decreases down a group (top to bottom)

Fluorine is in Group 17 (halogens) and Period 2 — it is the most electronegative element in the entire periodic table, with a value of 3.98 on the Pauling scale. It strongly attracts electrons in bonds, more than oxygen or chlorine. Sodium, a metal, has very low electronegativity.

3. For the first-order reaction below, the initial concentration of A is 0.80 M. What is the half-life of the reaction if the concentration of A decreases to 0.10 M in 54 seconds?

A → B

rate = k[A]

18 s

24 s

36 s

48 s

51 s

Solving: ln[A]0/[A] = kt ln0.80/0.10 = 54 k k = 2.079/54 =0.0385 t1/2 =0.693/0.0385 = 18 s

Explanation

Solving: ln[A]0/[A] = kt ln0.80/0.10 = 54 k k = 2.079/54 =0.0385 t1/2 =0.693/0.0385 = 18 s

4. 10.0 g of ice at 0.00 ⁰C is mixed with 25.0 g of water at 35.00⁰C in a coffee-cup calorimeter. What is the final temperature of the mixture? The specific heat of water is 4.18 J/g ⁰C; the heat of fusion of water is 333 J/g.

0.00 0C

2.24 0C

5.22 0C

25.0 0C

47.8 0C

Solving: 10.0 g× 333 J/g = -25.0 g ×4.18 J/g 0C × (T – 35) 0C, 104.5 T = 3657.5 – 3330, T = 3.13 0C

Explanation

Solving: 10.0 g× 333 J/g = -25.0 g ×4.18 J/g 0C × (T – 35) 0C, 104.5 T = 3657.5 – 3330, T = 3.13 0C

5. Dinitrogen pentaoxide decomposes nitrogen dioxide and oxygen according to the following balanced chemical equation and rate expression.
2N₂O₅(g) → 4NO₂(g) + O₂(g)
rate = k[N₂O₅]
What is the overall reaction order?

0

1

2

5

7

The overall reaction order is 1 because the rate expression only depends on the concentration of N2O5. The coefficient of N2O5 in the balanced chemical equation is 2, which means that the rate of the reaction is directly proportional to the concentration of N2O5 raised to the power of 1. Therefore, the overall reaction order is 1.

Explanation

The overall reaction order is 1 because the rate expression only depends on the concentration of N2O5. The coefficient of N2O5 in the balanced chemical equation is 2, which means that the rate of the reaction is directly proportional to the concentration of N2O5 raised to the power of 1. Therefore, the overall reaction order is 1.

6. If 50.0 g of benzene, C₆H₆, at 25.0⁰C absorbs 2.71 kJ of energy in the form of heat, what is the final temperature of the benzene? The specific heat of benzene is 1.72 J/g·⁰C.

25.00C

31.50C

56.50C

32.30C

57.30C

Solution: q = mass ×c ×Δt
2.71 kJ = 50.0 g× 1.72 J/g 0C × (Tf – 25 0C), Tf = 56.5 0C

Explanation

Solution: q = mass ×c ×Δt
2.71 kJ = 50.0 g× 1.72 J/g 0C × (Tf – 25 0C), Tf = 56.5 0C

7. Nitrosyl chloride decomposes according to the chemical equation below.
2NOCl(g)

2NO(g) + Cl₂(g)
A pressure of 0.320 atm of nitrosyl chloride is sealed in a flask and allowed to reach equilibrium. If 22.6% of the NOCl decomposes, what is the equilibrium constant for the reaction?

0.00153

0.00308

0.00611

0.00730

0.02471

Solving: K = NO2·Cl /NOCl2 =

Explanation

Solving: K = NO2·Cl /NOCl2 =

8. If 495 J is required to change the temperature of 12.7 g of sodium chloride from 75.0⁰C to 135 ⁰C, what is the specific heat of sodium chloride?

