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Chapter 5: Problem 6

Define these terms: thermochemistry, exothermic process, endothermic process.

Short Answer

Expert verified
Thermochemistry studies heat changes in reactions; exothermic processes release heat, while endothermic processes absorb heat.

Step by step solution

01

Define Thermochemistry

Thermochemistry is the scientific study of the energy and heat associated with chemical reactions and/or physical transformations. It considers the exchange of energy, particularly in the form of heat, and how it influences matter during these changes.
02

Define Exothermic Process

An exothermic process is a type of chemical reaction or physical change that releases heat energy to its surroundings. In exothermic processes, the energy needed to start the reaction is less than the energy released, making the surroundings feel warmer.
03

Define Endothermic Process

An endothermic process is a chemical reaction or physical change wherein the system absorbs heat energy from its surroundings. In these processes, the energy required to initiate the reaction is greater than the energy released, leading to cooling of the surroundings.

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Key Concepts

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

Exothermic Process
In chemistry, an exothermic process is a type of chemical reaction or physical change that emits energy in the form of heat. This energy release usually results in the surroundings feeling warmer. One common example is the combustion of wood in a fireplace. Here鈥檚 how the process works:
  • During the reaction, bonds in the reactants break, releasing energy.
  • The formation of new bonds in the products releases even more energy.
  • Overall, the energy released is greater than the energy required to start the reaction.
An interesting aspect of exothermic processes is that they are self-sustaining after initiation, unlike endothermic processes. This means once started, they continue as long as there is reactant available.
Endothermic Process
An endothermic process is a chemical reaction or physical change that requires the absorption of heat from the surroundings. This usually makes the surroundings cooler. A typical example is the melting of ice cubes. Here is the basic mechanism behind it:
  • Energy is absorbed to break the bonds in the reactants, such as the hydrogen bonds in ice.
  • This energy absorption is crucial as it allows the phase change from solid to liquid.
  • In endothermic processes, the absorbed energy is greater than any energy released.
These processes are not self-sustaining. They need a continuous supply of energy from the environment to proceed. Hence, you might need to keep adding heat to the system to maintain the reaction.
Energy Exchange in Reactions
Energy exchange is a critical aspect of chemical reactions, greatly influencing reaction products and feasibility. The fundamental principle lies in the first law of thermodynamics, which states that energy cannot be created or destroyed, but can change forms. In chemical reactions, this involves the transformation between chemical energy and heat.
  • In exothermic reactions, energy is transferred from the reactants to the surroundings, typically as heat.
  • Conversely, in endothermic reactions, energy is transferred from the surroundings to the reactants.
  • The direction and mode of the energy exchange determine whether the reaction feels hot or cool to the touch.
Understanding energy exchange helps predict the spontaneity of reactions. It can indicate how much energy is needed to start a reaction or how much energy will be released. This is especially relevant in applications such as energy-efficient chemical processes and controlling environmental conditions in industries.

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Most popular questions from this chapter

You are given the following data: \(\begin{aligned} \mathrm{H}_{2}(g) & \longrightarrow 2 \mathrm{H}(g) & & \Delta H^{\circ}=436.4 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{Br}_{2}(g) & \longrightarrow 2 \mathrm{Br}(g) & & \Delta H^{\circ}=192.5 \mathrm{~kJ} / \mathrm{mol} \\\ \mathrm{H}_{2}(g)+\mathrm{Br}_{2}(g) & \longrightarrow 2 \mathrm{HBr}(g) & \Delta H^{\circ} &=-72.4 \mathrm{~kJ} / \mathrm{mol} \end{aligned}\) Calculate \(\Delta H^{\circ}\) for the reaction\(\mathrm{H}(g)+\mathrm{Br}(g) \longrightarrow \mathrm{HBr}(g)\).

For reactions in condensed phases (liquids and solids), the difference between \(\Delta H\) and \(\Delta U\) is usually quite small. This statement holds for reactions carried out under atmospheric conditions. For certain geochemical processes, however, the external pressure may be so great that \(\Delta H\) and \(\Delta U\) can differ by a significant amount. A well-known example is the slow conversion of graphite to diamond under Earth's surface. Calculate \(\Delta H-\Delta U\) for the conversion of 1 mole of graphite to 1 mole of diamond at a pressure of 50,000 atm. The densities of graphite and diamond are \(2.25 \mathrm{~g} / \mathrm{cm}^{3}\) and \(3.52 \mathrm{~g} / \mathrm{cm}^{3},\) respectively.

The combustion of \(0.4196 \mathrm{~g}\) of a hydrocarbon releases \(17.55 \mathrm{~kJ}\) of heat. The masses of the products are \(\mathrm{CO}_{2}=1.419 \mathrm{~g}\) and \(\mathrm{H}_{2} \mathrm{O}=0.290 \mathrm{~g}\). (a) What is the empirical formula of the compound? (b) If the approximate molar mass of the compound is \(76 \mathrm{~g} / \mathrm{mol}\), calculate its standard enthalpy of formation.

Consider the following data:$$\begin{array}{lcc}\text { Metal } & \text { Al } & \text { Cu } \\\\\hline \text { Mass }(\mathrm{g}) & 10 & 30 \\\\\text { Specific heat }\left(\mathrm{J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right) & 0.900 & 0.385 \\\\\text { Temnerature }{ }^{\circ}{ }^{\circ} \mathrm{C} \text { ) } & 40 & 60\end{array}$$ When these two metals are placed in contact, which of the following will take place? (a) Heat will flow from \(\mathrm{Al}\) to Cu because \(\mathrm{Al}\) has a larger specific heat. (b) Heat will flow from \(\mathrm{Cu}\) to \(\mathrm{Al}\) because \(\mathrm{Cu}\) has a larger mass. (c) Heat will flow from \(\mathrm{Cu}\) to \(\mathrm{Al}\) because \(\mathrm{Cu}\) has a larger heat capacity (d) Heat will flow from Cu to Al because Cu is at a higher temperature. (e) No heat will flow in either direction.

Given the thermochemical data,$$\begin{array}{ll}\mathrm{A}+6 \mathrm{~B} \longrightarrow 4 \mathrm{C} & \Delta H_{1}=-1200 \mathrm{~kJ} / \mathrm{mol} \\\\\mathrm{C}+\mathrm{B} \longrightarrow \mathrm{D} & \Delta H_{1}=-150 \mathrm{~kJ} / \mathrm{mol}\end{array}$$ Determine the enthalpy change for each of the following: a) \(\mathrm{D} \longrightarrow \mathrm{C}+\mathrm{B}\) d) \(2 \mathrm{D} \longrightarrow 2 \mathrm{C}+2 \mathrm{~B}\) b) \(2 \mathrm{C} \longrightarrow \frac{1}{2} \mathrm{~A}+3 \mathrm{~B}\) e) \(6 \mathrm{D}+\mathrm{A} \longrightarrow 10 \mathrm{C}\) c) \(3 \mathrm{D}+\frac{1}{2} \mathrm{~A} \stackrel{\longrightarrow}{\longrightarrow} \mathrm{C}\)
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