Which change of state is exothermic? 

A. CO₂(s) → CO₂(g)
B. H₂O(l) → H₂O(g) 
C. NH₃(g) → NH₃(l) 
D. Fe(s) → Fe(l)

C

Which expression gives the mass, in g, of ethanol required to produce 683.5 kJ of heat upon complete combustion?

(M_r for ethanol = 46.0, $\mathrm{\Delta }{H}_c^{\theta }=-1367\text{ kJ}\text{mo}{\text{l}}^{-1}$)

A.     $\frac{683.5}{1367\times 46.0}$

B.     $\frac{1367}{683.5\times 46.0}$

C.     $\frac{683.5\times 46.0}{1367}$

D.     $\frac{1367\times 46.0}{683.5}$

C

Which statement is correct for this reaction?

Fe₂O₃ (s) + 3CO (g) → 2Fe (s) + 3CO₂ (g)       ΔH = −26.6 kJ

A. 13.3 kJ are released for every mole of Fe produced.

B. 26.6 kJ are absorbed for every mole of Fe produced.

C. 53.2 kJ are released for every mole of Fe produced.

D. 26.6 kJ are released for every mole of Fe produced.

A

What is the enthalpy change, in J, when 5 g of water is heated from 10°C to 18°C?

Specific heat capacity of water: 4.18 kJ kg⁻¹ K⁻¹

A.  5 × 4.18 × 8

B.  5 × 10⁻³ × 4.18 × 8

C.  5 × 4.18 × (273 + 8)

D.  5 × 10⁻³ × 4.18 × (273 + 8)

A

What can be deduced from this reaction profile?

A.     The reactants are less stable than the products and the reaction is exothermic.

B.     The reactants are less stable than the products and the reaction is endothermic.

C.     The reactants are more stable than the products and the reaction is exothermic.

D.     The reactants are more stable than the products and the reaction is endothermic.

A

Hydrazine reacts with oxygen.

N₂H₄(l) + O₂(g) → N₂(g) + 2H₂O(l)      ΔH^θ = -623 kJ

What is the standard enthalpy of formation of N₂H₄(l) in kJ? The standard enthalpy of formation of H₂O(l) is -286 kJ.

A. -623 - 286
B. -623 + 572
C. -572 + 623
D. -286 + 623

C

What is correct about energy changes during bond breaking and bond formation?

C

55% of the candidates selected the correct statements about bond breaking and bond formation. The most commonly chosen distractor reversed them. The question discriminated well between high-scoring and low-scoring candidates.

Which combination will give you the enthalpy change for the hydrogenation of ethene to ethane, $∆H_3$?

A.  $-∆H_2+∆H_1-∆H_4$

B.  $∆H_2\mathit{-}∆H_1+∆H_4$

C.  $∆H_2+∆H_1-∆H_4$

D.  $-∆H_2\mathit{-}∆H_1+∆H_4$

A

A good majority of candidates could select the Hess' Law cycle for hydrogenation of ethene to ethane.

What can be deduced from the facts that ozone absorbs UV radiation in the region of 340 nm and molecular oxygen in the region of 242 nm?

A.     The bond between atoms in molecular oxygen is a double bond.

B.     The bonds in ozone are delocalized.

C.     The bonds between atoms in ozone are stronger than those in molecular oxygen.

D.     The bonds between atoms in molecular oxygen need more energy to break.

D

Consider the following reaction:

N₂ (g) + 3H₂ (g) $\rightleftharpoons$ 2NH₃ (g)

Which calculation gives ΔH^Θ, in kJ, for the forward reaction?

