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Chapter 5: Problem 67

(a) What is meant by the term standard conditions with reference to enthalpy changes? (b) What is meant by the term enthalpy of formation? (c) What is meant by the term standard enthalpy of formation?

Short Answer

Expert verified
(a) Standard conditions for enthalpy changes refer to a set of physical conditions (pressure of 100 kPa and temperature of 298.15 K) under which thermodynamic quantities are measured, ensuring consistent comparisons between experiments. (b) Enthalpy of formation (ΔHf) is the heat absorbed or released during the formation of one mole of a species from its constituent elements in their most stable states at a specific temperature and pressure. (c) Standard enthalpy of formation (ΔHf°) is the enthalpy change when one mole of a compound is formed from its constituent elements in their most stable states at standard conditions (100 kPa and 298.15 K). It helps predict reaction energetics and determine the direction of a reaction.

Step by step solution

01

(a) Define standard conditions

The standard conditions, in the context of enthalpy changes, refer to a set of physical conditions under which thermodynamic quantities are measured or experimentally determined. It ensures that results from different experiments can be consistently compared. The standard conditions typically include a pressure of 100 kPa (1 atm) and a temperature of 298.15 K (25°C).
02

(b) Define enthalpy of formation

The enthalpy of formation, written as ΔHf, is the amount of heat absorbed or released during the formation of a single species (one mole) from its constituent elements in their most stable states at a specified temperature and pressure. The enthalpy of formation is a measure of the stability of the compound and has a direct impact on the spontaneity of a chemical reaction.
03

(c) Define the standard enthalpy of formation

The standard enthalpy of formation, written as ΔHf°, is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their most stable states at standard conditions (a pressure of 100 kPa and a temperature of 298.15 K). This value helps in comparing the relative enthalpy changes during the formation of different compounds under the standard conditions, enabling chemists to predict reaction energetics and determine the direction in which a reaction is more likely to proceed.

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Most popular questions from this chapter

A sample of a hydrocarbon is combusted completely in \(\mathrm{O}_{2}(g)\) to produce \(21.83 \mathrm{g} \mathrm{CO}_{2}(g), 4.47 \mathrm{g} \mathrm{H}_{2} \mathrm{O}(g),\) and 311 \(\mathrm{kJ}\) of heat. (a) What is the mass of the hydrocarbon sample that was combusted? (b) What is the empirical formula of the hydrocarbon? (c) Calculate the value of \(\Delta H_{f}^{\circ}\) per empirical-formula unit of the hydrocarbon. (d) Do you think that the hydrocarbon is one of those listed in Appendix C? Explain your answer.

The Sun supplies about 1.0 kilowatt of energy for each square meter of surface area \(\left(1.0 \mathrm{kW} / \mathrm{m}^{2},\) where a watt \(=1 \mathrm{J} / \mathrm{s}\right)\) Plants produce the equivalent of about 0.20 \(\mathrm{g}\) of sucrose \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\) per hour per square meter. Assuming that the sucrose is produced as follows, calculate the percentage of sunlight used to produce sucrose. $$\begin{array}{c}{12 \mathrm{CO}_{2}(g)+11 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+12 \mathrm{O}_{2}(g)} \\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad {\Delta H=5645 \mathrm{kJ}}\end{array}$$

(a) Which releases the most energy when metabolized, 1 \(\mathrm{g}\) of carbohydrates or 1 \(\mathrm{g}\) of fat? (b) A particular chip snack food is composed of 12\(\%\) protein, 14\(\%\) fat, and the rest carbohydrate. What percentage of the calorie content of this food is fat? (c) How many grams of protein provide the same fuel value as 25 of fat?

A \(201-\) lb man decides to add to his exercise routine by walking up three flights of stairs \((45 \mathrm{ft}) 20\) times per day. He figures that the work required to increase his potential energy in this way will permit him to eat an extra order of French fries, at 245 Cal, without adding to his weight. Is he correct in this assumption?

A 2.200 -g sample of quinone \(\left(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{O}_{2}\right)\) is burned in a bomb calorimeter whose total heat capacity is 7.854 \(\mathrm{kJ} / \mathrm{c}\) . The temperature of the calorimeter increases from 23.44 to \(30.57^{\circ} \mathrm{C}\) . What is the heat of combustion per gram of quinone? Per mole of quinone?
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