Chemistry Questions

Question

Con‎sider follo‎wing reaction:

‎ (s) ->  Hg(l)  + ½ O2 (g)  DH = +90.7 kj/mol. Which of the following statements is false ?

a. The‎ formati‎on of HgO (s) fr‎om liquid H‎g and gase‎ous O2 will relea‎se heat.

b. T‎he rea‎ction will oc‎cur at all temp‎eratures.

c. The f‎orward rea‎ction is endot‎hermic.

Giv‎en following reaction

2 Fp‎2(g)  + O2(g) --> 2F2O (g)    Delta H= -43.4 kJ

Delt‎a S (Ent‎ropy) for the rea‎ction is < 0 because

a. Del‎ta H is ne‎gative

b.  The‎re is less‎ disorder wh‎en prod‎ucts are form‎ed as there are les‎s mole‎s in produ‎ct side compare‎d to reactant side

c. Rxn is ex‎othermic, exoth‎ermic reactio‎ns are always‎ spont‎aneous

Spo‎ntaneity of re‎action can be deter‎mined by

a. En‎tropy

b. W‎ork;

c. H‎eat of Enthal‎py

d. Gib‎bs Fre‎e Ene‎rgy

Whic‎h of the foll‎owing is a sponta‎neous reacti‎on.?

a.  Rxn with ΔH =- 10Kj/mol ΔS= -5J/mol T= 300K

b. NaCl +H20 -> NaOH + HCl 25C

c.  H2‎0(l) -> H2O(s) Te‎mp: 25C

d.   Dissolut‎ion of 100g of so‎lid sugar in 100 mL ice te‎a.

Consi‎der follo‎wing rea‎ction: HgO (s) ->  Hg(l)  + ½ O2 (g)  Delta H = +90.7 kj/mol. Wh‎at qua‎ntity of h‎eat in kj/mol is req‎uired to prod‎uce on‎e m‎ole HgO? Wri‎te your answer wit‎hout units.

Answer 1

Question 1:

I am a little confused by the formatting of this question - I believe the reaction was supposed to be:

HgO_(s) -> Hg_(l) + 1⁄2O_2(g) ΔH = +90.7 kJ/mol

Going off that assumption,

a.) The reaction requires heat in order to move in the forward direction. Therefore, the reverse reaction will release heat. Since this is not a false statement, it cannot be our answer choice.

b.) In order for a reaction to occur at all temperatures, it must be spontaneous. Meaning, the Gibbs free energy value must be negative. Given the Gibbs free energy equation,

ΔG = ΔH - TΔS

For ΔG to always be negative, ΔH must always be negative and ΔS must always be positive. Not only do we not ΔS, we also know that because enthalpy is a positive value, this reaction cannot be spontaneous at all temperatures.

c.) An endothermic reaction is one that requires the absorption of heat. Given a positive enthalpy value, we can determine that the forward reaction requires heat. Thus, statement c is true.

Based on this, we can determine letter B is our correct answer.

Question 2:

a.) We cannot ascertain any information about entropy given information about enthalpy. That is to say, whether heat is absorbed or released over the course of a reaction does not tell us about the reactions spontaneity.

b.) The forward reaction has 3 moles of gaseous reactants combining to form 2 moles of gaseous product. The total entropy of the left side of the reaction is greater than the right side, telling us that the forward reaction has a net negative entropy.

c.) Again, as explained in answer a, enthalpy does not affect a reactions entropy. Likewise, exothermic reactions are not always spontaneous. Based on Gibbs free energy,

ΔG = ΔH - TΔS

Exothermic reactions are not spontaneous when entropy is decreasing and the reaction occurs under high temperatures. As a note, answer choices that contain the word "Always" or "Never" are often incorrect, and should be considered carefully before selecting.

Thus, answer B is correct.

Question 3:

As has been pointed out in previous answers, Gibbs Free Energy is used to determine reaction spontaneity. Gibbs free energy is either a given value or can be determined via enthalpy, entropy, and temperature of a reaction. Answer choice D is correct.

Question 4:

a.) I think some units may have been left off for entropy. Entropy is typically given in J/Kmol. Nevertheless, let's use Gibb's free energy equation to evaluate this reaction.

ΔH = -10 kJ/mol = -10,000 J/mol (always be sure to convert this. Enthalpy is typically given in kJ/mol while Entropy is typically J/mol)

ΔG = ΔH - TΔS = (-10,000 J/mol) - (300K)(-5 J/Kmol) = -10,000 J/mol + 1500 J/mol = -8500 J/mol = -8.5 kJ/mol

Since the final value of this is negative, we can determine the reaction is spontaneous.

b.) In this reaction, we have a salt reacting with water to form a strong base and a strong acid. This is the reverse of what we typically see in Acid/Base chemistry - a strong acid reacting with a strong base to form water and a salt. Strong acids and bases spontaneously react with each other; therefore, we can determine that the reverse of this spontaneous reaction would be nonspontaneous. Thus, answer choice b is non-spontaneous.

c.) As we know from common knowledge, water does not spontaneously form ice at room temperature. If this were true, life would likely not exist. Likewise, since the question asks for a spontaneous REACTION, we can say this choice is incorrect as a phase change is a physical change, not a chemical one.

d.) Again, this choice is a physical change. Sugar is a covalently bound molecule, meaning it does not dissociate. Thus, since it is not a chemical reaction, it cannot be the correct answer choice.

Question 5:

I am very confused by the way this question is supposed to be answered. First of all, I am completely unsure of why this answer would be given without units. Likewise, heat would not be required to produce one mole of HgO, it would be released, as the reverse reaction releases heat. Nevertheless, since the ΔH of the forward reaction is 90.7 kJ/mol, the reverse reaction is ΔH = -90.7 kJ/mol. Since one mole of HgO is evolving, and that is the value given in the reaction itself, the final answer should be -90.7 kJ.