Unit 2: Topic 2 - Intramolecular Force and Potential Energy Flashcards
How is a potential energy vs. internuclear distance graph useful (what information can you get out of one)?
A potential energy vs. internuclear distance graph describes the amount of energy in a bond and the distance between the two atoms bonded. The lowest point in the graph shows the equilibrium bond length (or internuclear distance) that is the most stable for the atoms (releases the most energy when bonded and requires the most energy to break the bond).
Why is a potential energy vs. internuclear distance graph shaped as it is?
The graph’s trend is best explained by Coulomb’s law: F=(kq₁q₂)/r²
The potential energy approaches 0 as the distance is larger because the atoms neither attract nor repulse each other. r (distance) increases, so F (attraction) approaches 0.
As the distance gets smaller, the bond beomes stronger because of the increased attraction between the nucleus and electrons (positive-negative). The potential energy is negative because the atoms require energy to be pushed apart/the bond to be broken.
However, when the distance between the atoms become too small, the atoms start to repel each other because of the positive-positive and negative-negative repulsions between the atoms. The potential energy is positive because the atoms will repel without the addition of energy.
Bonds want the least potential energy to become the most stable.
How is bond length influenced by the size of the atoms and the order of the bond they share?
Atoms that have a smaller radius have a shorter bond length. Therefore, they have larger bond energies because the atoms are much more attracted to each other.
Bonds with higher order are shorter and have larger bond energies as well. Triple bonds are shorter than double bonds, which are shorter than single bonds. Triple bonds have the largest bond energies while single bonds have the smallest bond energies.
(imagine friends with more bonds are closer together and harder to break apart)
How does the magnitude of the ions’ charge affect bond strength (Coulomb’s Law)?
The greater the charges of the ions (positive and negative), the greater the lattice energy because greater magnitudes of charges result in greater bond strength (Coulomb’s Law: F=(kq₁q₂)/r², increase in q₁q₂ results in an increase of F).
Lattice energy is a measure of the strength of attraction between ions and is the energy required to separate one mole of the solid into its component gaseous ions.
How does the distance between the ions affect bond strength (Coulomb’s Law)?
The greater the distance between the ions, the smaller the lattice energy because the ions are not as attracted to each other from far distances. Therefore, smaller ions lead to stronger interactions because the ions are closer and experience more attraction (Coulomb’s Law: F=(kq₁q₂)/r², decrease in r results in an increase of F).
Lattice energy is a measure of the strength of attraction between ions and is the energy required to separate one mole of the solid into its component gaseous ions.
What is the difference between intramolecular bonds and intermolecular bonds?
Here is a diagram to help visualize the difference.
For this topic, we are looking at intramolecular forces, not intermolecular forces.
Intramolecular forces are those within a molecule. (Ex: Covalent and ionic bonds)
Intermolecular forces are those that are between molecules. (Unit 3 covers this)
What are the components to the potential energy vs internuclear distance graph?
Click here to view the potential energy vs internuclear distance graph.
Let’s look at the different stages of the graph:
1) No overlap/attraction: Since the internuclear distance (The distance between the nuclei of the 2 atoms) is too large, bonds aren’t able to form. The potential energy is 0.
2) Some overlap/attraction: This is where the attractions and repulsions are at equilibrium, creating the ideal bond length. Notice: The energy is negative because when bonds are formed, energy is released, which is exothermic. The more negative the energy, (lowest part of the graph) the more stable the bond is.
3) Repulsion: The atoms are too close to each other, causing a lot of nuclei and electron cloud repulsion. This is really unstable and leads to a potential energy greater than zero.
What are the different types of bonds and what are their characteristic?
Molecules can have single, double, or triple bonds (covalent). Each line on a Lewis diagram represents a pair of shared electrons. Bond energy is the energy stored in bonds. (larger bond energy = stronger bonds)
Single bonds (bond order of 1): 2 electrons shared. It has the longest bond length but smallest bond energy (weakest bond)
Double bonds (bond order of 2): 4 electrons shared. Its characteristics are between those of single and triple bonds. (Longer than triple but shorter than single bonds. Stronger than single bonds but weaker than triple bonds)
Triple bonds (bond order of 3): 6 electrons shared. Has the shortest bond length but largest bond energy (strongest bond)
Notice: Bond length and bond energy are inversely related!
How do single, double, triple bonds look like on a bond energy graph?
Here is the graph for single and double bonds.
You may notice 2 things:
1) The red curve (double bond) has a lower potential energy. This means that more energy is released when double bonds are formed. This makes sense because double bonds are stronger than single bonds.
2) The minimum on the red curve is also closer to zero than the blue curve. This makes sense because double bonds are shorter in length than single bonds!
Knowing that a lower minimum means stronger bond and a curve closer to zero means shorter bonds, what will a triple bond look like? Click here (https://docs.google.com/document/d/1Lm6izwspd2uRo4z52qR80l1xyCxLL5u0y9hJ7jr44qY/edit) for the answer. These trends can also be used to compare different molecules based on their strength and bond lengths.
You can see how the triple bond has the lowest minimum out of the three bonds (because it is the strongest) and the curve is closest to zero (as it is the shortest bond).
What is lattice energy?
Lattice energy is how strongly an ionic crystal is held together (larger value means stronger bond). Think of it as bond energy but for ionic compounds rather than covalent! It is negative (exothermic) when bonds are formed and positive (endothermic) when bonds are broken.
Here is the equation for calculating lattice energy BUT just know that we AREN’T using this equation to calculate the lattice energy but to use it to understand how lattice energy works.
Think about Coulomb’s law!
1) Lattice energy is greater with more highly charged ions. (Greater Q1 and Q2 values)
2) Lattice energy increases with shorter ionic distance (Smaller value of r)
If the compound you are comparing has the same magnitude of charged ion, compare the sizes to find which compound has greater lattice energy. Use periodic trends to figure out which is smaller.
