Problem 5 - Chemistry: Principles and Reactions

Question

On the basis of your experience, predict which of the following reactions are spontaneous. (a) \(\mathrm{Zn}(s)+2 \mathrm{H}^{+}(a q) \longrightarrow \mathrm{Zn}^{2+}(a q)+\mathrm{H}_{2}(g)\) (b) \(\mathrm{CaCO}_{3}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s)+\mathrm{H}_{2} \mathrm{CO}_{3}(a q)\) (c) \(\mathrm{CH}_{4}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)\) (d) \(\mathrm{Ag}^{+}(a q)+\mathrm{Cl}^{-}(a q) \longrightarrow \mathrm{AgCl}(s)\)

Step-by-Step Solution

Answer

a) Zn + 2H+ -> Zn2+ + H2 b) CaCO3 + 2H2O -> Ca(OH)2 + H2CO3 c) CH4 + O2 -> CO2 + 2H2O d) Ag+ + Cl- -> AgCl Answer: Reactions (a), (c), and (d) are spontaneous, while reaction (b) is not.

Step 1: Recognize the type of reaction

This is a redox reaction, where one species is oxidized (loses electrons) while the other is reduced (gains electrons). In this case, Zn(s) is oxidized to Zn2+(aq) and H+(aq) is reduced to H2(g).

Step 2: Analyze standard reduction potentials

We can analyze this redox reaction by comparing the standard reduction potentials of the half-reactions: Zn2+(aq) + 2e- -> Zn(s) [E° = -0.76 V] 2H+(aq) + 2e- -> H2(g) [E° = 0 V] Since the reduction potential of Zn2+ is lower than that of the H+, Zn(s) will be more likely to be oxidized, and H+ is more likely to be reduced.

Step 3: Predict spontaneity

As we just pointed out, Zn(s) is more likely to be oxidized, and H+ is more likely to be reduced. Thus, this reaction is spontaneous. #b) Reaction 2: CaCO3 + 2H2O -> Ca(OH)2 + H2CO3#

Step 1: Recognize the type of reaction

This is an acid-base reaction, where water is acting as a base and is reacting with the acidic component CaCO3.

Step 2: Analyze reaction based on basic chemical reactivity

Because this reaction involves CaCO3(s) dissolving in water, this suggests we may be dealing with solubility rather than spontaneity. We know that calcium carbonates are less soluble in water, so the reaction will not proceed to a large extent.

Step 3: Predict spontaneity

Since CaCO3 is expected to be relatively insoluble in water, this reaction will not be spontaneous. #c) Reaction 3: CH4 + O2 -> CO2 + 2H2O#

Step 1: Recognize the type of reaction

This is a combustion reaction, where methane (CH4) reacts with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).

Step 2: Analyze reaction based on basic chemical reactivity

The key is recognizing that combustion reactions release a large amount of energy, mainly due to the formation of strong O-H and C-O bonds in water and carbon dioxide.

Step 3: Predict spontaneity

Since combustion reactions release energy and the products formed show higher stability, we can confidently predict that this reaction is spontaneous. #d) Reaction 4: Ag+ + Cl- -> AgCl#

Step 1: Recognize the type of reaction

This is a precipitation reaction, where two ions in solution react to form an insoluble solid product.

Step 2: Analyze the formation of a precipitate

Precipitation reactions are dependent on the solubility product of the formed solid. In this case, silver chloride (AgCl) is known to have a low solubility product, making it insoluble in water.

Step 3: Predict spontaneity

Since AgCl is insoluble in water and will form a precipitate, this reaction is spontaneous. In conclusion, reactions (a), (c), and (d) are spontaneous, while reaction (b) is not.

Key Concepts

Redox Reactions

Redox reactions are a fascinating area of chemistry where you see the transfer of electrons take center stage. In these reactions, one species loses electrons, also known as oxidation, while another species gains those electrons, which is called reduction. For a clearer understanding, let's break down a common example from our exercise.
The reaction involves zinc metal ( Zn(s) ) being oxidized to zinc ions ( Zn^{2+}(aq) ), and hydrogen ions ( H^+(aq) ) being reduced to hydrogen gas ( H_2(g) ). This reaction not only represents the dual nature of electron transfer but can also be predicted to be spontaneous based on standard reduction potentials.

• Use of the oxidation state: The species that increases its oxidation state is said to be oxidized.
• Electronic flow: Electrons move from the oxidized species to the reduced one, driving the reaction forward.

Standard reduction potentials help in predicting the spontaneity of a reaction; where each half-reaction has an associated voltage. A more positive voltage signifies a higher tendency to gain electrons. Evaluating these potentials like dieting for electrons determines which direction the redox reaction will proceed.
Remember, the spontaneity can often depend on whether the overall cell potential (the net voltage) is positive.

Combustion Reactions

Combustion reactions are energy-releasing experiences where a substance combines with oxygen to produce heat and light, usually in the form of fire. These are some of the most recognizable chemical reactions and are essential in everyday life, from heating a home to powering a car.
In our exercise, methane ( CH_4(g) ) is combusted in the presence of oxygen ( O_2(g) ), producing carbon dioxide ( CO_2(g) ) and water ( H_2O(g) ). The distinguishing feature of combustion reactions is their exothermic nature, meaning they release energy.

• Heat and light release: Combustion typically results in visible flames and the release of heat, which manifests as light and warmth.
• Exothermic stability: The reaction forms products that are lower in energy than the reactants, resulting in a release of energy.

Combustion reactions tend to be spontaneous because they favor chemical states with lower potential energy and higher stability, aligning with the universal trend where nature favors the establishment of stable, lower-energy configurations.

Precipitation Reactions

Precipitation reactions are a classic example of distinct chemical processes where soluble reactants in solution form an insoluble product, known as a precipitate. This transformation is especially useful in qualitative analysis to identify the presence of certain ions in a solution.
In the case from the exercise, silver ions ( Ag^+(aq) ) react with chloride ions ( Cl^-(aq) ) to form solid silver chloride ( AgCl(s) ). The process results in the "falling out" of a solid from the solution, hence the term precipitation.

• Solubility principles: The formation of a precipitate can be predicted using rules of solubility, where certain ionic products have low solubility in water.
• Spontaneity based on solubility: The extremely low solubility of silver chloride drives the reaction towards forming the solid, making it spontaneous.

These reactions are spontaneous because they reach a state that forms a stable product (the precipitate) where no further dissolution into the liquid state is favorable. These processes help understand purification and separation in chemical processes, laboratories, and industries.

Acid-Base Reactions

Acid-base reactions showcase the critical interaction between substances that modify the concentration of hydrogen ions within a solution. These reactions form the backbone of various natural and industrial processes.
In one example from the exercise, calcium carbonate ( CaCO_3(s) ) and water ( H_2O(l) ) participate, where water acts like a base. However, this reaction is deemed non-spontaneous based on the insolubility of CaCO_3.

• Proton transfer: Acid-base reaction typically involves transfer reactions where protons ( H^+ ) are exchanged.
• Acid as a proton donor: An acid is a substance that donates protons, whereas a base accepts them.

The spontaneity of these reactions largely depends on the presence and interaction strength of acids and bases involved. The natural tendency for acids and bases to stabilize through neutralization plays a key role, but this is influenced by their solubility and the reaction environment.