2.4 structure and bonding chem

Drawing lewis structure (example NH₃)

1. Count total VE present

2. Choose least electronegative atom as the central atom (never choose H)

3. Create a single bond between the center atom and each outer atom. (each bond represents 2 electrons)

4. Place remaining electrons in pairs around outer atoms, then only central atoms. (using octet rule)

5. Check that each atom has the correct number of full VE. If not, change the lone pair on the outer atom to create double/triple bonds.

should look like this… (for nh3)

Electronegativity

- Electronegativity increases from left to right across the table, and decreases down a group.

- δ+ = lowest electronegativity 

- δ− = highest electronegativity

4 regions names + bonding angles

Tetrahedral (four regions of negative charge, all regions bonded and a bond angle of approximately 109/109.5 degrees).

Bent 4 regions (four regions of negative charge, two bonded and two unbonded, and a bond angle of approximately 109/109.5 degrees)

Trigonal Pyramidal (four regions of negative charge, three bonded and one unbonded, and a bond angle of approximately 109/109.5 degrees)

3 regions + bonding angles + name

Trigonal Planar (three regions of negative charge, all regions bonded and with a bond angle of approximately 120 degrees)

Bent 3 regions (three regions of negative charge, two bonded and one unbonded, and a bond angle of approximately 120 degrees)

2 regions

Linear - 2 regions of negative charge, all bonded, 180 degrees

justifying molecule shape

1. Identify regions (each outer atom + lone pairs) AROUND CENTRAL __ ATOM

2. Maximise separation to minimise repulsion, giving parent geometry + bond angle

3. Bonded / non-bonded regions, giving overall shape

In NH₃, there are four regions of electron density around the central nitrogen atom. These regions of electron density maximise separation to minimise repulsion according to the Valence Shell Electron Pair Repulsion (VSEPR) Theory, giving NH₃ a parent geometry of tetrahedral with a bond angle of 109.5 degrees. There are three bonding regions and one non-bonding region (one lone pair), therefore the overall shape of NH₃ is a trigonal pyramid. 

state why __ has the same bond angle as __

1. First say:

The bond angles are determined by the number of electron density regions around the central atom, which are arranged to minimise repulsion by maximising separation.

2. Then do everything in justification for both molecules

3. Then say:

Both and have (same number of) regions of electron density, which gives them bond angles of (the angle for the number of regions) even though their shapes are different.

Explaning polar molecules

Polar due to presence of another atom/bond

In [compound], there are __ polar [bond] bonds due to the difference in electronegativity between __ and __ (__ being more electronegative) [list all bonds]. Because of the __ bond, the molecule has an asymmetrical shape. Therefore, the effect of the bond dipoles do not cancel out. This means that __ has an overall dipole and is polar molecule.

Polar due to bent shape

In [compound], there are __ polar [bond] bonds due to the difference in electronegativity between __ and __ (__ being more electronegative) [list all bonds]. Due to __ having a bent shape, it is asymmetrical. Therefore the bond dipoles do not cancel out. __ has an overall molecular dipole and is therefore a polar molecule. 

• ALWAYS LINK TO SHAPE

explaining non-polar molecules

Non-polar due to same polar bonds (symmetrical)

In [compound], there are __ polar [bond] bonds due to the difference in electronegativity between __ and __ (__ being more electronegative). However, the molecule has a symmetrical __ shape (specify what shape), so the bond dipoles are arranged symmetrically and cancel out. Therefore, __ has no overall dipole and is a non-polar molecule.

Non-polar due to same non-polar bonds

In [compound], there are __ non-polar [bond] bonds. As there is no difference in electronegativity between the two __ atoms, the bonding electrons are shared equally and no dipole forms. Therefore, __ is non-polar.

• ALWAYS LINK TO SHAPE

Ionic substance structure

__ is an ionic substance. It consists of alternating positively charged X ions (letter form) and negatively charged X ions (letter form). These oppositely charged ions are held together by strong electrostatic forces of attraction (ionic bonding) in a regular 3D crystalline lattice structure. 

• metal + non-metal

• strong electrostatic force (ionic bond)

• ions

Metallic substance structure

__ is a metallic substance. It is made of positively charged [metal] cations arranged in a 3D crystalline lattice structure, surrounded by a sea of delocalised valence electrons. The [metal] cations and delocalised electrons are held together by the strong electrostatic attractions, forming metallic bonds. These metallic bonds are non-directional.

