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IB DP Chemistry: SL復(fù)習(xí)筆記5.2.4 Using ?Hf° to Find Enthalpy Changes

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年8月4日下午2:48

Using ?Hf° to Find Enthalpy Changes

Standard Enthalpy of Formation is defined as

“The enthalpy change when one mole of a compound is formed from its elements under standard conditions”

We can use enthalpy of formation of substances to find an unknown enthalpy change using a Hess cycle
In this type of cycle the elements are always placed at the bottom of the diagram

5.1.8-Enthalpy-of-formation-and-Hess-cycles-Part-1

Enthalpy changes using enthalpy of formation

In this cycle the arrows will always be pointing upwards because the definition of the enthalpy of formation must go from elements to compounds
This means the Hess's Law calculation of ΔH will always be in the same arrangement

5.1.8-Enthalpy-of-formation-and-Hess-cycles-Part-2

Try the following worked example:

Worked Example

Given the data: 5.1.8-Worked-Example-Table Calculate the enthalpy of combustion of gaseous diborane given that it burns according to the following equation:

B₂H₆(g) + 3O₂(g)??→?B₂O₃(s) + 3H₂O(g)

Answer

Step 1:?Find the sum of the enthalpies of combustion of the products

ΔH_f?= + ( -1270) + ( - 242 x 3) = - 1996 kJ

Step 2:?Find the sum of the enthalpies of combustion of the reactants

ΔH_f?= + (+31.4) + 0? = + 31.4 kJ

There is no enthalpy of formation for oxygen as ΔH_f?of elements by definition is zero

Step 3:?Calculate the enthalpy change

ΔH = ΔH_{f products}? - ΔH_{f reactants}? = - 1996 - (+ 31.4) =?-2027.4 kJ

Exam Tip

In Paper 1, Enthalpy of Formation data will given in the question. For Paper 2, you may need to refer to Section 12 of the Data Booklet where you will find Thermodynamic Data for Selected Compounds

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