Problem 4 - Chemistry and Chemical Reactivity

Question

Identify whether the following processes are exothermic or endothermic. Is the sign on \(q_{\mathrm{sys}}\) positive or negative? (a) the reaction of \(\mathrm{Na}(\mathrm{s})\) and \(\mathrm{Cl}_{2}(\mathrm{g})\) (b) cooling and condensing gaseous \(\mathrm{N}_{2}\) to form liquid \(\mathrm{N}_{2}\) (c) cooling a soft drink from \(25^{\circ} \mathrm{C}\) to \(0^{\circ} \mathrm{C}\) (d) heating \(\mathrm{HgO}\) (s) to form \(\mathrm{Hg}(\ell)\) and \(\mathrm{O}_{2}(\mathrm{g})\)

Step-by-Step Solution

Answer

(a) Exothermic, \( q_{\mathrm{sys}} < 0 \); (b) Exothermic, \( q_{\mathrm{sys}} < 0 \); (c) Exothermic, \( q_{\mathrm{sys}} < 0 \); (d) Endothermic, \( q_{\mathrm{sys}} > 0 \).

Step 1: Understand Exothermic and Endothermic Processes

Exothermic processes release heat into the surroundings, so the sign of \(q_{\mathrm{sys}}\) is negative. Endothermic processes absorb heat from the surroundings, so the sign of \(q_{\mathrm{sys}}\) is positive.

Step 2: Analyze Process (a)

The formation of \( \text{NaCl} \) from \( \text{Na(s)} \) and \( \text{Cl}_2(\text{g}) \) releases energy as bonds are formed, making it an exothermic reaction. Thus, \( q_{\mathrm{sys}} \) for this process is negative.

Step 3: Analyze Process (b)

Cooling and condensing \( \text{N}_2 \) from gas to liquid releases heat energy, as energy must be removed for condensation. This makes it an exothermic process with \( q_{\mathrm{sys}} \) being negative.

Step 4: Analyze Process (c)

Cooling a soft drink from \( 25^{\circ} \text{C} \) to \( 0^{\circ} \text{C} \) involves heat being removed from the drink, so the surroundings gain this heat. This describes an exothermic process, thus \( q_{\mathrm{sys}} \) is negative.

Step 5: Analyze Process (d)

The decomposition of \( \text{HgO} \) to form \( \text{Hg}(\ell) \) and \( \text{O}_2(\text{g}) \) requires energy input to break the bonds in \( \text{HgO} \), making it an endothermic process. Therefore, \( q_{\mathrm{sys}} \) is positive.

Key Concepts

Exothermic Process

An exothermic process is one that releases heat into its surroundings. This is commonly observed in reactions where bonds are being formed, as these processes often release energy. Think of a bonfire, where the burning wood releases heat that you can feel. Another example is the formation of NaCl from Na and Cl2, a strongly exothermic reaction. In exothermic processes, the system loses heat, making the sign of heat transfer, represented as \( q_{\mathrm{sys}} \), negative. This means that the system's thermal energy decreases, transferring energy to its surroundings.

• Releases heat
• System's energy decreases
• \( q_{\mathrm{sys}} \) is negative

This concept is crucial in processes like combustion, certain chemical reactions, and phase changes where heat is emitted.

Endothermic Process

Endothermic processes absorb heat from their surroundings. Instead of releasing energy, these processes require energy input, often to break chemical bonds. Think of photosynthesis, where plants absorb sunlight to convert CO2 and water into glucose and oxygen. The decomposition of HgO to Hg and O2 is another example of an endothermic process. Here, energy is absorbed to break the bonds in HgO. In these processes, the system gains heat, so the sign of \( q_{\mathrm{sys}} \) is positive. This means the system's thermal energy increases, drawing energy from its surroundings.

• Absorbs heat
• System's energy increases
• \( q_{\mathrm{sys}} \) is positive

This understanding is key in processes like melting ice, boiling water, and other changes where heat must be absorbed.

Heat Transfer

Heat transfer is the movement of thermal energy from one body or system to another due to a temperature difference. It occurs in every chemical reaction and physical process. Heat always flows from a warmer object to a cooler one until equilibrium is reached. In an exothermic process, the system transfers heat to the surroundings. For instance, when a soft drink cools from 25°C to 0°C, the drink loses heat to the environment, illustrating heat transfer out of the system.

• Occurs due to temperature difference
• Moves from warm to cool
• Critical in understanding energy changes

Recognizing how heat is transferred helps in predicting the direction and magnitude of energy changes during chemical and physical processes.

Chemical Reactions

Chemical reactions are processes that lead to the transformation of one set of chemical substances to another. These reactions are accompanied by changes in energy, typically involving either the release or absorption of heat. They can be broadly classified based on energy changes as exothermic or endothermic. For example, the reaction of Na(s) with Cl2(g) to form NaCl releases energy, making it exothermic.

• Transform matter
• Involve energy changes
• Can release or absorb heat

Understanding these reactions is essential as they are the basis of energy changes observed in many natural and industrial processes.