Measuring enthalpy changes: notes and practice questions

A student is investigating the thermal properties of ethanol. They heat a sample of ethanol with a mass of $25.0\ \text{g}$ from an initial temperature of $20.0\ \text{\textdegree C}$ to a final temperature of $65.0\ \text{\textdegree C}$. The specific heat capacity of ethanol is $2.44\ \text{J}\ \text{g}^{-1}\ \text{\textdegree C}^{-1}$.

How much thermal energy, in joules, is absorbed by the ethanol during this heating process?

A $110\ \text{J}$

B $1220\ \text{J}$

C $2750\ \text{J}$

D $1130\ \text{J}$

A student measures the temperature change when a salt is dissolved in water to determine the enthalpy of solution, $ΔH_{sol}$. What is the expression for the enthalpy of solution for ammonium nitrate in kJ mol$^{−1}$?

Data:

• Initial temperature of water: 25.0 °C

• Final temperature of solution: 21.0 °C

• Mass of water: 50.0 g

• Mass of ammonium nitrate dissolved: 4.00 g

• Molar mass, $M_r$, of ammonium nitrate, $NH_4 NO_3$: 80.06 g mol$^{-1}$

• Specific heat capacity of water, $c$: 4.18 J g$^{−1}$ K$^{−1}$

$q = mcΔT$

A. $\frac{ 50.0 \times 4.18 \times 4.0 }{ \frac{ 4.00 }{ 80.06 } \times 1000 }$

B. $- \frac{ 50.0 \times 4.18 \times 4.0 }{ \frac{ 4.00 }{ 80.06 } \times 1000 }$

C. $\frac{ 50.0 \times 4.18 \times 4.0 }{ \frac{ 4.00 }{ 80.06 } }$

D. $\frac{ 4.00 \times 4.18 \times 4.0 }{ \frac{ 50.0 }{ 80.06 } \times 1000 }$

The reaction of solid calcium oxide with water is an exothermic process, represented by the equation: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$

What will happen to the temperature of the mixture when water is added to solid calcium oxide in an insulated container?

A. The temperature will decrease.

B. The temperature will increase.

C. The temperature will not change.

D. The reaction will absorb thermal energy from the surroundings.

The decomposition of calcium carbonate, $CaCO_3(s)$, into calcium oxide, $CaO(s)$, and carbon dioxide, $CO_2(g)$, is an important industrial process. The balanced chemical equation for this reaction is:

$CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$

Using standard enthalpy of formation values:

$\Delta H_f^{\ominus}(CaCO_3(s) ) = -1207.6\ \text{kJ}\ \text{mol}^{-1}$

$\Delta H_f^{\ominus}(CaO(s) ) = -635.1\ \text{kJ}\ \text{mol}^{-1}$

$\Delta H_f^{\ominus}(CO_2(g) ) = -393.5\ \text{kJ}\ \text{mol}^{-1}$

The enthalpy change associated with the decomposition of $25.0\ \text{g}$ of $CaCO_3(s)$ is closest to:

A $-44.7\ \text{kJ}$

B $+53.2\ \text{kJ}$

C $+44.7\ \text{kJ}$

D $+179\ \text{kJ}$

For the complete combustion of propane gas, $C_3H_8(g)$:

$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$

Which statement correctly describes this process?

A. The process is endothermic and $\Delta H$ is positive.

B. The process is endothermic and $\Delta H$ is negative.

C. The process is exothermic and $\Delta H$ is positive.

D. The process is exothermic and $\Delta H$ is negative.

When $2.00$ g of ammonium nitrate, $NH_4NO_3(s)$, was dissolved in $100$ $cm^3$ of water in a calorimeter, the temperature of the solution changed from $298.15$ K to $296.65$ K.

What is the molar enthalpy change of dissolution, $\Delta H_{diss}$, for ammonium nitrate in $kJ$ $mol^{-1}$? Assume the specific heat capacity of the solution is $4.18$ J $g^{-1}$ $K^{-1}$ and the density of water is $1.00$ g $cm^{-3}$.

Relative atomic masses: N = $14.01$, H = $1.01$, O = $16.00$.

A. $+25.1$

B. $-25.1$

C. $+12.5$

D. $-12.5$

A student conducts an experiment to determine the specific heat capacity of an unknown metal. A $250\ \text{g}$ sample of the metal is heated, causing its temperature to rise from $20.0\ \text{\textdegree C}$ to $35.0\ \text{\textdegree C}$. If $1440\ \text{J}$ of thermal energy were absorbed by the metal, what is its specific heat capacity?

A $0.384\ \text{J}\ \text{g}^{-1}\ \text{\textdegree C}^{-1}$

B $3.84\ \text{J}\ \text{g}^{-1}\ \text{\textdegree C}^{-1}$

C $0.0384\ \text{J}\ \text{g}^{-1}\ \text{\textdegree C}^{-1}$

D $5.76\ \text{J}\ \text{g}^{-1}\ \text{\textdegree C}^{-1}$

The energy profile for the thermal decomposition of a metal carbonate is shown. Identify the correct statement describing the reaction.

