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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

Consider the following data: $$ \begin{array}{lcc} \text { Metal } & \text { Al } & \text { Cu } \\ \hline \text { Mass (g) } & 10 & 30 \\ \text { Specific heat }\left(\mathrm{J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right) & 0.900 & 0.385 \\ \text { Temperature }\left({ }^{\circ} \mathrm{C}\right) & 40 & 60 \end{array} $$ When these two metals are placed in contact, which of the following will take place? (a) Heat will flow from Al to Cu because Al has a larger specific heat. (b) Heat will flow from Cu to Al because Cu has a larger mass. (c) Heat will flow from Cu to Al because 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.

Determine the amount of heat (in kJ) given off when \(1.26 \times 10^{4} \mathrm{~g}\) of \(\mathrm{NO}_{2}\) are produced according to the equation $$ \begin{aligned} 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow & 2 \mathrm{NO}_{2}(g) \\ \Delta H &=-114.6 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$

A 0.1375 -g sample of solid magnesium is burned in a constant-volume bomb calorimeter that has a heat capacity of \(3024 \mathrm{~J} /{ }^{\circ} \mathrm{C}\). The temperature increases by \(1.126^{\circ} \mathrm{C}\). Calculate the heat given off by the burning \(\mathrm{Mg}\), in \(\mathrm{kJ} / \mathrm{g}\) and in \(\mathrm{kJ} / \mathrm{mol}\).

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

The internal energy of an ideal gas depends only on its temperature. Do a first-law analysis of this process. A sample of an ideal gas is allowed to expand at constant temperature against atmospheric pressure. (a) Does the gas do work on its surroundings? (b) Is there heat exchange between the system and the surroundings? If so, in which direction? (c) What is \(\Delta E\) for the gas for this process?
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