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Calculate the standard enthalpy of formation of $\mathrm{SO}_{2}(g)$ from the standard enthalpy changes of the following reactions:$$\begin{aligned}2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{SO}_{3}(g) & & \Delta H_{\mathrm{rxn}}^{\circ}=-196 \mathrm{kJ} \\\frac{1}{4} \mathrm{S}_{8}(s)+3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{SO}_{3}(g) & & \Delta H_{\operatorname{man}}^{\circ}=-790 \mathrm{kJ} \\\frac{1}{8} \mathrm{S}_{8}(s)+\mathrm{O}_{2}(g) \rightarrow \mathrm{SO}_{2}(g) & \Delta H_{5}^{\circ} &=?\end{aligned}$$

Chemistry 101

Chapter 5

Thermochemistry: Energy Changes in Reactions

Thermochemistry

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Lectures

05:27

In chemistry, a chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Both reactants and products are involved in the chemical reactions.

06:42

In chemistry, energy is what is required to bring about a chemical reaction. The total energy of a system is the sum of the potential energy of its constituent particles and the kinetic energy of these particles. Chemical energy, also called bond energy, is the potential energy stored in the chemical bonds of a substance. Chemical energy is released when a bond is broken during chemical reactions.

03:21

Calculate the standard mol…

04:41

02:02

Use the following themoche…

02:26

Calculate the standard ent…

01:54

Use Hess's law and th…

02:09

Given that$$\begin…

02:10

Given that$\mathrm{S}(…

01:13

The standard enthalpy chan…

were given to reactions on were asked to solve for the Delta h of the other reaction for this is just a traditional Hess's law calculation. The two reactions that are given to us are two moles of S 02 plus two going to two moles of eso three Delta H Negative 1 96 a fourth of a mole of s eight solid plus three moles of +02 going to two moles of eso three Delta H is negative 7 90 killer jewels. If we reverse the first reaction, multiply it by 1/2 which is what is necessary in order for us to get eso to product and in the target reaction we need s 02 as a product. That's why we need to reverse it within divided by two. Because we only need one bowl of eso to and we're starting out with two moles of eso to so we'll reverse it, multiply it by 1/2 and get this chemical reaction with the Delta h of positive 1 96 divided by two. Then for the second reaction, we're going to need to multiply it by 1/2 because we need to have 18 of s eight on the react inside. Where we do that, the Delta h will then be negative. 7 90 Then we can sum up these reactions and the eso threes will cancel. We had three halfs of 02 here and 1/2 of 02 here, so 1/2 will cancel from both of them. Remember, because this reaction reversed and what we left with just one mole of 02 So we get our 1/8 mole s eight and one mole eso to reacting to get so I want eight ball s eight and one mole. Oh, to react to get one mole of eso to to sum up our modified don't each values and we get negative to 97.

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