L7: Alloyed Materials Flashcards
What is entropy?
For a favourable process will the entropy change be positive or negative?
A measure of disorder
Positive
What is Gibbs free energy?
What is the equation for it?
When is the process thermodynamically favourable?
The free energy of the change i.e. the total amount of energy that has been freed up by the change - energy that can do useful work
G= H- TS
If G is negative
What four Situations may occur when two liquids are mixed together?
- Complete solubility
- partial solubility
- complete insolubility
- a reaction may take place producing a new compound
What is complete solubility
One liquid mixes completely with the other in all proportions to form a homogeneous solution e.g. ethanol and H2O
What is partial solubility
A small amount of liquid essay will dissolve into liquid be but addition of more a causes of separation of the mixture into a layer of be containing some a and a layer of a containing some be e.g. phenol and water
What is complete insolubility
In practice it doesn’t occur as a tiny amount of solubility will take place however oil and water are a good example
In a mixture which is the solvent and which is the solute
The solvent is the material in greater proportion the solute is the material in smaller proportion
What are the two main ways atoms in the alloy can combine
Substitution all or interstitial solid Solutions- Involves the atoms of one element mixing with the atoms of the other element
Intermediate compounds- Involves a reaction taking place between the elements
How do you substitutional solid solutions take place?
When will greatest solubility occur?
Solute atoms replaced the solvent atoms in the crystal structure
When atoms have similar size ( within 14%) same valence similar electro negativity, Same crystal structure
When does interstitial solution occur?
What atoms generally fill the spaces?
Solute atoms fitting into the interstices within the crystal structure
Can only occur for small solute atoms- large ones would distort crystal
Generally only nonmetals occupy the spaces e.g. hydrogen oxygen nitrogen and carbon and B
What are intermediate compounds
Combinations of elements metals or nonmetals that combine chemically to produce a compound
Aka intermetallic compounds when all elements are metals
Why are intermediate compounds important for engineering
They have very different properties to the alloy and individual components they are often hard and brittle
Describe the mixing process in terms of entropy
If a single atom of one is added to pew or two we increase the disorder of the system
If we continue to add one the entropy increases until the new material is 50 percent one
If we add more one the entropy starts to decrease as we now have two dissolved in one
Considering entropy alone mixing is the thermodynamically favourable process
How will bond strength affect the entropy when mixing two compounds
If the AB ones are weaker than the BB bonds Then the enthalpy change will be positive
If AB bonds are stronger than BB bonds then enthalpy will be negative
How would adding an atom of a lead to an increase in energy and a positive enthalpy change?
A distortion In the crystal structure may occur this can be because the atom is a different size or has a different bonding arrangement this can change BB bond Lengths And angles leading to an increase in energy
