Gibbs Free Energy of Formation

You can use standard enthalpy change (ΔH°) and standard absolute entropy (S°) data to calculate the change in Gibbs free energy (ΔG°) for a chemical reaction or physical change using the equation:

 ΔG° = ΔH° - TΔS°

... OR ....
you can use tables of values for the standard Gibbs free energy of formation of compounds (ΔG_ƒ°).
For a chemical reaction or physical change:

reactants → products 

ΔG° = ΣΔG_ƒ°(products) - ΣΔG_ƒ°(reactants)

If you would like to understand where this comes from and how it can be applied solve problems in chemistry, visit our new tutorial Standard Gibbs Free Energy of Formation Calculations

AUS-e-TUTE Members should log-in to use the new tutorial, game, test and exam resources.

Effect of Temperature on GIbbs Free Energy

Imagine a reaction in which you decompose liquid water to produce oxygen gas and hydrogen gas:

H₂O_(l) → H_2(g) + ½O_2(g)

The reaction is endothermic, ΔH > 0, and the change in entropy is also positive, ΔS > 0
At room temperature and pressure this reaction is not spontaneous ( ΔG > 0).
But could I heat, or cool, the reaction sufficiently to make the reaction spontaneous?
Can I turn a nonspontaneous reaction into a spontaneous reaction by changing the temperature?
Want to know?
Go to https://www.ausetute.com.au/gibbstemp.html

AUS-e-TUTE Members can access the tutorial, game, test and exam by logging-in to the Members Only area of the website.

Gibbs Free Energy Calculations

For a chemical system, either a chemical reaction or a physical change, at a constant temperature and pressure we define a function called the Gibbs Free Energy (G) so that we can determine whether the system will be spontaneous or non-spontaneous:

• spontaneous if ΔG < 0 (ΔG is negative)
• non-spontaneous if ΔG > 0 (ΔG is positive)

In this new tutorial we will calculate the change in Gibbs free energy of a reaction at constant temperature and pressure (ΔG) using:

• standard absolute entropy of each reactant and product (S°)
• standard enthalpy of formation of each reactant and product (ΔH_ƒ°)

For a chemical system under standard conditions, we can calculate the change in standard Gibbs free energy using the equation shown below:

ΔG° = ΔH° - TΔS°

AUS-e-TUTE members can access the new Gibbs free energy calculations tutorial, game, test and exam when they log-in (Go to Physical Chemistry Heading, then "Thermodynamics").

If you are not an AUS-e-TUTE member, there is a "free-to-view" Gibbs free energy calculations tutorial currently available for evaluation purposes at https://www.ausetute.com.au/gibbscalc.html

Calculating Entropy Changes

If you stick some clean reactive metal, like magnesium, into an aqueous solution of hydrochloric acid... Hey presto! Bubbles of hydrogen gas! This gas production is a dead giveaway that the entropy of the system has increased.
But what if all the reactants and products are soluble in water and the entire chemical process occurs in aqueous solution. Has the entropy of the system increased or decreased?
Thankfully, chemists have a tool they can use to make quantitative predictions about the changes in entropy of a chemical system (ΔS). This tool is called the Standard Absolute Entropy (S°), and we have just added a new tutorial, game, test and exam to help you understand what this is and how it can be used.
AUS-e-TUTE Members should log-in to access the new resources under the heading "Thermodynamics".
If you are not already an AUS-e-TUTE there is a "free-to-view" tutorial currently available at https://www.ausetute.com.au/entropychange.html

Gibbs Free Energy and Spontaneity of Reactions

Whether or not a chemical reaction proceeds in a particular direction depends on a balance between the enthalpy of the system and its entropy.
Gibbs Free Energy allows us to quantify this relationship, and determine whether a particular reaction will be spontaneous.
AUS-e-TUTE members can now access a new tutorial, game, test and exam on this topic.
Not a member?
There is a "free-to-view" tutorial currently available at http://www.ausetute.com.au/freeenergy.html

Spontaneous Chemical Reactions

What makes a chemical reaction spontaneous?
If I add hydrochloric acid (HCl_(aq)) to metallic magnesium (Mg_(s)), bubbles of hydrogen gas (H_2(g)) are given off and the remaining solution contains chloride ions and magnesium ions (MgCl_2(aq)):

Mg_(s) + 2HCl_(aq) → H_2(g) + MgCl_2(aq)

But if I try to bubble hydrogen gas through an aqueous solution of magnesium chloride, nothing happens. No solid magnesium forms! Why?
AUS-e-TUTE has just added a new tutorial, game, test and exam to help you understand what makes a reaction spontaneous, nonspontaneous, reversible or irreversible.
AUS-e-TUTE Members should log-in to use these new resources (listed under Thermodynamics in the Physical Chemistry section).