A.   2z − y − 3x

B.   y + 3x − 2z

C.   y + 3x − 6z

D.   6z − y − 3x

C

5.35g of solid ammonium chloride, NH₄Cl(s), was added to water to form 25.0g of solution. The maximum decrease in temperature was 14 K. What is the enthalpy change, in kJmol^-1, for this reaction? (Molar mass of NH₄Cl = 53.5gmol^-1; the specific heat capacity of the solution is 4.18 Jg^-1K^-1)

A. $\mathrm{\Delta }H=+\frac{25.0\times 4.18\times \left(14+273\right)}{0.1\times 1000}$

B. $\mathrm{\Delta }H=-\frac{25.0\times 4.18\times 14}{0.1\times 1000}$

C. $\mathrm{\Delta }H=+\frac{25.0\times 4.18\times 14}{0.1\times 1000}$

D. $\mathrm{\Delta }H=+\frac{25.0\times 4.18\times 14}{1000}$

C

Which equation represents the standard enthalpy of formation of lithium oxide?

A.  4Li (s) + O₂(g) → 2Li₂O (s)

B.  2Li (s) + $\frac{1}{2}$O₂(g) → Li₂O (s)

C.  Li (s) + $\frac{1}{4}$O₂(g) → $\frac{1}{2}$Li₂O (s)

D.  Li (g) + $\frac{1}{4}$O₂(g) → $\frac{1}{2}$Li₂O (g)

B

The enthalpy of combustion of a fuel was determined using the calorimeter shown. The final result was lower than the literature value.

Which factors could have contributed to this error?

I.   Not all heat from the combustion was transferred to the calorimeter.
II.  Incomplete combustion occurred.
III. The temperature probe touched the bottom of the calorimeter.

A.  I and II only

B.  I and III only

C.  II and III only

D.  I, II and III

A

Which quantity is likely to be the most inaccurate due to the sources of error in this experiment?

A.  Mass of ethanol burnt

B.  Molecular mass of ethanol

C.  Mass of water

D.  Temperature change

D

Methane undergoes incomplete combustion.

2CH₄ (g) + 3O₂ (g) → 2CO (g) + 4H₂O (g)

What is the enthalpy change, in kJ, using the bond enthalpy data given below?

A. [2(1077) + 4(463)] − [2(414) + 3(498)]

B. [2(414) + 3(498)] − [2(1077) + 4(463)]

C. [8(414) + 3(498)] − [2(1077) + 8(463)]

D. [2(1077) + 8(463)] − [8(414) + 3(498)]

C

65 % of the candidates chose the correct expression for determining the enthalpy change using bond enthalpies. The most commonly chosen distractor (C) had the signs reversed. The question had a good discrimination index.

What is the enthalpy change of the following reaction?

CH₂CHCH₂CH₃ + HBr → CH₃CHBrCH₂CH₃

A.  –119.6 kJ

B.  +119.6 kJ

C.  –119.8 kJ

D.  +119.8 kJ

C

What is the enthalpy change of reaction for the following equation?

A. x + y + z

B. −x − y + z

C. x − y − z

D. x − y + z

D

83 % of candidates answered this Hess Law question correctly. This topic was obviously well covered.

Consider the following equations.

2Al (s) + $\frac{3}{2}$O₂ (g) → Al₂O₃ (s)    ΔH^Ɵ = −1670 kJ
Mn (s) + O₂ (g) → MnO₂ (s)    ΔH^Ɵ = −520 kJ

What is the standard enthalpy change, in kJ, of the reaction below?

4Al (s) + 3MnO₂ (s) → 2Al₂O₃ (s) + 3Mn (s)

A. −1670 + 520

B. $\frac{3}{2}$(−1670) + 3(520)

C. 2(−1670) + 3(−520)

D. 2(−1670) + 3(520)

D

It is pleasing that 83 % of the candidates were confident in applying Hess’s law to obtain an expression for the standard enthalpy change of reaction. The most commonly chosen distractor (C) had the correct coefficients but one of the signs was incorrect.

When equal masses of X and Y absorb the same amount of energy, their temperatures rise by 5 °C and 10 °C respectively. Which is correct?