0.866 J/g·0C

2.60 J/g·0C

0.650 J/g·0C

1.15 J/g·0C

2.83 × 105 J/g·0C

Solving: q = mass ×c ×Δt
495 J = 12.7 g ×c× (135 – 75)0C c = 495 J / 12.7g · 60 0C = 0.650 J/g ·0C

Explanation

Solving: q = mass ×c ×Δt
495 J = 12.7 g ×c× (135 – 75)0C c = 495 J / 12.7g · 60 0C = 0.650 J/g ·0C

9. When 10.0 g KOH is dissolved in 100.0 g of water in a coffee-cup calorimeter, the temperature rises from 25.18 ⁰C to 47.53 ⁰C. What is the enthalpy change per gram of KOH dissolved in the water? Assume that the solution has a specific heat capacity of 4.18 J/g⁰C.

–116 J/g

–934 J/g

–1.03 × 103 J/g

–2.19 × 103 J/g

–1.03 × 104 J/g

Solving: ΔH/g =-(100.0 g+ 10.0g)×4.18 J/g0C ×(47.53 -25.18) 0C /10 .0 g = –1.03 × 103 J/g

Explanation

Solving: ΔH/g =-(100.0 g+ 10.0g)×4.18 J/g0C ×(47.53 -25.18) 0C /10 .0 g = –1.03 × 103 J/g

10. The rate constant of a first-order decomposition reaction is 0.0147 s^–1. If the initial concentration of reactant is 0.178 M, what is the concentration of reactant after 30.0 seconds?

8.72 × 105 M

0.0645 M

0.115 M

0.0785 M

0.643 M

Solving: first order rate = k[A], ln[A]0/[A] = kt
ln0.178M/[A] = 0.0147 s–1 × 30.0s = 0.441 0.178M/[A] = 1.554 A = 0.115 M

Explanation

Solving: first order rate = k[A], ln[A]0/[A] = kt
ln0.178M/[A] = 0.0147 s–1 × 30.0s = 0.441 0.178M/[A] = 1.554 A = 0.115 M

11. Calcium carbonate decomposes to calcium oxide and carbon dioxide.

CaCO₃(s)

CaO(s) + CO₂(g)

ΔH⁰ = 179 kJ

The equilibrium constant for this reaction is 9.7 × 10^24 at 298 K. What is the equilibrium constant at 575 K? (R = 8.31 J/mol·K)

7.5 × 10^-16

1.3 × 10^-8

1.4 × 10^-38

1.3 × 10^-15

1.0 * 10^-23

The equilibrium constant (K) is given by the equation: K = e^(-ΔH0/RT), where ΔH0 is the enthalpy change, R is the gas constant, and T is the temperature in Kelvin. To find the equilibrium constant at 575 K, we can substitute the given values into the equation. Since the enthalpy change (ΔH0) is given as +179 kJ, we need to convert it to J by multiplying by 1000. Plugging in the values, we get K = e^(-179000/(8.31*575)). Evaluating this expression gives us an equilibrium constant of approximately 1.3 × 10^-8.

Explanation

The equilibrium constant (K) is given by the equation: K = e^(-ΔH0/RT), where ΔH0 is the enthalpy change, R is the gas constant, and T is the temperature in Kelvin. To find the equilibrium constant at 575 K, we can substitute the given values into the equation. Since the enthalpy change (ΔH0) is given as +179 kJ, we need to convert it to J by multiplying by 1000. Plugging in the values, we get K = e^(-179000/(8.31*575)). Evaluating this expression gives us an equilibrium constant of approximately 1.3 × 10^-8.

12. The standard molar enthalpy of formation of NH₃(g) is –45.9 kJ/mol. What is the enthalpy change if 9.51 g N₂(g) and 1.96 g H₂(g) react to produce NH₃(g)?

–10.3 kJ

–20.7 kJ

–29.8 kJ

–43.7 kJ

–65.6 kJ

Solving: N2(g) + 3H2(g) → 2NH3(g) 9.51/28 = 0.3396 mol N2(g), 1.96 /2 = 0.98 molH2(g) so H2 is limiting reactant, need 0.3267 mol N2(g) only and produce 0.6534 mol NH3(g). ΔH = 0.6534mol × (–45.9 kJ/mol) = -29.9 kJ

Explanation

Solving: N2(g) + 3H2(g) → 2NH3(g) 9.51/28 = 0.3396 mol N2(g), 1.96 /2 = 0.98 molH2(g) so H2 is limiting reactant, need 0.3267 mol N2(g) only and produce 0.6534 mol NH3(g). ΔH = 0.6534mol × (–45.9 kJ/mol) = -29.9 kJ

13. For the system
CO(g) + H₂O(g)

CO₂(g) + H₂(g)
K is 1.6 at 900 K. If 0.400 atm CO(g) and 0.400 atm H₂O(g) are combined in a sealed flask, what is the partial equilibrium pressure of CO₂(g)?

