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Chapter 6: Problem 10

Decomposition reactions are usually endothermic, whereas combination reactions are usually exothermic. Give a qualitative explanation for these trends.

Short Answer

Expert verified
Decomposition reactions are usually endothermic because they involve breaking chemical bonds, which requires energy absorption. Conversely, combination reactions are usually exothermic because they involve the formation of chemical bonds, which releases energy.

Step by step solution

01

Defining the types of reactions

Start by defining what is meant by a decomposition reaction and a combination reaction. A decomposition reaction is a type of chemical reaction where one compound breaks down into two or more simpler substances. On the other hand, a combination reaction is a type of chemical reaction where two or more substances combine to form a single complex product.
02

Defining endothermic and exothermic reactions

Next, you must understand the difference between endothermic and exothermic reactions. In an endothermic reaction, heat is absorbed from the surroundings into the system, resulting in a decrease in temperature. In an exothermic reaction, heat is released from the system into the surroundings, leading to an increase in temperature.
03

Relating reaction types to heat changes

Now the key is to connect these definitions together. In a decomposition reaction, a compound is breaking apart and this tends to require an input of energy, hence they are often endothermic. Conversely, in a combination reaction, simpler substances are coming together to form a more complex substance, releasing energy in the process, hence they are often exothermic.

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Most popular questions from this chapter

Calculate the standard enthalpy change for the reaction $$ 2 \mathrm{Al}(s)+\mathrm{Fe}_{2} \mathrm{O}_{3}(s) \longrightarrow 2 \mathrm{Fe}(s)+\mathrm{Al}_{2} \mathrm{O}_{3}(s) $$ given that $$ \begin{aligned} 2 \mathrm{Al}(s)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow & \mathrm{Al}_{2} \mathrm{O}_{3}(s) \\ \Delta H_{\mathrm{rxn}}^{\circ} &=-1669.8 \mathrm{~kJ} / \mathrm{mol} \\ 2 \mathrm{Fe}(s)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow & \mathrm{Fe}_{2} \mathrm{O}_{3}(s) \\ \Delta H_{\mathrm{rxn}}^{\circ} &=-822.2 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$

Explain what is meant by a state function. Give two examples of quantities that are state functions and two that are not.

How are the standard enthalpies of formation of an element and of a compound determined?

Portable hot packs are available for skiers and people engaged in other outdoor activities in a cold climate. The air-permeable paper packet contains a mixture of powdered iron, sodium chloride, and other components, all moistened by a little water. The exothermic reaction that produces the heat is a very common one- the rusting of iron: $$ 4 \mathrm{Fe}(s)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{Fe}_{2} \mathrm{O}_{3}(s) $$ When the outside plastic envelope is removed, \(\mathrm{O}_{2}\) molecules penetrate the paper, causing the reaction to begin. A typical packet contains \(250 \mathrm{~g}\) of iron to warm your hands or feet for up to \(4 \mathrm{~h}\). How much heat (in \(\mathrm{kJ}\) ) is produced by this reaction? (Hint: See Appendix 2 for \(\Delta H_{\mathrm{f}}^{\circ}\) values.

Suggest ways (with appropriate equations) that would enable you to measure the \(\Delta H_{\mathrm{f}}^{\circ}\) values of \(\mathrm{Ag}_{2} \mathrm{O}(s)\) and \(\mathrm{CaCl}_{2}(s)\) from their elements. No calculations are necessary.
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