A. The specific heat capacity of X is twice that of Y.

B. The specific heat capacity of X is half that of Y.

C. The specific heat capacity of X is one fifth that of Y.

D. The specific heat capacity of X is the same as Y.

A

Answered correctly by 53 %, this question relating specific heat capacity to temperature rise was handled better by higher scoring candidates.

Consider the following reactions:

Fe₂O₃ (s) + CO (g) → 2FeO (s) + CO₂ (g)       ΔH^Θ = −3 kJ

Fe (s) + CO₂ (g) → FeO (s) + CO (g)               ΔH^Θ = +11 kJ

What is the ΔH^Θ value, in kJ, for the following reaction?

Fe₂O₃ (s) + 3CO (g) → 2Fe (s) + 3CO₂ (g)

A.   −25

B.   −14

C.   +8

D.   +19

A

Which statement is correct about identical pieces of magnesium added to two solutions, X and Y, containing hydrochloric acid at the same temperature?

A.  Solution X will reach a higher maximum temperature.

B.  Solution Y will reach a higher maximum temperature.

C.  Solutions X and Y will have the same temperature rise.

D.  It is not possible to predict whether X or Y will have the higher maximum temperature because we cannot identify the limiting reactant.

A

A nice question that required thinking. There were two factors to consider: amount of reactants and temperature rise. Most candidates focused on the same amount of Mg and HCl hence concluded that the temperature rise would be the same. In fact, it is the same amount of heat that is released, however with a larger volume of solution in Y, the temperature rise is higher for X. 40% of the candidates answered correctly.

Which describes the reaction shown in the potential energy profile?

A.     The reaction is endothermic and the products have greater enthalpy than the reactants.

B.     The reaction is endothermic and the reactants have greater enthalpy than the products.

C.     The reaction is exothermic and the products have greater enthalpy than the reactants.

D.     The reaction is exothermic and the reactants have greater enthalpy than the products.

A

What is the heat change, in kJ, when 100.0 g of aluminium is heated from 19.0 °C to 32.0 °C?

Specific heat capacity of aluminium: 0.90 J g⁻¹K⁻¹

A.  $0.90\times 100.0\times 13.0$

B.  $0.90\times 100.0\times 286$

C.  $\frac{0.90\times 100.0\times 13.0}{1000}$

D.  $\frac{0.90\times 100.0\times 286}{1000}$

C

In which order does the oxygen–oxygen bond enthalpy increase?

A.     H₂O₂ < O₂ < O₃

B.     H₂O₂ < O₃ < O₂

C.     O₂ < O₃ < H₂O₂

D.     O₃ < H₂O₂ < O₂

B

A student obtained the following data to calculate $q$, using $q=mc\mathrm{\Delta }T$.

$m=20.2 \mathrm{g}\pm 0.2 \mathrm{g}$

$∆T=10°\mathrm{C}\pm 1°\mathrm{C}$

$c=4.18 \mathrm{J} {\mathrm{g}}^{-1} {\mathrm{K}}^{-1}$

What is the percentage uncertainty in the calculated value of $q$?

A.  $0.2$

B.  $1.2$

C.  $11$

D.  $14$

C

Just over ½ of the candidates could propagate uncertainties from absolute to relative.

Which combustion reaction releases the least energy per mole of C₃H₈?

Approximate bond enthalpy / kJ mol⁻¹
O=O    500
C=O    800
 C≡O   1000

A.  C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O (g)

B.  C₃H₈(g) + $\frac{9}{2}$O₂(g) → 2CO₂(g) + CO (g) + 4H₂O (g)

C.  C₃H₈(g) + 4O₂(g) → CO₂(g) + 2CO (g) + 4H₂O (g)

D.  C₃H₈(g) + $\frac{7}{2}$O₂(g) → 3CO (g) + 4H₂O (g)

Chemistry: Atoms First 2e, https://openstax.org/books/chemistry-atoms-first-2e/pages/9-4-strengths-of-ionic-andcovalent-bonds © 1999–2021, Rice University. Except where otherwise noted, textbooks on this site are licensed under a Creative Commons Attribution 4.0 International License.
(CC BY 4.0) https://creativecommons.org/licenses/ by/4.0/.

