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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 as energy is required to break the bonds in the molecule. Combination reactions are generally exothermic because energy is released when new bonds are formed.

Step by step solution

01

Understand Endothermic and Exothermic Reactions

First, the definitions of these two types of reactions need to be recognized. An endothermic reaction is one in which heat is absorbed from the surroundings, usually making the surroundings colder. An exothermic reaction, on the other hand, is one in which heat is released into the surroundings, usually making them warmer.
02

Apply Concepts to Decomposition and Combination Reactions

Decomposition reactions involve the breakdown of a single entity (normal molecule, radical, ions, etc.) into two or more fragments. This process generally requires energy to break the bonds in the molecule, so it's usually endothermic. On the other hand, combination reactions involve two or more reactants combining to form a product. Since energy is often released when bonds are formed, it's usually exothermic.
03

Provide Qualitative Explanation

Considering the energy changes involved in bond breaking and formation provides a qualitative explanation as follows: Decomposition reactions tend to be endothermic as energy is required to break the existing bonds in the molecule, while combination reactions are generally exothermic because energy is released during the formation of new bonds.

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Most popular questions from this chapter

Describe how chemists use Hess's law to determine the \(\Delta H_{\mathrm{f}}^{\circ}\) of a compound by measuring its heat (enthalpy of combustion.

What is meant by the standard-state condition?

lime is a term that includes calcium oxide (CaO, also called quicklime) and calcium hydroxide \(\left[\mathrm{Ca}(\mathrm{OH})_{2},\right.\) also called slaked lime \(] .\) It is used in the steel industry to remove acidic impurities, in airpollution control to remove acidic oxides such as \(\mathrm{SO}_{2}\), and in water treatment. Quicklime is made industrially by heating limestone \(\left(\mathrm{CaCO}_{3}\right)\) above \(2000^{\circ} \mathrm{C}:\) $$ \begin{array}{r} \mathrm{CaCO}_{3}(s) \longrightarrow \mathrm{CaO}(s)+\mathrm{CO}_{2}(g) \\ \Delta H^{\circ}=177.8 \mathrm{~kJ} / \mathrm{mol} \end{array} $$ Slaked lime is produced by treating quicklime with water: $$ \begin{array}{r} \mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s) \\ \Delta H^{\circ}=-65.2 \mathrm{~kJ} / \mathrm{mol} \end{array} $$ The exothermic reaction of quicklime with water and the rather small specific heats of both quicklime \(\left(0.946 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\) and slaked lime \(\left(1.20 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\) make it hazardous to store and transport lime in vessels made of wood. Wooden sailing ships carrying lime would occasionally catch fire when water leaked into the hold. (a) If a \(500-\mathrm{g}\) sample of water reacts with an equimolar amount of \(\mathrm{CaO}\) (both at an initial temperature of \(25^{\circ} \mathrm{C}\) ), what is the final temperature of the product, \(\mathrm{Ca}(\mathrm{OH})_{2} ?\) Assume that the product absorbs all of the heat released in the reaction. (b) Given that the standard enthalpies of formation of \(\mathrm{CaO}\) and \(\mathrm{H}_{2} \mathrm{O}\) are \(-635.6 \mathrm{~kJ} / \mathrm{mol}\) and \(-285.8 \mathrm{~kJ} / \mathrm{mol}\) respectively, calculate the standard enthalpy of formation of \(\mathrm{Ca}(\mathrm{OH})_{2}\)

State Hess's law. Explain, with one example, the usefulness of this law in thermochemistry.

Which of the following standard enthalpy of formation values is not zero at \(25^{\circ} \mathrm{C} ? \mathrm{Na}(s), \operatorname{Ne}(g)\) \(\mathrm{CH}_{4}(g), \mathrm{S}_{8}(s), \mathrm{Hg}(l), \mathrm{H}(g)\)
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