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Chapter 14: Problem 88

What is a heterogeneous chemical reaction? Where does a heterogeneous reaction occur?

Short Answer

Expert verified
A heterogeneous chemical reaction is a reaction that involves the interaction of two or more different phases, such as solid, liquid, or gas, and typically occurs at the interface between these phases. Examples include solid-liquid reactions like the dissolution of a salt in water, solid-gas reactions like the reaction between carbon and oxygen, liquid-gas reactions like the reaction between hydrogen and oxygen gases, and liquid-liquid reactions occurring between immiscible liquids. The interface between the different phases plays a crucial role in determining the rate of the heterogeneous reaction.

Step by step solution

01

Definition of a heterogeneous chemical reaction

A heterogeneous chemical reaction is a reaction that involves the interaction of two or more different phases. These phases can be solid, liquid, or gas. It typically occurs at the interface between the different phases, where the reactants are in direct contact with each other. The term "heterogeneous" refers to the fact that the reactants are not uniformly distributed throughout the reaction mixture.
02

Examples of Heterogeneous reactions

Heterogeneous reactions can occur in various situations, such as: 1. Solid-liquid reactions: When a solid reacts with a liquid, the reaction occurs at the surface of the solid where it is in contact with the liquid. Examples include the dissolution of a salt in water or the corrosion of metals in the presence of water. 2. Solid-gas reactions: Here, the reaction occurs at the surface of a solid exposed to a gas. An example of this is the reaction between carbon and oxygen to form carbon dioxide (CO2), which occurs at the surface of the solid carbon. 3. Liquid-gas reactions: These reactions involve the interaction between a liquid and a gas, such as the dissolution of a gas in a liquid or the reaction between a gas and a dissolved substance in a liquid. An example would be the reaction between hydrogen gas and oxygen gas, which occurs at the liquid-gas interface when the two gases are bubbled through water. 4. Liquid-liquid reactions: These reactions occur at the interface between two immiscible liquids. An example is the reaction between oil and water when a surfactant is present to facilitate the reaction at the liquid-liquid interface.
03

Importance of the interface in heterogeneous reactions

In heterogeneous chemical reactions, the interface between the different phases plays a crucial role in determining the rate of the reaction. This is because the reactants need to come into contact with one another for the reaction to take place. As a result, the rate of a heterogeneous reaction is often limited by the available interfacial area and the transport processes (such as diffusion) that bring the reactants into contact at the interface.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Chemical Reaction Phases
Understanding the phases involved in chemical reactions is crucial for grasping how heterogeneous reactions work. Phases refer to the distinct states of matter present鈥攕olid, liquid, and gas. In a heterogeneous chemical reaction, these phases are not uniformly mixed.
Instead, you find distinct areas where different phases come into contact, usually causing a reaction at these interfaces.
  • Solid Phase: Has a fixed shape and volume.
  • Liquid Phase: Has a fixed volume but takes the shape of its container.
  • Gas Phase: Takes both the shape and volume of its container.
These differences create varying environments where particles interact, which is the essence of heterogeneous reactions.
Interface in Reactions
The interface in a chemical reaction refers to the boundary where different phases meet and react. This boundary is a hotspot for reactions in heterogeneous systems. It is critical because it is where the actual reaction takes place as particles from different phases make contact.
In heterogeneous reactions, the properties of the interface, such as its size, surface area, and nature, affect the rate and efficiency of the reaction.
  • An increased interfacial area typically enhances reaction rates due to more opportunities for particle interaction.
  • Interfaces are also influenced by external factors like temperature and pressure, which can affect how well substances diffuse across the boundary.
Solid-Liquid Reactions
Solid-liquid reactions occur when a solid comes into contact with a liquid, and the reaction happens at the surface of the solid. The reactants need to be at this interface for the reaction to proceed.
Consider the example of salt dissolving in water. The process occurs at the salt's surface, where water molecules break down the salt crystals, leading to a solution. Key factors affecting these reactions include:
  • Surface area of the solid: More surface area means more exposure to the liquid, increasing reaction rate.
  • Temperature: Higher temperatures can increase the reaction rate by causing more kinetic energy in the molecules.
Solid-Gas Reactions
When solids interact with gases, the reaction often occurs at the solid's surface in contact with the gas. These reactions are common in industrial processes like combustion. For instance, when carbon reacts with oxygen to form carbon dioxide (CO鈧�), the process happens where the gas touches the solid carbon.
Factors impacting solid-gas reactions include:
  • Surface texture of the solid: Rough surfaces offer more interaction points.
  • Pressure of the gaseous reactants: Influences the concentration and the reaction rate.
  • Temperature: Warmer conditions can accelerate the reaction by increasing the energy of particles involved.
Liquid-Gas Reactions
Liquid-gas reactions happen at the interface where a gas and a liquid meet. An interesting case is when a gas dissolves in a liquid, such as when carbon dioxide bubbles through water to form part of carbonated beverages.
The dynamics of liquid-gas interactions can depend on several factors:
  • Solubility of the gas in the liquid: Higher solubility results in faster reaction rates.
  • Presence of catalysts or substances that can facilitate the interaction and bonding between gas and liquid molecules.
  • Agitation: Stirring or shaking can increase the rate by exposing more gas to the liquid.
Liquid-Liquid Reactions
Liquid-liquid reactions occur at the interface between two immiscible liquids. These interactions are often seen in processes involving oil and water. Surfactants often enhance these reactions by increasing the contact area between the two liquids.
Examining factors affecting liquid-liquid reactions, we find:
  • The properties of the individual liquids, including viscosity and concentration, substantially affect reaction rates.
  • The presence of surfactants which lower surface tension and allow the liquids to mix more readily, enabling more interaction.
  • Temperature adjustments, which can alter the solubility and reaction speed between the liquids.