Not an AUS-e-TUTE Member?
A "free-to-view" spontaneous reactions tutorial is currently available for evaluation purposes at:
http://www.ausetute.com.au/spontaneous.html

Introduction to Entropy

What is entropy?
What is meant by a chemical system having low entropy or high entropy?
What is the relationship between disorder, energy and entropy?

If you are asking these questions, then you will find AUS-e-TUTE's new entropy introductory tutorial, game and test very helpful! AUS-e-TUTE Members should log in to use these new resources (under the topic heading Thermodynamics in the Test Centre).

Not an AUS-e-TUTE Member?
A "free-to-view" tutorial is currently available at http://www.ausetute.com.au/entropy.html

Law of Conservation of Energy

We use the Law of Conservation of Energy all the time.
Every time I plug the kettle in to boil some water (for a nice hot cup of tea), electrical energy is converted into thermal energy. As I sit and watch steam rising up from my mug of tea I realise that thermal energy is being transformed into kinetic energy. The sugar I added to my tea might get converted (eventually) into the mechanical energy required to walk from my chair to the kitchen, or it might get stored (eventually) as chemical potential energy in my body (probably as fat).
How important is the Law of Energy Conservation (otherwise known as the First Law of Thermodynamics) in Chemistry?
So important that we've just added a new tutorial, game, test and exam on this topic to the  AUS-e-TUTE website.

AUS-e-TUTE members should log-in to use the new resources in the Members ONLY area of the website.

If you are not a member, there is a "free-to-view" tutorial currently available at http://www.ausetute.com.au/econserve.html

If you like what you see .... you'll love being an AUS-e-TUTE even more because we have lots of interactive resources to help you learn some Chemistry.
Find out more about AUS-e-TUTE membership at http://www.ausetute.com.au/membership.html

Exploding Pool Chemicals

Sydney Morning Herald, Sunday 12^th January 2014, "Sydney man severely injured mixing chlorine for pool"
In summary, the victim of this accident had a backyard pool which had turned green. He had been advised to add 2 kilograms of "chlorine" to his pool to turn it sparkling blue again. He went home, put 2 kg of the "chlorine" granules into a bucket, and added water. Within 30 seconds the mixture exploded. The man was taken to hospital in a critical condition.

Now, for some chemistry....

From the description of the "chlorine" it sounds like it was calcium hypochlorite, Ca(OCl)₂ , a white solid at room temperature and pressure that is commonly sold as "pool chlorine".
When added to water, calcium hypochlorite produces hypochlorous acid, HOCl, and calcium hydroxide, Ca(OH)₂(aq).

Word equation:	calcium hypochlorite	+	water	gives	hypochlorous acid	+	calcium hydroxide
Chemical equation:	Ca(OCl)2	+	2H2O	→	2HOCl	+	Ca(OH)2

Dissolving calcium hypochlorite in water produces energy. The heat (or enthalpy) of solution is about 630 kJ mol^-1
We can now calculate how much energy was released when the man added water to 2 kg of calcium hypochlorite in the bucket.
First we need to calculate how many moles of calcium hypochlorite were in the bucket:
moles (Ca(OCl)₂ ) = mass ÷ molar mass
            and mass (Ca(OCl)₂) = 2 kg = 2 x 1000 = 2,000 g
            and molar mass (Ca(OCl)₂) = 40 + 2(16 + 35.5) = 143 g mol^-1
moles (Ca(OCl)₂ )  = 2000 ÷ 143 ≈ 14 mol
Now,  from the heat (enthalpy) of solution, we know that 1 mole of calcium hypochlorite dissolves in water to produce 630 kJ of energy.
We can calculate how much energy is released when 14 mol of calcium hypochlorite dissolves in water:
energy released = moles x energy in kJ mol^-1
energy released  = 14 x 630 = 8820 kJ
which is equivalent to exploding about 100 g of TNT in the bucket !