0.22 atm

0.31 atm

0.47 atm

0.65 atm

Solving: Assuming forming CO2(g) has pressure is x, same for H2(g), reactant 0.400 atm CO(g) will become 0.400 atm – x, same for H2O(g) .Now we get K = x ·x /(0.4- x)·(0.4 – x) 1.6 = x2 /(0.4 – x)2 1.26 = x / (04 – x) 2.26 x = 0.506 x = 0.22 atm

Explanation

Solving: Assuming forming CO2(g) has pressure is x, same for H2(g), reactant 0.400 atm CO(g) will become 0.400 atm – x, same for H2O(g) .Now we get K = x ·x /(0.4- x)·(0.4 – x) 1.6 = x2 /(0.4 – x)2 1.26 = x / (04 – x) 2.26 x = 0.506 x = 0.22 atm

14. The rate of reaction for the formation of carbon monoxide is measured at 1.24 mol/L·hr. What is the rate of formation of carbon monoxide in units of mol/L·s?
CH₃CHO(g) → CH₄(g) + CO(g)

3.44 × 10-4 mol/L·s

2.07 × 102 mol/L·s

1.24 mol/L·s

74.4 mol/L·s

4.64 × 103 mol/L·s

Solving: 1.24 mol/L·hr = 1.24 mol/L·3600s = 3.44 × 10-4 mol/L·s

Explanation

Solving: 1.24 mol/L·hr = 1.24 mol/L·3600s = 3.44 × 10-4 mol/L·s

15. The initial rates method was used to study the reaction below.
2A + B + C → D + E

[A] (mol/L)	[B] (mol/L)	[C] (mol/L)	Δ[D]/Δt (mol/L·s)
0.150	0.250	0.300	1.47 × 10^6
0.150	0.125	0.300	3.68 × 10^7
0.150	0.250	0.600	2.94 × 10^6
0.300	0.125	0.300	7.35 × 10^7

Rate = 8.71 × 104[A]2 × [B] × [C]

Rate = 5.23 × 104[A] × [B]2 × [C]

Rate = 2.90 × 103[A]2 × [B] × [C]2

Rate = 8.71 × 104[A] × [B] × [C]2

The correct answer for this question is rate = 5.23 × 104[A] × [B]2 × [C]. This can be determined by comparing the given values for [A], [B], [C], and Δ[D]/Δt with the rate equations provided. By substituting the values into each equation, it can be observed that only the rate equation rate = 5.23 × 104[A] × [B]2 × [C] yields the correct value of Δ[D]/Δt (mol/L·s). Therefore, this is the correct answer.

Explanation

The correct answer for this question is rate = 5.23 × 104[A] × [B]2 × [C]. This can be determined by comparing the given values for [A], [B], [C], and Δ[D]/Δt with the rate equations provided. By substituting the values into each equation, it can be observed that only the rate equation rate = 5.23 × 104[A] × [B]2 × [C] yields the correct value of Δ[D]/Δt (mol/L·s). Therefore, this is the correct answer.

16. When 66.0 g of an unknown metal at 28.5⁰C is placed in 83.0 g H₂O at 78.5⁰C, the water temperature decreases to 75.9⁰C. What is the specific heat capacity of the metal? The specific heat capacity of water is 4.184 J/g⁰C.

0.055 J/g· 0C

0.29 J/g · 0C

0.69 J/g · 0C

0.18 J/g · 0C

2.6 J/g · 0C

Solving: qreceive = -qrelease
66.0g ×c × (78.5 – 28.5) 0C = -83.0 g ×4.184 J/g0C ×(75.9 – 78.5) 0C,
c = 83.0 ×4.184 J/g0C× 2.6 / 66.0 × 50 = 0.27 J/g0C

Explanation

Solving: qreceive = -qrelease
66.0g ×c × (78.5 – 28.5) 0C = -83.0 g ×4.184 J/g0C ×(75.9 – 78.5) 0C,
c = 83.0 ×4.184 J/g0C× 2.6 / 66.0 × 50 = 0.27 J/g0C

17. The reaction A → B follows first-order kinetics with a half-life of 21.7 hours. If the concentration of A is 0.023 M after 48.0 hours, what is the initial concentration of A?