D

Which statement describes an endothermic reaction?

A.  The bonds broken are stronger than the bonds formed.

B.  The enthalpy of the reactants is higher than the enthalpy of the products.

C.  The temperature of the surroundings increases.

D.  The products are more stable than the reactants.

A

The enthalpy changes for two reactions are given.

Br₂ (l) + F₂ (g) → 2BrF (g)         ΔH = x kJ
Br₂ (l) + 3F₂ (g) → 2BrF₃ (g)      ΔH = y kJ

What is the enthalpy change for the following reaction?

BrF (g) + F₂ (g) → BrF₃ (g)

A.  x – y

B.  –x + y

C.  $\frac{1}{2}$(–x + y)

D.  $\frac{1}{2}$(x – y)

C

Which equation shows the enthalpy of formation, $∆H_{\mathrm{f}}$, of ethanol?

A.  $2\mathrm{C} \left(\mathrm{s}\right)+3{\mathrm{H}}_2 \left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2 \left(\mathrm{g}\right)\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \left(\mathrm{g}\right)$

B.  $4\mathrm{C} \left(\mathrm{s}\right)+6{\mathrm{H}}_2 \left(\mathrm{g}\right)+{\mathrm{O}}_2 \left(\mathrm{g}\right)\to 2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \left(\mathrm{g}\right)$

C.  $2\mathrm{C} \left(\mathrm{s}\right)+3{\mathrm{H}}_2 \left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2 \left(\mathrm{g}\right)\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \left(\mathrm{l}\right)$

D.  $4\mathrm{C} \left(\mathrm{s}\right)+6{\mathrm{H}}_2 \left(\mathrm{g}\right)+{\mathrm{O}}_2 \left(\mathrm{g}\right)\to 2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \left(\mathrm{l}\right)$

C

Less than half of the candidates selected the correct equation for the enthalpy of formation of ethanol. The most common wrong answer involved ethanol formed as a gas.

What is the enthalpy change of the reaction?

C₆H₁₄ (l) → C₂H₄ (g) + C₄H₁₀ (g)

A.  + 1411 + 2878 + 4163

B.  + 1411 − 2878 − 4163

C.  + 1411 + 2878 − 4163

D.  − 1411 − 2878 + 4163

C

When sodium carbonate powder is added to ethanoic acid, the beaker becomes cooler.

Possible enthalpy diagrams are shown.

Which correctly describes the reaction?

C

The C=N bond has a bond length of 130 pm and an average bond enthalpy of 615kJmol^-1. Which values would be most likely for the C-N bond?

A

Which is correct when Ba(OH)₂ reacts with NH₄Cl?

Ba(OH)₂ (s) + 2NH₄Cl (s) → BaCl₂ (aq) + 2NH₃ (g) + 2H₂O (l)       ΔH^Θ = +164 kJ mol⁻¹

C

In which reaction do the reactants have a lower potential energy than the products? 

A. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
B. HBr(g) → H(g) + Br(g) 
C. Na⁺(g) + Cl^-(g) → NaCl(s) 
D. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

B

What is the enthalpy change, in kJ, of the following reaction?

3H₂ (g) + N₂ (g) $\rightleftharpoons$ 2NH₃ (g)

A. (6 × 391) − [(3 × 436) + 945]

B. (3 × 391) − (436 + 945)

C. −[(3 × 436) + 945] + (3 × 391)

D. −(6 × 391) + [(3 × 436) + 945]

D

Which is the enthalpy change of reaction, ΔH?

A

Which equation represents the N–H bond enthalpy in NH₃?