• metal cations + delocalised electrons

• strong, non-directional electrostatic force (metallic bond)

Molecular substance structure

__ is a molecular substance composed of discrete X molecules. Within each molecule, X atoms are held together by strong covalent bonds. However between the X molecules, there are weak intermolecular forces of attraction holding the molecules together.

• molecules

• weak intermolecular forces

• primarily non-metal

Intramolecular force

forces WITHIN the molecule are very strong covalent bonds that link atoms within individual molecules.

Covalent bonds = intramolecular bonds!

For my understanding: H₂ linking to O to form H₂O

Intermolecular force

forces BETWEEN the molecule are weak intermolecular forces that link individual molecules to each other. 

For my understanding: H₂O being linked to another H₂O

Covalent Network - linear chain structure

Carbon polymers consist of long, straight linear chains of __ ??, allowing them to be packed closely together.

• atoms

• covalent bonding

Covalent Network - 2d layer (graphite) structure

Graphite is a covalent network composed of 2D layers of carbon atoms. Each carbon atom is covalently bonded to three other carbon atoms in a hexagonal lattice, forming flat sheets. The layers are held together by weak intermolecular forces.

• atoms

• covalent bonding

Covalent Network - Diamond/SiO2 (3D network) structure

Diamond/SiO2 is a covalent network composed of atoms covalently bonded together in a tetrahedral 3D crystalline lattice structure. 

• diamond = carbon atoms (just say atoms)

• sio2 = just atoms (not carbon)

• covalent bonding

Ionic solid bp and mp

A lot of heat energy is required to overcome these strong ionic bonds, therefore X has a relatively high melting and boiling point.

Ionic solid hardness and britleness

__ is very hard because the ions are held strongly together by ionic bonds in their closely packed 3D crystalline lattice structure. __ is brittle although it is very hard. When the force applied to the ionic solid exceeds the strength of the electrostatic forces holding the ions together, ions of like charges come in contact and the repulsive electrostatic forces between the ions of like charges will make the ionic solid shatter.

ionic solid electrical conductivity

Solid __ does not conduct electricity because its ions (charged particles) are in a fixed position and unable to move freely. However, molten __ conducts electricity because its ions are able to move.

ionic solid solubility

In order for a substance to dissolve, the solute and solvent particles need to form sufficiently strong attractive forces to overcome the existing forces of attraction in the solute and the solvent.

[ionic structure description]. __ is a polar ionic substance and is therefore soluble in polar solvents such as water. When reacted with water, the partially negative oxygen atoms of the water molecules are attracted to the positive [positive ions] ions in the lattice, and the partially positive hydrogen atoms of the water molecule are attracted to the negative [negative ions] in the lattice. These attractions are strong enough to overcome the ionic bonds in __, allowing it to dissolve.

example answer for ionic solid properties

1. describe what it’s made of

2. BP and MP

3. brittle/hardness

4. electrical conductivity

5. solubility

molecular solid BP and MP

Because the intermolecular forces of attraction between the molecules are relatively weak, not much heat energy is required to overcome these weak forces of attraction and separate the molecules. Therefore __ has a relatively low melting and boiling point.

molecular solid brittleness / hardness

__ is usually soft, brittle, and dull because they are made of discrete molecules held together by relatively weak intermolecular forces. The __ is brittle because the weak intermolecular forces between the molecules break easily under stress.

molecular solid conductivity

Since __ consists of discrete molecules and have no charged particles (ions or delocalised electrons) available to carry an electrical charge, they show no electrical conductivity.

molecular solid solubility

In order for a substance to dissolve, the solute and solvent particles need to form sufficiently strong attractive forces to overcome the existing forces of attraction in the solute and the solvent. [Compound] is a polar/non-polar molecular substance. As water is a polar solvent, strong attractions form/do not form between the water molecules and the [compound] molecules. These attractions are strong enough/not strong enough to overcome the intermolecular forces between the [compound] molecules, so the substance is soluble/insoluble in water.