Reference:
http://www.smh.com.au/nsw/sydney-man-severely-injured-mixing-chlorine-for-pool-20140111-30nl3.html

Suggested Reading:
Defining Enthalpy Change 
Energy Profiles 
Strength of Acids
Strength of Bases 
pH of Aqueous Salt Solutions 


Suggested Study Questions:

1.  What is the name given to the type of reaction that produced energy?
2.  What name is given to the type of reaction that absorbs energy in order to produce products?
3. Sketch an energy profile diagram for the reaction between calcium hypochlorite and water.
4. Sketch an energy profile diagram for the explosion of TNT.
5. For 2 kg of calcium hypochlorite completely dissolving in water, calculate the moles of calcium hydroxide that would be produced.
6. For 2 kg of calcium hypochlorite dissolving in water, calculate the mass of hypochlorous acid that would be produced.
7. Assume the man used a 30 L of water in a 40 L bucket, what is the concentration of calcium hydroxide expected to be?
8. Hypochlorous acid is considered to be a very weak acid. What does this mean? 
9. Calcium hydroxide is considered to be a strong base. What does this mean?
10. What is the approximate pH of the solution when 2 kg of calcium hypochlorite is dissolved in  30 L of water?
11. What safety precautions do you think pool owners should take when mixing calcium hypochlorite granules with water?
12. Design a safety poster that could be displayed in a pool shop clearly showing the safest way to mix calcium hypochlorite granules with water. 

Brownian Motion Machine

In 1912, Marian Smoluchowski proposed a prototype for an engine at the molecular scale which he thought could convert Brownian motion into work. It consisted of a series of vanes mounted on an axis and set into motion by molecular bombardment. An asymmetrical cog ensuring that the axis could only rotate in one direction so that the device could perform work such as lifting a weight.

In 1963, Richard Feynman demonstrated that the second law of thermodynamics would prevent the device from working in a system that was in a state of thermal equilibrium.

Scientists from the University of Twente, the University of Patras in Greece and the Foundation for Fundamental Research on Matter (FOM) have now demonstrated the first working Brownian Motion engine. Using a granular gas, a solid suspended in air that is constantly vibrated, a constant supply of energy is required to maintain the granular gaseous state so that the system is never at thermal equilibrium. Once the vanes of the engine start rotating, they induce a rotating motion in the gas known as a convection roll, which reinforces the movement of the device and allows for virtually continuous rotation.

Reference:
Peter Eshuis, Ko van der Weele, Detlef Lohse and Devaraj van der Meer. Experimental Realization of a Rotational Ratchet in a Granular Gas. Phys. Rev. Lett., 104, 248001 (2010) DOI: 10.1103/PhysRevLett.104.248001

Study Questions:

1. What is brownian motion?
2. What are the three laws of thermodynamics?
3. What is meant by term thermal equilibrium?
   
4. Why would the second law of thermodynamics prevent this device from working in a system that was at thermal equilibrium?
5. List the ways in which a granular gas is similar to compound in the gaseous state.
6. List the ways in which a granular gas is different to a compound in the gaseous state.
   

Uniqueness of Helium

Helium. He, is used to fill balloons, in lasers for eye surgery, as a cooling agent in nuclear reactors, and as a pressurizing agent for liquid fuel rockets in space exploration.

Helium has a number of characteristics that make it special. It is the most stable of all the elements and has the lowest boiling point. It becomes a fluid at temperatures close to absolute zero (0K) while most other materials are solid. In fact, helium is a liquid even at 0K and becomes a solid only under high pressure, and, helium is the only substance to exhibit superfluidity.

Of all the elements, helium is closest to the ideal gas. Two helium atoms form the weakest bound diatomic molecule, or dimer. All the properties of temperature, a measure of the kinetic energy of particles in matter, can be modeled if the force acting between a pair of helium atoms is known.
University of Delaware scientists have now predicted that the average separation between the helium atoms is 47 angstroms, compared to a typical bond length of 1 angstrom (one ten billionth of a meter or 0.0001 micron), and that the binding energy is 6,790 times smaller than the potential depth.

Reference:
M. Przybytek, W. Cencek, J. Komasa, G. %u0141ach, B. Jeziorski, K. Szalewicz. Relativistic and Quantum Electrodynamics Effects in the Helium Pair Potential. Physical Review Letters, 2010; 104 (18): 183003 DOI: 10.1103/PhysRevLett.104.183003