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Most popular questions from this chapter

Write the definition of equilibrium in terms of the general rate laws Rate \(_{\text {forward rxn }}=k_{\text {forward rxn }}[\text { Reactants }]^{\text {order }}\) and Rate \(_{\text {reverse rxn }}=k_{\text {reverse rxn }}[\text { Products }]^{\text {order }}\)

At \(25^{\circ} \mathrm{C}\), the solubility of \(\mathrm{Al}(\mathrm{OH})_{3}\) in water is \(2.86 \times 10^{-9} \mathrm{M}\). What are the equilibrium concentrations of the cation and the anion in a saturated solution?

When the reaction \(\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \rightleftarrows 2 \mathrm{NO}(g)\) is run at \(2000^{\circ} \mathrm{C}\), appreciable amounts of reactants and product are present at equilibrium. (a) A sealed 2.00-L container at \(2000{ }^{\circ} \mathrm{C}\) is filled with \(1.00\) mole of \(\mathrm{NO}(g)\) and nothing else. At that moment, which reaction is faster, forward or reverse? Justify your answer. (b) At equilibrium, the concentration of \(\mathrm{NO}(g)\) is \(0.0683 \mathrm{M}\) and the concentration of \(\mathrm{N}_{2}(g)\) is \(0.2159 \mathrm{M}\). What is the value of \(K_{\mathrm{eq}}\) at \(2000^{\circ} \mathrm{C} ?\)

Consider the gas-phase reaction \(3 \mathrm{O}_{2}(g) \rightleftarrows 2 \mathrm{O}_{3}(g) .\) Suppose \(K_{\mathrm{eq}}\) for this reaction is \(\sim 1\) (it is not, but assume it is for this problem). Suppose you want pure ozone \(\left(\mathrm{O}_{3}\right)\) that is uncontaminated with oxygen \(\left(\mathrm{O}_{2}\right)\). (a) Why can't you simply remove the oxygen from the reaction vessel once the reaction has come to equilibrium to obtain pure ozone? (b) In fact, \(K_{\text {eq }}\) for this reaction at room temperature is \(2.5 \times 10^{-29}\). Knowing this, how important would you say Le Ch芒telier's principle is for this reaction when it comes to influencing the amount of ozone present at equilibrium? Explain.

(a) Write the equilibrium constant expression for the reaction $$ \mathrm{PbI}_{2}(s) \leftrightarrows \mathrm{Pb}^{2+}(a q)+2 \mathrm{I}^{-}(a q) $$ (b) How would the equilibrium be affected if \(\mathrm{PbI}_{2}(s)\) were added? (c) How would the equilibrium be affected if \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(s)\) were added? (Hint: Don't forget that \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) is a water-soluble salt.)
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