• you will know if strong enough just based on polarity!

example answer for molecular solid properties

1. describe what it’s made of

2. BP and MP

3. brittle/hardness

4. electrical conductivity

5. solubility

metallic BP and MP

There are strong, non-directional electrostatic attractions between the delocalised electrons and the positively charged [metal] cations of the atoms. This metallic bonding is difficult to overcome, meaning __ has a relatively high melting and boiling point.

metallic malleability and ductile + hard

__ is usually malleable (able to be shaped using force) and ductile (able to be drawn into thin wires) due to its non-directional bonding. When a force is applied to the __ ,the positively charged [metal] cations are able to change positions and still experience strong metallic bonding to nearby delocalised valence electrons. This makes the metal malleable and ductile as layers of atoms can slide over each other without disrupting the bonding. __ is hard due to its strong metallic bonding in a closely packed 3D lattice structure.

metallic electrical conductivity

__ has high electrical conductivity. This is because the delocalised valence electrons of metal atoms are free to move throughout the metal lattice, allowing them to carry an electrical current when voltage is applied to the metal. As these electrons are free to move, __ metal will conduct electricity as a solid and when molten.

metallic substance structure answer properties

1. explain what it’s made of

2. BP and MP

3. malleability

4. electrical conductivity

graphite brittleness/ leaving marks answer

Graphite is soft and slippery because sheets of graphite can move over each other due to the weak intermolecular forces holding the sheets together. When graphite is applied to the paper, the weak intermolecular forces holding the sheets together break, and a graphite layer is left on the page. 

graphite bp / mp

Due to the strong covalent bonds between the carbon atoms within each layer, a very high amount of heat energy is required to break these bonds, therefore graphite has a high melting point.

graphite electrical conductor + solubility (i can’t be bothered to make another card)

Graphite is an electrical conductor as each carbon atom has one remaining electron that becomes delocalised within the layer, allowing it to carry an electrical charge.

In order for a substance to dissolve, the solute and solvent particles need to form sufficiently strong attractive forces to overcome the existing forces of attraction in the solute and solvent. Graphite is a non-polar substance and is therefore insoluble in polar solvents such as water. The attractive forces between the carbon atoms in graphite are stronger than the attractive forces between the graphite and water molecules. Therefore, graphite is insoluble in water.

Diamond/SiO₂ structure + brittleness answer

Diamond/SiO₂ is composed of atoms covalently bonded together in a tetrahedral 3D crystalline lattice structure. (say carbon atom covalently bonded to 4 other carbon atoms for diamond in a tetrahedral 3D lattice). There are only strong covalent bonds in the structure so the atoms cannot be easily removed so they do not leave a mark on paper (very hard).

Diamond/SiO₂ solubility

In order for a substance to dissolve, the solute and solvent particles need to form sufficiently strong attractive forces to overcome the existing forces of attraction in the solute and in the solvent. Diamond/SiO₂ is a non-polar giant covalent substance and is therefore insoluble in polar solvents such as water. The strong covalent bonds between the atoms of in Diamond/Sio2 are much stronger than the attractive forces that can form between the substance and the solvent molecules. Therefore, diamond/SiO₂ is insoluble in water and most other solvents.

Diamond/SiO2 BP/MP + electrical conductivity

Because Diamond/SiO2 has strong covalent bonds holding the atoms together in a 3D array, a very high amount of heat energy is required to break these bonds, therefore it has a high melting point.

Diamond/SiO2 is an electrical insulator as there are no free charged particles within the network.

Endothermic

The reaction is endothermic because more energy is required to break the forces of attractions in the reactants than is released when new forces of attraction are formed in the products. Therefore, energy is absorbed from the surroundings.

• just say bonds if you know it's covalent shape

• products have more energy than the reactants.

• Reactants absorb energy from surroundings (gains heat).

• Positive ΔH. 
  
  

Phase changes:

• melting, boiling (energy is absorbed to weaken/break bonds)

[phase change] is an endothermic process because heat energy is absorbed from the surroundings to break the forces of attraction between the __ molecules in the [state], separating the molecules to form a gas/allows the molecules to slide past each other.

• or just say wtv is changing states from __ (phase change) then continue
  
  

Graph:

Starts low, goes high, goes medium

• (products end up with more energy so will be higher than the reactant energy)

Exothermic

The reaction is exothermic because less energy is required to break the forces of attraction in the reactants than is released when new forces of attraction are formed in the products. Therefore, energy is released to the surroundings.