A.  NH₃ (g) → N (g) + 3H (g)

B.  $\frac{1}{3}$NH₃ (g) → $\frac{1}{3}$N (g) + H (g)

C.  NH₃ (g) → $\frac{1}{2}$N₂ (g) + $\frac{3}{2}$H₂ (g)

D.  NH₃ (g) → •NH₂ (g) + •H (g)

B

Two 100 cm³ aqueous solutions, one containing 0.010 mol NaOH and the other 0.010 mol HCl, are at the same temperature.

When the two solutions are mixed the temperature rises by y °C.

Assume the density of the final solution is 1.00 g cm⁻³.

Specific heat capacity of water = 4.18 J g⁻¹ K⁻¹

What is the enthalpy change of neutralization in kJ mol⁻¹?

A.     $\frac{200\times 4.18\times y}{1000\times 0.020}$

B.     $\frac{200\times 4.18\times y}{1000\times 0.010}$

C.     $\frac{100\times 4.18\times y}{1000\times 0.010}$

D.     $\frac{200\times 4.18\times (y+273)}{1000\times 0.010}$

B

The energy from burning 0.250 g of ethanol causes the temperature of 150 cm³ of water to rise by 10.5 °C. What is the enthalpy of combustion of ethanol, in kJ mol^–1?

Specific heat capacity of water: 4.18 J g^–1K^–1.

A.  $\frac{150\times 4.18\times 10.5}{{\displaystyle \frac{0.250}{46.08}}}$

B.  $\frac{150\times 4.18\times 10.5}{{\displaystyle \frac{0.250}{46.08}}\times 1000}$

C.  $\frac{150\times 4.18\times \left(273+10.5\right)}{{\displaystyle \frac{0.250}{46.08}}}$

D.  $\frac{150\times 4.18\times \left(273+10.5\right)}{{\displaystyle \frac{0.250}{46.08}}\times 1000}$

B

Which describes an exothermic reaction?

D

What is the enthalpy of combustion of butane in kJ mol⁻¹?

2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l)

A.     4x + 5y − z 

B.     4x + 5y + z 

C.     8x + 10y − 2z 

D.     8x + 5y + 2z

A

Why is the value of the enthalpy change of this reaction calculated from bond enthalpy data less accurate than that calculated from standard enthalpies of formation?

2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(g)

A.     All the reactants and products are gases.

B.     Bond enthalpy data are average values for many compounds.

C.     Elements do not have standard enthalpy of formation.

D.     Standard enthalpies of formation are per mole.

B

What is the enthalpy change of combustion of urea, (NH₂)₂CO, in kJ mol⁻¹?

2(NH₂)₂CO(s) + 3O₂(g) → 2CO₂(g) + 2N₂(g) + 4H₂O(l)

A.     2 × (−333) −2 × (−394) −4 × (−286)

B.     $\frac{1}{2}$[2 × (−394) + 4 × (−286) −2 × (−333)]

C.     2 × (−394) + 4 × (−286) −2 × (−333)

D.     $\frac{1}{2}$[2 × (−333) −2 × (−394) −4 × (−286)]

B

Which is correct for the reaction?

2Al (s) + 6HCl (aq) → 2AlCl₃ (aq) + 3H₂ (g)         ΔH = −1049 kJ

A. Reactants are less stable than products and the reaction is endothermic.

B. Reactants are more stable than products and the reaction is endothermic.

C. Reactants are more stable than products and the reaction is exothermic.

D. Reactants are less stable than products and the reaction is exothermic.

D

This question on enthalpy change was also well answered.

What is the $\mathrm{H}-\mathrm{H}$ bond enthalpy, in $\mathrm{kJ} {\mathrm{mol}}^{-1}$, in the ${\mathrm{H}}_2$ molecule?

