Can someone check my answers please?
I had specific trouble with 7, 13, and 27 (I'm pretty sure my other answers are correct; some reassurance would be nice though). And I have a small question about 8.
1. The ΔHf of an element in its standard state is defined to be
0 kJ/mol
4. Which substance has a standard enthalpy of formation, ΔHf, equal to zero?
a) gold, Au(s)
b) water H2O (l)
c) carbon monoxide, CO(s)
d) zinc, Zn(g)
e) water, H2O
I chose a) because Au is an element and (s) is its standard state.
5. Which of he following statements are true?
I. The reaction vessel cools when an endothermic reaction occurs.
II. An endothermic reaction has a negative value of ΔH.
III. Heat is liberated when an exothermic reaction occurs.
a) I and II
b) I, II, and III
c) I and III only
d) II and III only
e) none of them
I chose c).
6. Which of the following processes are exothermic?
I. boiling water
II. freezing water
III. condensing steam
IV. melting ice
I chose II and III because the reaction needs to release heat, i.e. become cooler, in order for water to freeze and for steam to condense.(?)
7. Which factor does not affect the rate of a chemical reaction in aqueous solution?
a) the enthalpy change of the reaction
b) the activation energy of the reaction
c) the collision frequency of the reacting particles
d) the relative orientation of the colliding particles
e) the temperature of the solution
What's the significance of it being an aqueous solution? How does that change things?
I THINK the answer is a)? Because even if the enthalpy change is super high or super low, the activation energy (which I believe DOES have an effect) won't be affected, and the activation energy is what matters, right...?
8. Which statement about an activated complex is true?
a) It is a stable substance.
b) It has a lower chemical potential energy, or enthalpy, than reactants or products.
c) It occurs only in endothermic reactions.
d) It occurs at the transition state of the reaction.
e) It always breaks down to form product molecules.
I chose d) because I'm sure of that, but why is e) false?
9. A catalyst changes the
I. mechanism of a reaction
II. enthalpy change of a reaction
III. activation energy of a reaction
I said I and III.
10. The overall rate of any chemical reaction is most closely related to
a) the number of steps in the reaction mechanism
b) the overall reaction
c) the fastest step in the reaction mechanism
d) the slowest step in the reaction mechanism
e) the average rate of all the steps in the reaction mechanism
I chose d).
13. In a chemical reaction, bonds are formed and broken.
a) How would you characterize the enthalpy change of bond breaking?
I'm not totally sure how to answer this question because isn't "enthalpy change" the difference in the enthalpies of the reactants and the products? So can the breaking of bonds have an "enthalpy change"...? Anyway, I just wrote:
Bond breaking results in energy absorption so its enthalpy change is positive?
b) How wold you characterize the enthalpy change of bond formation?
Again, I'm not sure how to answer, but:
Bond formation results in energy being released, so its enthalpy change is negative.
c) State the relationship between the enthalpy change of the overall reaction (exothermic and endothermic) and bond breakage and formation.
The enthalpy change of the overall reaction depends on whether more energy is released from bonds forming or if more energy is absorbed from bonds breaking. If more is absorbed, then it's endothermic; if more is released, then it's exothermic.
19. C4H10(g) + 6.5 O2(g) -> 4CO2(g) + 5H2O(l)
a) Write a separate balanced chemical equation for the formation of C4H10, CO2, and H2O, directly from the elements in their standard states.
4C(s) + 5H2(g) -> C4H10(g)
C(s) +O2(g -> CO2(g)
H2(g) + 1/2 O2(g) -> H2O(l)
b) Algebraically combine these equations to get the balanced chemical equation for the complete combustion of C4H10.
I reversed 4C(s) + 5H2(g) -> C4H10(g),
multiplied C(s) +O2(g -> CO2(g) by 4,
and multiplied H2(g) + 1/2 O2(g) -> H2O(l) by 5. (then added the equations)
27. A student dissolves 1.96 g of NaOH in 100.0 mL water in a coffee-cup calorimeter. The initial temperature of the water is 23.4 C. After the NaOH dissolves, the temperature of the water rises to 28.7 C.
a) Determine the enthalpy of dissolution of sodium hydroxide, in kJ/mol NaOH. Assume the heat capacity of the calorimeter is negligible.