• say bonds if you know it’s covalent

• reactants have more energy than the products

• Reactants release energy into the surroundings (loses heat).

• Negative ΔH. 

Phase changes:

• condensation, freezing (energy is released when forming bonds)

[Phase change] is an exothermic process because energy is released to the surroundings as forces of attraction form between the [state] particles, changing them from a ___ to a ___.

• or just say wtv is changing states from __ (phase change) then continue

• always say forces of attraction?

Graph:

Starts mid, goes high, goes low

• (reactants end up with more energy so will be higher than the product energy)

Activation energy (E_a)

The minimum amount of energy that must be absorbed by reactants to initiate a chemical reaction. 

Δ_rH

the difference in enthalpy between reactants and products. 

• Measured in kJ mol^-1

ΔH

change in enthalpy 

• Measured in J or kJ

Bond Energy Calculations

Formula: Δ_rH = Σ_breaking – Σ_making (reactants - products)

 You will be given a bond table (it will show X–X bonds = X kJ mol^-1)

1. Try drawing the lewis structure if you can. 

2. Write the equation given in the form of how many bonds there are. (Δ_rH becomes the answer to the equation). Change the arrow into a minus symbol.

3. Write the equation as numbers according to the bond table.

4. Calculate the left side of the equation (energy required to break reactant bonds)

5. Calculate the right side of the equation (energy released when bonds in the product are formed)

6. Input answers into the formula (Δ_rH = Σ_breaking – Σ_making,)

N₂ reacts with H₂ to produce NH₃. Calculate the average bond enthalpy of N–H bond in NH₃.

You are given: N₂ + 3H₂ → 2NH₃ | Δ_rH = -92.0 kJ mol^-1

Bond table:

N≡N : 945 kJ mol^-1 H–H = 436 kJ mol^-1

1. N₂ = 1N≡N bond 3H₂ = 3H–H bonds 2NH₃ = 6N–H bonds (1 NH₃ = 3 N–H bonds, 2 NH₃ = 3x2 = 6 N–H) write bonds out

2. N≡N + 3(H–H) – 6(N–H) = -92.0 kJ mol^-1  —> becomes (–) sign! 

3. 945 kJ mol^-1 + 3 x 436 kJ mol^-1 – 6 (N–H) = –92.0 kJ mol^-1

4. Bond breaking = 945 + 3 x 436 

= 945 + 1308

= 2253 kJ mol^-1 

5. 2253 kJ mol^-1 – 6(N–H) = –92.0 kJ mol^-1

– 6(N–H) = -92 – 2253 kJ mol^-1 move 2253 to the other side to solve for N–H

    N–H = –2345 kJ mol^-1 / –6   divide by -6

    N–H = 390.83 kJ mol^-1 

    N–H = 391 kJ mol^-1 (3 SF) MUST ROUND!!

Estimate the ΔH for CH₄ + 2Cl₂ → CH₂Cl₂ + 2HCl

Bond table:

H–C = 414 kJ mol^-1 H–Cl = 431 kJ mol^-1 Cl–Cl = 243 kJ mol^-1 C–Cl = 327 kJ mol^-1

1. CH₄ = 4H–C bond 2Cl₂ = 2Cl–Cl bond CH₂Cl₂ = 2H–C bond, 2C–Cl bond 2HCl = 2H–Cl bond

2. 4(H–C) + 2(Cl-Cl) – [2(H–C) + 2(C–Cl)] + 2(H–Cl) 

3. 4 x 414 + 2 x 243 – [(2 x 414) + (2 x 327)] + 2 x 431

4. Bond breaking: 4 x 414 + 2 x 243

= 1656 + 486

= 2142 kJ mol^-1

5. Bond making: [2 x 414 + 2 x 327] + 2 x 431

= 828 + 654 + 862

= 2344 kJ mol^-1

6. 2142 – 2344 = –202 kJ mol^-1

mole

•  A mole is represented by the symbol (n), measured in (mol)

molar mass

• Molar mass is the mass of one mole of a substance (element/ compound).