0.0050 M

0.051 M

0.51 M

0.11 M

2.0 × 102 M

Solving: t1/2 = 21.7 hr = 0.693/ k k = 0.693/21.7hr ln[A]0/[A] = kt ln[A]0/0.023M = 0.693/21.7hr × 48.0hr = 1.533 ln[A]0/0.023M =1.533 [A]0/0.023M=4.63 [A]0 = 4.63 × 0.023M = 0.11 M

Explanation

Solving: t1/2 = 21.7 hr = 0.693/ k k = 0.693/21.7hr ln[A]0/[A] = kt ln[A]0/0.023M = 0.693/21.7hr × 48.0hr = 1.533 ln[A]0/0.023M =1.533 [A]0/0.023M=4.63 [A]0 = 4.63 × 0.023M = 0.11 M

18. For the reaction below, relate the rate of disappearance of hydrogen to the rate of formation of ammonia.
N₂(g) + 3H₂(g) → 2NH₃(g)

A: +

B: 3

C: 1/2

D: -1/3

E: -

The reaction given is N2(g) + 3H2(g) → 2NH3(g). According to the stoichiometry of the reaction, for every 3 moles of H2 that react, 2 moles of NH3 are formed. Therefore, the rate of disappearance of hydrogen is directly related to the rate of formation of ammonia, but with the opposite sign. Since the coefficient of H2 in the balanced equation is 3, the rate of disappearance of hydrogen is 3 times the rate of formation of ammonia. Hence, the correct answer is d: -1/3.

Explanation

The reaction given is N2(g) + 3H2(g) → 2NH3(g). According to the stoichiometry of the reaction, for every 3 moles of H2 that react, 2 moles of NH3 are formed. Therefore, the rate of disappearance of hydrogen is directly related to the rate of formation of ammonia, but with the opposite sign. Since the coefficient of H2 in the balanced equation is 3, the rate of disappearance of hydrogen is 3 times the rate of formation of ammonia. Hence, the correct answer is d: -1/3.

19. For the reaction below, if the rate of appearance of Br₂ is 0.180 mol/L·s, what is the rate of disappearance of NOBr?
2NOBr(g) → 2NO(g) + Br₂(g)

–0.360 mol/L·s

–0.090 mol/L·s

0.090 mol/L·s

0.180 mol/L·s

0.360 mol/L·s

The rate of appearance of Br2 is given as 0.180 mol/L·s. Since the stoichiometric coefficient of Br2 in the balanced chemical equation is 1, the rate of disappearance of NOBr, which also has a stoichiometric coefficient of 1, will be equal in magnitude but opposite in sign. Therefore, the rate of disappearance of NOBr is -0.180 mol/L·s.

Explanation

The rate of appearance of Br2 is given as 0.180 mol/L·s. Since the stoichiometric coefficient of Br2 in the balanced chemical equation is 1, the rate of disappearance of NOBr, which also has a stoichiometric coefficient of 1, will be equal in magnitude but opposite in sign. Therefore, the rate of disappearance of NOBr is -0.180 mol/L·s.

20. The rate constant for the decomposition of cyclobutane is 2.08 × 10^-2 s^-1 at high temperatures.
C₄H₈(g) → 2C₂H₄(g)
How many seconds are required for an initial concentration of 0.100 M C₄H₈(g) to decrease to 0.0450 M?

