View
271
Download
5
Category
Preview:
DESCRIPTION
Learn the basics of Enthalpy, State functions and Calorimetry.
Citation preview
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
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Requirement
1.Balanced chemical equation2.The value of ΔHrxn per 1
mole of the limiting reagent
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Important Conventions
Specify the states of the reactants and products. They are important in the
determination of standard enthapy values.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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/
blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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)
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Nose bleeds
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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/
blog/
Prepared: Kemikal Drills by Rea Abuan 2014
ΔHf⁰ values
1. Zero for elements in their standard states
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
ΔHf⁰ values
1. Non-zero for compounds, ions and elements in their non-standard states
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Ok, are you still following us here?
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
When you’re ready, we’ll continue with calculating
ΔHf⁰ from a given reaction
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
In general
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
For this reaction:
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
PRODUCTS REACTANTS
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Take time to absorb these.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Formation of CO2(g)
Recall this equation from earlier.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Let’s set-up the equation for its enthalpy of formation
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Let’s apply the general equation
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Now, where can you find the ΔHf⁰ values of the reactants / products?
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Guess? Kidding. Consult your book or the internet for the values. Do not
guess, ok?
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
So, calculating..
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Formation of CO2(g)
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Try this on your own:
(l)Find the .
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Okay, let’s up the game for a bit.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
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
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Exercises:
H2O2(l) → H2O(l) + O½ 2(g)
-98.0 kJ / molwww.reaabuan.com/
blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Let’s end here. Check out the exercises on my blog.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Also, check the tutorial videos of problem solving on my blog.
www.reaabuan.com/blog/
Prepared: Kemikal Drills by Rea Abuan 2014
Until the next post!
www.reaabuan.com/blog/
Recommended