• Molar mass is represented by the symbol (M), measured in g mol^-1

• Molar mass and the atomic mass number are different concepts, but they have the same values. (e.g. Cl has a mass number of around 35.5, and has a molar mass of 35.5 g mol^-1) 

• Molar mass of a compound can be calculated by adding together the molar masses of all the elements in the compound. (e.g. molar mass of NH₃ = molar mass of N + molar mass of 3 x H). 

• The molar mass of an ion is the same as the mass of the original atom. (e.g. M(Na⁺) = M(Na) ) 

Thermochemical calculations

1. Calculate the number of moles in the mass of the substance stated.

2. Determine how many moles of the substance releases Δ_rH for equation (just look if there’s a coefficient in front of the substance) “__ mole of __ releases [number]”

3. Multiply the number of moles (in step 1) by Δ_rH, and divide the answer by step 2, to find the amount of energy released/absorbed. 

4. State whether the energy was released or absorbed, and whether it is endothermic or exothermic. (DO NOT include the – symbol when stating this. The unit is also kJ, NOT kJ mol^-1)

5. round things to 3 sf

6. ‘__ kJ (without minus if have) of energy is released/absorbed (3 SF)’

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O | Δ_rH = –1235 kJ mol^-1 

1. Calculate the energy released when 23.0g of ethanol (C₂H₅OH) is burned

You are given: M(C₂H₅OH) = 46.0 g mol^-1

1. n = m/M

  = 23 g / 46.0

               = 0.5 mol

2. 1 mole of C₂H₅OH releases 1235 kJ of energy   there’s no coefficient in front!

3. (0.5 x –1235) / 1 

= –617.5 / 1

= –617.5 kJ

4. 617.5 kJ of energy was released when 23.0g of ethanol was burned. This is an exothermic reaction. 

Table particle stuff

Metal + non-metal = ionic substance

• particles = ions

• bonding = ionic bonding (strong electrostatic force)

Metal = metallic substance

• particles = cations + delocalised electrons

• bonding = metallic bonding (strong, non-directional electrostatic force)

Covalent network

• particles = atoms

• bonding = covalent bonding

primarily non-metal (or has metalloid idk) = molecular substance

• particles = molecules

• bonding = weak intermolecular forces

Explain why ethanol readily evaporates, and why the evaporation of ethanol results in the student’s hands feeling cool. (just an example question)

evaporate = phase change

why easily phase change = molecular substance so weak intermolecular forces, which is why only a small amount of heat energy is required to break bonds, therefore readily blah blah

why evaporation = hands cool (endothermic or exothermic part)

As the evaporation is endothermic, this heat energy is absorbed from the student’s hand, therefore making their hand feel cool

• just remember think about the phase change, and then just go what happens in endothermic reaction or liek wtv vice versa

By referring to the attractive forces between particles, explain why lithium bromide, LiBr(s), is unable to dissolve in non-polar solvents such as cyclohexane, C6 H12(l), but can dissolve in polar solvents such as water, H2 O(l).

In order for a substance to dissolve, the solute and solvent particles need to form sufficiently strong attractive forces to overcome the already existing forces of attraction. As cyclohexane is a non-polar molecule, the attractive forces it forms with the ions in the ionic lattice of lithium fluoride are insufficiently strong to overcome the strong ionic bonds. This means the ionic lattice remains intact, and the lithium fluoride does not dissolve. However, as water is a polar molecule, it has a negative pole, which can attract the positive ions in the lattice, and a positive pole, which can attract the negative ions in the lattice. These attractive forces are strong enough to overcome the ionic bonds present, allowing the substance to dissolve.

• basically just say how dissolving occurs

• state non-polar, and insufficiently strong to overcome forces, so will not dissolve, opposite for polar

unit of mass in molar mass question stuff

ALWAYS BE CAREFUL TO CONVERT TO GRAMS!!!!! IF ITS KG CONVERT!!!

What makes a chemical bond polar

A polar bond is a covalent bond that contains a dipole due to a difference in electronegativity between the bonded atoms. The more electronegative atom attracts the bonding electrons more strongly, creating a partial negative charge (δ⁻) on one atom and a partial positive charge (δ⁺) on the other.

• include bond

• more electronegative = negative dipole

drawing solubility diagram

Draw:

• Just draw the separate ions themselves

• Water molecules surrounding each ion.

  • Oδ⁻ points towards the positive ion (Na⁺).

  • Hδ⁺ points towards the negative ion (Cl⁻).