0.00114 s

1.07 s

2.64 s

38.4 s

874 s

Solving: ln0.100M/0.0450M = 2.08 × 10-2 s–1 × t 0.799= 2.08 × 10-2 s–1 × t t =38.4s

Explanation

Solving: ln0.100M/0.0450M = 2.08 × 10-2 s–1 × t 0.799= 2.08 × 10-2 s–1 × t t =38.4s

21. The initial rates method was used to study the reaction below. A + 3B =2C [A] (mol/L) [B] (mol/L) -D[A]/Dt (mol/Lms) 0.210 0.150 3.41 x 10^3 0.210 0.300 1.36 x 10^2 0.420 0.300 2.73 x 10^-2

Rate = 0.515[A] x [B]

Rate = 0.515[A]^2 x [B]

Rate = 0.721[A]^2 x [B]

Rate = 0.721[A] x [B]^2

Rate = 0.721[A]^2 x [B]^2

1.36 10-2/3.41  10-3= (0.210/0.210)m (0.300/0.150)n 4 = (2)n n = 2
2.73  10 -2/1.36  10-2 =(0.420/0.210)m (0.300/0.300)n 2 =(2)m m = 1
2.73  10 -2= k (0.420)m (0.300)n = k (0.420) (0.300)2 k = 0.721

Explanation

1.36 10-2/3.41  10-3= (0.210/0.210)m (0.300/0.150)n 4 = (2)n n = 2
2.73  10 -2/1.36  10-2 =(0.420/0.210)m (0.300/0.300)n 2 =(2)m m = 1
2.73  10 -2= k (0.420)m (0.300)n = k (0.420) (0.300)2 k = 0.721

22. Which of the following statements is/are CORRECT?
   1.   Product concentrations appear in the numerator of an equilibrium constant expression.
   2.   A reaction favors the formation of products if K >> 1.
   3.   Stoichiometric coefficients are used as exponents in an equilibrium constant expression.

1 only

2 only

3 only

2 and 3

1, 2, and 3

The correct answer is 1, 2, and 3.

1. The statement is correct because in an equilibrium constant expression, the concentrations of products are placed in the numerator.
2. The statement is correct because if the equilibrium constant (K) is much greater than 1, it indicates that the reaction strongly favors the formation of products.
3. The statement is correct because the stoichiometric coefficients of the reactants and products are used as exponents in the equilibrium constant expression.

Explanation

The correct answer is 1, 2, and 3.

1. The statement is correct because in an equilibrium constant expression, the concentrations of products are placed in the numerator.
2. The statement is correct because if the equilibrium constant (K) is much greater than 1, it indicates that the reaction strongly favors the formation of products.
3. The statement is correct because the stoichiometric coefficients of the reactants and products are used as exponents in the equilibrium constant expression.

23. Determine the heat of reaction for the combustion of ammonia,
        4NH₃(g) + 7O₂(g) → 4NO₂(g) + 6H₂O(l)
using molar enthalpies of formation.

NH3(g)   –45.9
NO2(g)   +33.1
H2O(l)   –285.8

+30.24 kJ

–206.9 kJ

–298.6 kJ

–1398.8 kJ

–1663.6 kJ

Solving: ΔH = Σ ΔfH0 products - Σ ΔfH0reactants = 4× 33.1 + 6×(-285.8) - 4×(–45.9) = –1398.8 kJ

Explanation

Solving: ΔH = Σ ΔfH0 products - Σ ΔfH0reactants = 4× 33.1 + 6×(-285.8) - 4×(–45.9) = –1398.8 kJ

24. Water has a specific heat of 4.18 J/g · ⁰C. If 35.0 g of water at 98.8 ⁰C loses 4.94 kJ of heat, what is the final temperature of the water?

32.00C

46.20C

47.20C

57.20C

65.0C

Solving: loses heat, so -4.94 kJ = 35.0 g ×4.18 J/ g ·0C × (Tf – 98.8 0C), Tf = 65.0 0C

Explanation

Solving: loses heat, so -4.94 kJ = 35.0 g ×4.18 J/ g ·0C × (Tf – 98.8 0C), Tf = 65.0 0C

25. Use the equilibrium constants for the following reactions at 700 ⁰C

	2SO₂(g) + O₂(g) 2SO₃(g)	K₁ = 4.8
	2NO(g) + O₂(g) 2NO₂(g)	K₂ = 16

to determine the equilibrium constant for the following reaction. SO₃(g) + NO(g)

SO₂(g) + NO₂(g)