A. The enthalpy change is positive and the products are more stable than the reactants.

B. The enthalpy change is positive and the reactants are more stable than the products.

C. The enthalpy change is negative and the products are more stable than the reactants.

D. The enthalpy change is negative and the reactants are more stable than the products.

A student performs an experiment to determine the enthalpy of combustion of ethanol, $C_2H_5OH$. When $0.450$ g of ethanol is completely burned, the thermal energy released causes the temperature of $250$ $cm^3$ of water to increase by $8.00$ K.

What is the enthalpy of combustion of ethanol in $kJ$ $mol^{-1}$?

Specific heat capacity of water: $4.18$ $Jg^{-1}K^{-1}$

$M_r$ ethanol $= 46.08$ $g$ $mol^{-1}$

A. $-856$

B. $-428$

C. $-1710$

D. $-85.6$

Which statement about an endothermic reaction is correct?

A. Temperature decreases and the products have higher enthalpy than the reactants.

B. Temperature decreases and the products have lower enthalpy than the reactants.

C. Temperature increases and the products have higher enthalpy than the reactants.

D. Temperature increases and the products have lower enthalpy than the reactants.

The Haber process for the synthesis of ammonia is represented by the equation: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$. The reaction is exothermic and requires a catalyst, indicating a high activation energy for the uncatalysed reaction.

Which potential energy profile best represents the forward uncatalysed reaction of the Haber process?

A neutralization reaction is carried out in a calorimeter, which is in thermal equilibrium with the laboratory. The temperature inside the calorimeter is observed to rise.

Which row correctly describes the energy changes for this process?

A student investigated the dissolution of ammonium chloride, $NH_4Cl$, in water. $5.35\text{ g}$ of solid ammonium chloride was added to $100.0\text{ cm}^3$ of distilled water in an insulated calorimeter.

The dissolution reaction is given by:

$NH_4Cl(s) \rightarrow NH_4^+(aq) + Cl^-(aq)$

The initial temperature of the water was $298.00\text{ K}$ and the final temperature of the solution was $294.40\text{ K}$.

Determine the enthalpy of this reaction in $kJ \text{ mol}^{-1}$. Assume the specific heat capacity of the solution is $4.18\text{ J g}^{-1} \text{ K}^{-1}$ and the density of water is $1.00\text{ g cm}^{-3}$.

A. $+7.50$

B. $-7.50$

C. $+15.0$

D. $-15.0$

The energy profile for a chemical reaction is shown.

What is the correct interpretation of this energy profile?

A student performed an experiment to determine the enthalpy change of dissolution for ammonium nitrate, $NH_4NO_3(s)$. They dissolved $0.050$ mol of solid ammonium nitrate in $150$ g of water in a coffee cup calorimeter.

Initial temperature of water = $22.5^{\circ}C$

Final temperature of solution = $19.0^{\circ}C$

Specific heat capacity of water, $c_w = 4.18 \text{ J g}^{-1} \text{ K}^{-1}$

Assume the specific heat capacity of the solution is the same as water and the density of the solution is $1.00 \text{ g cm}^{-3}$. The thermal energy absorbed by the calorimeter is negligible.

What is the enthalpy change of dissolution, $\Delta H^{\ominus}_{dissolution}$, for ammonium nitrate according to these experimental results?

A. $-44 \text{ kJ mol}^{-1}$

B. $+2.2 \text{ kJ mol}^{-1}$

C. $+44 \text{ kJ mol}^{-1}$

D. $+2200 \text{ kJ mol}^{-1}$

The diagram shows the potential energy profile for a chemical reaction carried out in an insulated container.

What would be observed during this reaction?

A student conducted an experiment to determine the enthalpy of combustion of ethanol. They found that burning $0.250$ g of ethanol ($C_2H_5OH$) caused the temperature of $150$ $\text{cm}^3$ of water to increase by $8.00$ K.

What is the enthalpy of combustion of ethanol in $\text{kJ mol}^{-1}$?

Specific heat capacity of water: $4.18$ $\text{J g}^{-1} \text{ K}^{-1}$

$M_r$ ethanol: $46.08$ $\text{g mol}^{-1}$

$\text{A. } -925$

$\text{B. } -462$

$\text{C. } -1850$

$\text{D. } -92.5$

Which statement about an endothermic reaction is correct?

A. Temperature decreases and the products have higher enthalpy than the reactants.

B. Temperature decreases and the products have lower enthalpy than the reactants.

C. Temperature increases and the products have higher enthalpy than the reactants.

D. Temperature increases and the products have lower enthalpy than the reactants.