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General Chemistry Principles, Patterns, and Applications, 2011

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present in their st<strong>and</strong>ard states (their most stable forms at 1 atm pressure <strong>and</strong> the temperature of the<br />

measurement). The st<strong>and</strong>ard heat of formation of any element in its most stable form is defined to be<br />

zero. The st<strong>and</strong>ard enthalpy of reaction (can be calculated from the sum of the st<strong>and</strong>ard<br />

enthalpies of formation of the products (each multiplied by its stoichiometric coefficient) minus the<br />

sum of the st<strong>and</strong>ard enthalpies of formation of the reactants (each multiplied by its stoichiometric<br />

coefficient)—the “products minus reactants” rule. The enthalpy of solution (ΔHsoln) is the heat<br />

released or absorbed when a specified amount of a solute dissolves in a certain quantity of solvent at<br />

constant pressure.<br />

K E Y T A K E A W A Y<br />

<br />

Enthalpy is a state function whose change indicates the amount of heat transferred from a system to its<br />

surroundings or vice versa, at constant pressure.<br />

C O N C E PTUAL P R OBLEMS<br />

Please be sure you are familiar with the topics discussed in Essential Skills 4 () before proceeding to the<br />

Conceptual Problems.<br />

1. Heat implies the flow of energy from one object to another. Describe the energy flow in an<br />

a. exothermic reaction.<br />

b. endothermic reaction.<br />

2. Based on the following energy diagram,<br />

a. write an equation showing how the value of ΔH 2 could be determined if the values of ΔH 1 <strong>and</strong> ΔH 3 are<br />

known.<br />

b. identify each step as being exothermic or endothermic.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

447

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