0.30

0.55

0.85

1.8

3.3

Solving: K1 = SO3(g)2 / SO2(g)2· O2(g) K2 = NO2(g)2/ NO(g)2· O2(g)
K = SO2(g)· NO2(g) / NO(g)· SO3(g) = (SO2(g)2· O2(g)/ SO3(g)2 ) × (SO3(g) · NO(g)/ SO2(g) · NO 2(g)) × ( NO2(g)2/ NO(g)2· O2(g)) = 1/ K1 × 1/K × K2
K2 = K2 / K1 =16 /4.8 = 3.33 K = 1.8

Explanation

Solving: K1 = SO3(g)2 / SO2(g)2· O2(g) K2 = NO2(g)2/ NO(g)2· O2(g)
K = SO2(g)· NO2(g) / NO(g)· SO3(g) = (SO2(g)2· O2(g)/ SO3(g)2 ) × (SO3(g) · NO(g)/ SO2(g) · NO 2(g)) × ( NO2(g)2/ NO(g)2· O2(g)) = 1/ K1 × 1/K × K2
K2 = K2 / K1 =16 /4.8 = 3.33 K = 1.8

26. Which of the following may change the ratio of products to reactants in an equilibrium mixture for a chemical reaction involving gaseous species?
   1.   Increasing the temperature.
   2.   Adding a catalyst.
   3.   Adding gaseous reactants.

1 only

2 only

3 only

1 and 2

1 and 3

Increasing the temperature (1) can change the ratio of products to reactants in an equilibrium mixture because it causes the reaction to shift in the direction that consumes heat, which is either the forward or reverse reaction depending on whether the reaction is exothermic or endothermic. Adding gaseous reactants (3) can also change the ratio because it increases the concentration of reactants, causing the reaction to shift in the direction that consumes the additional reactants. Adding a catalyst (2) does not change the ratio of products to reactants in an equilibrium mixture, as it only speeds up the rate of the reaction but does not affect the position of the equilibrium.

Explanation

Increasing the temperature (1) can change the ratio of products to reactants in an equilibrium mixture because it causes the reaction to shift in the direction that consumes heat, which is either the forward or reverse reaction depending on whether the reaction is exothermic or endothermic. Adding gaseous reactants (3) can also change the ratio because it increases the concentration of reactants, causing the reaction to shift in the direction that consumes the additional reactants. Adding a catalyst (2) does not change the ratio of products to reactants in an equilibrium mixture, as it only speeds up the rate of the reaction but does not affect the position of the equilibrium.

27. All of the following statements are true EXCEPT

Hess' law states that ΔH for an overall reaction is the sum of the ΔH values for the individual equations.

The molar enthalpy of formation of a compound is equal to the enthalpy change when one mole of the compound is formed from elements.

A reaction with a negative enthalpy is exothermic.

The enthalpy of formation of an element in its most stable state is equal to zero.

The sum of the enthalpies of formation of the products in a chemical reaction is defined as the enthalpy of reaction.

The correct answer is "the sum of the enthalpies of formation of the products in a chemical reaction is defined as the enthalpy of reaction." This statement is not true because the enthalpy of reaction is defined as the difference between the enthalpies of the products and the reactants, not the sum of the enthalpies of formation of the products. The enthalpy of reaction takes into account the stoichiometric coefficients of the balanced chemical equation.

Explanation

The correct answer is "the sum of the enthalpies of formation of the products in a chemical reaction is defined as the enthalpy of reaction." This statement is not true because the enthalpy of reaction is defined as the difference between the enthalpies of the products and the reactants, not the sum of the enthalpies of formation of the products. The enthalpy of reaction takes into account the stoichiometric coefficients of the balanced chemical equation.

28. For the second-order reaction below, the rate constant of the reaction is 9.4 × 10^–3 M^–1s^–1. How long (in seconds) is required to decrease the concentration of A from 2.16 M to 0.40 M?

2A → B

rate = k[A]²

2.0 × 10^1 s

7.8 × 10^1 s

1.8 × 10^2 s

1.9 × 10^2 s

2.2 × 10^2 s

Solving: 1/[A] – 1/[A]0 = kt 1/0.40 – 1/2.16 = 9.4 × 10–3 × t T = 216 = 2.2 × 102 s

Explanation

Solving: 1/[A] – 1/[A]0 = kt 1/0.40 – 1/2.16 = 9.4 × 10–3 × t T = 216 = 2.2 × 102 s