$2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$

$∆H_{\mathrm{f}}\left({\mathrm{H}}_2\mathrm{O}\right)=x \mathrm{kJ} {\mathrm{mol}}^{-1}$

A.  $x-y+4z$

B.  $\frac{1}{2}\left(x-y+4z\right)$

C.  $x-y+2z$

D.  $\frac{1}{2}\left(x-y+2z\right)$

B

There was no correct answer to this question as the value $∆H_{\mathrm{f}}$ was given instead of $∆H_{\mathrm{r}}$. The majority of candidates answered as if it was $∆H_{\mathrm{r}}$

Which expression gives the enthalpy change, ΔH, for the thermal decomposition of calcium carbonate?

A.     ΔH = ΔH₁ − ΔH₂

B.     ΔH = 2ΔH₁ − ΔH₂

C.     ΔH = ΔH₁ − 2ΔH₂

D.     ΔH = ΔH₁ + ΔH₂

A

What is the enthalpy change of the reaction, in kJ?

2C (graphite) + O₂(g) → 2CO (g)

A.  −394 − 283

B.  2(−394) + 2(−283)

C.  −394 + 283

D.  2(−394) + 2(283)

D

Which combination of ΔH₁, ΔH₂, and ΔH₃ would give the enthalpy of the reaction?

CS₂(l) + 3O₂(g) → CO₂(g) + 2SO₂(g)

ΔH₁  C (s) + O₂(g) → CO₂(g)
ΔH₂  S (s) + O₂(g) → SO₂(g)
ΔH₃  C (s) + 2S (s) → CS₂(l)

A.  ΔH = ΔH₁ + ΔH₂ + ΔH₃

B.  ΔH = ΔH₁ + ΔH₂ − ΔH₃

C.  ΔH = ΔH₁ + 2(ΔH₂) + ΔH₃

D.  ΔH = ΔH₁ + 2(ΔH₂) − ΔH₃

D

A very well answered question using Hess’s law. 85% of the candidates used the enthalpies given to calculate the unknown enthalpy of reaction.

Which statement is correct?

A.     In an exothermic reaction, the products have more energy than the reactants.

B.     In an exothermic reversible reaction, the activation energy of the forward reaction is greater than that of the reverse reaction.

C.     In an endothermic reaction, the products are more stable than the reactants.

D.     In an endothermic reversible reaction, the activation energy of the forward reaction is greater than that of the reverse reaction.

D

What is the correct interpretation of the following potential energy profile?

A.  Endothermic reaction; products more stable than reactants.

B.  Exothermic reaction; products more stable than reactants.

C.  Endothermic reaction; products less stable than reactants.

D.  Exothermic reaction; products less stable than reactants.

C

What is the enthalpy of combustion, ΔH_c, of ethanol in kJ mol⁻¹?
Maximum temperature of water: 30.0°C
Initial temperature of water: 20.0°C
Mass of water in beaker: 100.0 g
Loss in mass of ethanol: 0.230 g
M_r (ethanol): 46.08
Specific heat capacity of water: 4.18 J g⁻¹ K⁻¹
q = mcΔT

A.  $-\frac{100.0\times 4.18\times (10.0\times 273)}{{\displaystyle \frac{0.230}{46.08}}\times 1000}$

B.  $-\frac{0.0230\times 4.18\times 10.0}{{\displaystyle \frac{100.0}{46.08}}\times 1000}$

C.  $-\frac{100.0\times 4.18\times 10.0}{{\displaystyle \frac{0.230}{46.08}}\times 1000}$

D.  $-\frac{100.0\times 4.18\times 10.0}{{\displaystyle \frac{0.230}{46.08}}}$

C

The enthalpy of combustion of ethanol is determined by heating a known mass of tap water in a glass beaker with a flame of burning ethanol.

Which will lead to the greatest error in the final result?

A.     Assuming the density of tap water is 1.0 g cm⁻³

B.     Assuming all the energy from the combustion will heat the water

C.     Assuming the specific heat capacity of the tap water is 4.18 J g⁻¹ K⁻¹

D.     Assuming the specific heat capacity of the beaker is negligible

B