Q = mc(ΔT)
= (100g)(4.184 J/gC)(28.7 C - 23.4 C)
=2217.52 J
=2.21752 kJ
n=m/MM
= 1.96 NaOH/(23+16+1.01)
= 0.049 mol
ΔH = -Q/n
= -2.21752 kJ/0.049 mol
= -45.3 kJ/mol
For this one, the book actually gives the answer: 47 kJ/mol.
Firstly, I got a negative (book's typo, or my error?). Secondly, I'm about 2 off, so I think I might've missed something in my calculations? I know 2 is a small difference, but it could mean I missed something important... (That's why i tried it again carrying all the decimals... Still got 45 though.)
b) Suppose that the heat capacity of the calorimeter was not negligible. Explain how the value of ΔH that you calculated in part (a) would compare with the actual ΔH.
I really have no idea.
Would the value of ΔH, calculated when the ΔH of the calorimeter was assumed to be negligible, be higher or lower than the actual ΔH value?
And would it be because the amount of heat absorbed by the calorimeter was not taken into account, or because the amount of heat LOST through the calorimeter was not taken into account?
If you somehow included the heat capacity of the calorimeter into the calculations, how would the result change?
To check your answers and address your questions, let's go through each question one by one:
1. You correctly chose option a) gold, Au(s). The standard enthalpy of formation, ΔHf, of an element in its standard state is defined to be zero kJ/mol.
5. You answered c) I and III only, and that is correct. When an endothermic reaction occurs, the reaction vessel cools (statement I) and heat is liberated when an exothermic reaction occurs (statement III). However, an endothermic reaction actually has a positive value of ΔH (statement II is false).
6. You correctly chose II and III. Boiling water and condensing steam are exothermic processes because heat is released when a substance changes from a higher energy phase to a lower energy phase. Freezing water and melting ice are actually endothermic as they require the absorption of heat.
7. The factor that does not affect the rate of a chemical reaction in aqueous solution is the enthalpy change of the reaction. The significance of it being an aqueous solution is that it does not alter the rate of the reaction. The rate of reaction is determined by factors such as activation energy, collision frequency, and the relative orientation of the colliding particles.
8. You correctly chose option d) It occurs at the transition state of the reaction. Option e) is false because an activated complex can either decompose to form product molecules or continue along the reaction pathway to eventually form product molecules.
9. You correctly chose options I and III. A catalyst can change the mechanism of a reaction (option I) and lower the activation energy of the reaction (option III). However, it does not change the enthalpy change of the reaction (option II).
10. The overall rate of any chemical reaction is most closely related to the slowest step in the reaction mechanism. Therefore, the correct answer is option d) the slowest step in the reaction mechanism.
13. a) For bond breaking, you correctly mentioned that energy is absorbed, so the enthalpy change is positive. b) For bond formation, you again correctly mentioned that energy is released, so the enthalpy change is negative. Your explanations are correct.
19. Your balanced equations for the formation of C4H10, CO2, and H2O from their elements are correct. To algebraically combine these equations, you'll need to multiply the equations by appropriate coefficients so that the number of moles of each species matches in the final balanced equation.
27. Your calculation for the enthalpy of dissolution of sodium hydroxide (NaOH) is correct. The negative sign indicates that the dissolution is exothermic. As for the difference in the magnitude, it could be due to rounding errors or discrepancies in the given values in the problem or the book. However, it's important to note that your calculation shows a negative value, while the book provides a positive value, so there might be an error in the book or an error in your calculation.
b) If the heat capacity of the calorimeter was not negligible, the value of ΔH calculated in part (a) would be lower than the actual ΔH. This is because the heat capacity of the calorimeter represents the amount of heat absorbed or lost by the calorimeter during the reaction. If you included the heat capacity of the calorimeter in the calculations, the result would be higher as it would account for the heat absorbed or lost by the calorimeter in addition to the heat associated with the dissolution of NaOH.
It's essential to be aware of uncertainties and potential errors in calculations, and sometimes small discrepancies can occur due to rounding or measurement errors.