Thermochemistry [ Enthalpy and Calorimetry]

Thermochemical Equations

Prepared: Kemikal Drills by Rea Abuan 2014

Thermochemical Equations

A thermochemical equation is a balanced chemical reaction + the value of its enthalpy of reaction

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Requirement

1.Balanced chemical equation2.The value of ΔHrxn per 1

mole of the limiting reagent



This equation means that 183.75 kJ of heat is released when 1 mole of sodium solid reacts with water to give 1 mole of aqueous sodium hydroxide and ½ moles of hydrogen gas.



Important Conventions

Specify the states of the reactants and products. They are important in the

determination of standard enthapy values.



Important Conventions

The mole of the limiting reactant is important in the determination of ΔH of the reaction. Convention follows a ΔHrxn per one mole of the LRwww.reaabuan.com/

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Thermochemical Standard States

They are the most stable pure form of the substance.

Standard Pressure = 1.00 atmSpecified temperature = usually at 25 ⁰C (unless otherwise stated)



Standard States

1. For pure substances (solid or liquid) – standard state is the pure solid or liquid

e.g. oxygen, O2 = gas

bromine, Br2 = gas



a. For gases, P = 1.00 atm

b. For gaseous mixtures, partial pressure must be at P=1.00 atm

c. For aqueous solutions, concentration = 1.00 M



Standard Enthalpy Change, ΔHrxn

The value of enthalpy change after the complete conversion of the specified

moles of reactants all at their standard states into the specified moles of the products all at the standard states as

well.



Standard Molar Enthalpies of Formation, ΔHf⁰

The name implies that there should be a formation reaction.

ΔHf⁰ is the standard molar enthalpy of formation of a one mole of substance formed from its elements in their standard states.



Nose bleeds



Formation of CO2(g)

Carbon dioxide is formed by reacting oxygen, O2(g) with carbon, C(graphite). Both oxygen and carbon are in their standard states. www.reaabuan.com/

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ΔHf⁰ values

1. Zero for elements in their standard states



ΔHf⁰ values

1. Non-zero for compounds, ions and elements in their non-standard states



Ok, are you still following us here?



When you’re ready, we’ll continue with calculating

ΔHf⁰ from a given reaction



In general



For this reaction:



PRODUCTS REACTANTS



Take time to absorb these.



Formation of CO2(g)

Recall this equation from earlier.



Let’s set-up the equation for its enthalpy of formation



Let’s apply the general equation





Now, where can you find the ΔHf⁰ values of the reactants / products?



Guess? Kidding. Consult your book or the internet for the values. Do not

guess, ok?



Going back here.

Evaluate first. 1. O2 and C(graphite) are in their pure states,

right? So ΔHf⁰ should be zero.

2. CO2(g) is a compound in its non-standard state. Recall, we want to synthesize it through this reaction. So ΔHf⁰ should be non-zero.



So, calculating..



Formation of CO2(g)



Try this on your own:

(l)Find the .



Okay, let’s up the game for a bit.



Exercise:

Calculate the for the following reaction using the following values for

H2O2(l) → H2O(l) + O½ 2(g)H2O2 (l) = -187.8 kJ / molH2O (l) = −285.8 kJ/mol



Exercises:

H2O2(l) → H2O(l) + O½ 2(g)

-98.0 kJ / molwww.reaabuan.com/

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Let’s end here. Check out the exercises on my blog.



Also, check the tutorial videos of problem solving on my blog.



Until the next post!


Education

Thermochemistry [ Enthalpy and Calorimetry]