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4.03 – Hess’s Law

One of the keys to thermodynamics is understanding state functions.  The practical application is seeing that if you know information about the initial and final state of a system, you can determine the change.  Sometimes, it is too complicated to directly measure that change, but we can measure smaller steps in the process and combine them to determine the overall change.  This is Hess’s Law.

Lesson Objectives

  • Use Hess’s law to determine enthalpy changes for reactions. (5.6)
  • Use standard enthalpies of formation to calculate ΔH° for reactions. (5.7)

In addition to the class discussion, you should familiarize yourself with the key ideas of this topic.  These slides relate to sections 5.6-5.7 of your textbook.

Now that you are familiar with these concepts, you should put them into practice.

When you have finished working out the practice problems, take a picture of your work and add it to your OneNote.

Complete the problems below, then take a picture of your work and post it in OneNote.  Be prepared to present these problems in class.

Mastery Problems

5.64

Given the data

N_2(g)+O_2(g)\rightarrow 2NO(g) ΔH = +180.7 kJ/mol
2NO(g)+O_2(g)\rightarrow 2NO_2(g) ΔH = -113.1 kJ/mol
2N_2O(g)\rightarrow 2N_2(g)+O_2(g) ΔH = -163.2 kJ/mol

Use Hess's law to calculate ΔH for the reaction
N_2O(g) + NO_2(g) \rightarrow 3NO(g)

 

5.70

Many cigarette lighters contain liquid butane, C_4H_10(l).  Using standard enthalpies of formation, calculate the quantity of heat produced when 5.00 g of butane is completely combusted in air under standard conditions.

5.117

The precipitation reaction between AgNO3(aq) and NaCl(aq) proceeds as follows:
AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s)
(a) By using Appendix C, calculate ΔH° for the net ionic equation of this reaction.
(b) What would you expect for the value of ΔH° of the overall molecular equation compared to that for the net ionic equation? Explain.
(c) Use the results from (a) and (b) along with data in Appendix C to determine the value of ΔHׄ°f for AgNO3(aq).

2002 FRQ #5

Answers to these mastery problems can be found in the Content Library within OneNote.Machine generated alternative text: 5. + OW(aq) * H20(/) A student is asked to determine the molar enthalpy of neutralization, AH lit, for the reaction represented above. The student combines equal volumes of 1.0 M HCI and 1.0 M NaOH in an open polystyrene cup calorimeter. The heat released by the reaction is determined by using the equation q = mcAT. Assume the following. • Both solutions are at the same temperature before they are combined. The densities of all the solutions are the same as that of water. • Any heat lost to the calorimeter orto the air is negligible. The specific heat capacity of the combined solutions is the same as that of water. (a) Give appropriate units for each of the terms in the equation q = mcAT. (b) List the measurements that must be made in order to obtain the value of q . (c) Explain how to calculate each of the following. (i) The number of moles of water formed during the experiment (ii) The value of the molar enthalpy of neutralization, AH ut, for the reaction between HCl(aq) and NaOH(aq) (d) The student repeats the experiment with the same equal volumes as before, but this time uses 2.0 M HCI and 2.0M NaOH. (i) Indicate whether the value of q increases, decreases, or stays the same when compared to the first experiment. Justify your prediction. (ii) Indicate whether the value of the molar enthalpy of neutralization, AHneut , increases, decreases, or stays the same when compared to the first experiment. Justify your prediction. (e) Suppose that a significant amount of heat were lost to the air during the experiment. What effect would this have on the calculated value of the molar enthalpy of neutralization, AH ? Justify your answer.

Three for three!

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