Chemical Energetics

Which row is correct for a chemical reaction in which $\Delta H$ is negative?

Options

A reaction pathway diagram is shown.

Which equation does the reaction pathway diagram show?

Options

A   $2\text{H(g)} + \text{O(g)} \rightarrow \text{H}_2\text{O(g)}$
B   $\text{H}_2\text{(g)} \rightarrow 2\text{H(g)}$
C   $\text{H}_2\text{O(g)} \rightarrow \text{H}_2\text{O(l)}$
D   $2\text{O(g)} \rightarrow \text{O}_2\text{(g)}$

Some bond energy data are given in the table.

Hydrazine, $\text{N}_2\text{H}_4$, reacts with oxygen, as shown.

A value for the enthalpy change, $\Delta H$, of this reaction is calculated by selecting and using data from the table.

What is the value of $\Delta H$?

Options

A   $-728\text{ kJ / mol}$
B   $-568\text{ kJ / mol}$
C   $-34\text{ kJ / mol}$
D   $+352\text{ kJ / mol}$

Which statement about reactions is correct?

Options

A   A reaction in which the number of bonds broken equals the number of bonds formed always has an enthalpy change, $\Delta H = 0$.
B   Combustion can be either exothermic or endothermic.
C   In exothermic reactions, thermal energy is transferred to the surroundings, so the temperature of the surroundings increases.
D   The activation energy, $E_a$, for a reaction is the minimum energy particles must have in order to collide.

The word equations for two reactions of ethene are shown.

$$\text{reaction 1} \quad \text{ethene} + \text{hydrogen} \rightarrow \text{ethane}$$ $$\text{reaction 2} \quad \text{ethene} + \text{bromine} \rightarrow \text{1,2-dibromoethane}$$

The bond energies of the bonds involved in the reactions are shown in the table.

What is the value of $(\Delta H_{\text{reaction 1}} - \Delta H_{\text{reaction 2}})$?

Options

A   $+240\text{ kJ / mol}$
B   $+3\text{ kJ / mol}$
C   $-3\text{ kJ / mol}$
D   $-246\text{ kJ / mol}$

Ammonium nitrate dissolves in water.

$$\text{NH}_4\text{NO}_3\text{(s)} \xrightarrow{\text{H}_2\text{O}} \text{NH}_4\text{NO}_3\text{(aq)} \quad \Delta H = +25\text{ kJ / mol}$$

Which statements are correct?

1. The process is endothermic.
2. The water gets colder during the process.
3. Thermal energy is absorbed by the ammonium nitrate from the water.

Options

A   1, 2 and 3
B   1 and 2 only
C   1 and 3 only
D   2 and 3 only

Hydrogen reacts with oxygen to produce water.

$$\begin{aligned} \text{H–H} &\\ \text{H–H} &+ \text{O=O} \rightarrow \begin{matrix} \text{H–O–H} \\ \text{H–O–H} \end{matrix} \end{aligned}$$

Some bond energies are shown.

Using the data in the table, what is the enthalpy change of reaction?

Options

A   –920 kJ / mol
B   –834 kJ / mol
C   –484 kJ / mol
D   +442 kJ / mol

Which statements about endothermic reactions are correct?

1. Energy is absorbed from the surroundings.
2. Energy is released to the surroundings.
3. The temperature of the reaction mixture falls.
4. The temperature of the reaction mixture rises.

Options

A   1 and 3
B   1 and 4
C   2 and 3
D   2 and 4

Hydrogen and chlorine react to form hydrogen chloride.

$$\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$$

Bond energy data is given in the table.

What is the enthalpy change, $\Delta H$, for this reaction?

Options

A   –247 kJ / mol
B   –184 kJ / mol
C   +184 kJ / mol
D   +247 kJ / mol

Which statement about exothermic and endothermic reactions is correct?

Options

A   In an endothermic reaction, energy is used to break bonds but no energy is released when bonds form.
B   In an endothermic reaction, energy is released when bonds form but more energy is used to break bonds.
C   In an exothermic reaction, energy is released both by breaking and by forming bonds.
D   In an exothermic reaction, energy is released when bonds form but no energy is needed to break bonds.