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IB DP Chemistry: SL復(fù)習(xí)筆記5.2.2 Using Hess's Law - Cycles

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年8月4日下午2:39

Hess's Law using Cycles

There are two common methods to solving Hess's Law problems, using cycles and using equations
To be successful in using cycles you need to follow carefully a step-by-step plan using the information in the question to construct a cycle and add the given information
The following example shows one way to lay out your solution:

Worked Example

Calculate the enthalpy of reaction for

2N₂(g) + 6H₂(g)?→?4NH₃(g)

Given the data:4NH₃(g) + 3O₂(g)?→?2N₂(g) + 6H₂O(l), ????????? ΔH₁?= -1530 kJ mol^-1

H₂(g) + ?O₂(g )→? H₂O(l),?????????????????? ??????????? ΔH₂?= -288 kJ mol^-1

Answer:

- Begin by writing the target enthalpy change at the top of your diagram from left to right:

5.1.6-Worked-Example-1-Part-1

- Next, write the alternative route at the bottom of your cycle and connect the top and bottom with arrows pointing in the correct directions:

5.1.6-Worked-Example-1-Part-2

- Add the enthalpy data and adjust, as necessary, for different molar amounts

5.1.6-Worked-Example-1-Part-3

- Write the Hess's Law calculation out:

ΔH_r?= +6ΔH₂?– ΔH_1?= + (-288 x 6) - ( -1530 ) = -198 kJ

- Two important rules:
  - If you?follow the direction of the arrow?you?ADD?the quantity
  - If you go?against the arrow?you?SUBTRACT?the quantity

Worked Example

What is the enthalpy change, in kJ, for the reaction below?

4FeO(s) + O₂(g) → 2Fe₂O₃(s)

Given the data:2Fe(s) + O₂(g) → 2FeO(s)? ? ? ? ? ? ? ? ? ?H = – 544 kJ

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)???????????? ?H = –1648 kJ

Answer:

- Draw the Hess cycle and add the known values

5.1.6-Hesss-Law-Worked-Example-2-Answer

- Write the Hess's Law calculation out:

Follow the alternative route and the process the calculation

ΔH_r?= - ( - 544 x 2) + (- 1648) = - 560 kJ

Exam Tip

It is very important you get the arrows in the right direction and that you separate the mathematical operation from the sign of the enthalpy change. Many students get these problems wrong because they confuse the signs with the operations. To avoid this always put brackets around the values and add the mathematical operator in front

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