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Chapter 6: Problem 44

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.

Short Answer

Expert verified
Hess’s Law is used to calculate the enthalpy of formation (ΔHf) of a compound, by adding up the enthalpies of the formation of products and subtracting the enthalpies of formation of reactants. The heat of combustion for a compound is measured using a calorimeter. This is then used to calculate ΔHf values, as the heat of combustion is essentially the negative of the ΔHf when the compound is formed from its elements in their standard states.

Step by step solution

01

Understanding Hess’s Law

Hess's Law, also known as the Law of Constant Heat Summation, states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in many steps. This is due to the state function of enthalpy, which depends only on the initial and final states and not on the pathway taken.
02

Applying Hess's Law to Calculate ΔHf

When calculating the enthalpy of formation (ΔHf), chemists use Hess's law. This is done by adding the enthalpies of formation for each product and subtracting the enthalpies of formation for each reactant. In mathematical terms: ΔHf = Σ (ΔHf products) - Σ (ΔHf reactants).
03

Measuring Heat of Combustion

The heat of combustion for a compound is measured using a calorimeter. The calorimeter captures and measures the heat produced when the compound is burned completely. This is a more practical way of determining ΔHf, as measuring the enthalpy change of formation directly is often more challenging.
04

Using Heat of Combustion in Hess's Law

Once the heat of combustion is measured, we can use it to calculate ΔHf values. This is because the heat of combustion is the negative of the enthalpy of formation when the compound is formed from its constituent elements in their standard states. So, by using Hess’s Law, we can manipulate other known reactions, such as combustion reactions, to obtain ΔHf.

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Most popular questions from this chapter

The \(\Delta H_{\mathrm{f}}^{\circ}\) values of the two allotropes of oxygen, \(\mathrm{O}_{2}\) and \(\mathrm{O}_{3}\), are 0 and \(142.2 \mathrm{~kJ} / \mathrm{mol}\), respectively, at \(25^{\circ} \mathrm{C}\). Which is the more stable form at this temperature?

Explain the meaning of this thermochemical equation: $$\begin{array}{r}4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g) \\\\\Delta H=-904 \mathrm{~kJ} / \mathrm{mol}\end{array}$$

Calculate the standard enthalpy of formation for diamond, given that $$\begin{array}{l} \text { C(graphite) }+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) \\ \qquad \begin{aligned} \Delta H^{\circ} &=-393.5 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{C}(\text { diamond })+\mathrm{O}_{2}(g) \longrightarrow & \mathrm{CO}_{2}(g) \\ \Delta H^{\circ} &=-395.4 \mathrm{~kJ} / \mathrm{mol} \end{aligned} \end{array}$$

What is heat? How does heat differ from thermal energy? Under what condition is heat transferred from one system to another?

After a dinner party, the host performed the following trick. First, he blew out one of the burning candles. He then quickly brought a lighted match to about 1 in above the wick. To everyone's surprise, the candle was relighted. Explain how the host was able to accomplish the task without touching the wick.
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