1.) Urea is used as a fertilizer. It is secreted by mammals.

a.) The structural formula for urea is shown below. State the electron domain geometry and molecular geometry for the Nitrogen and Carbon atoms.

Answer:
Nitrogen has a tetrahedral electron geometry structure and a trigonal pyramidal molecular geometry structure. Whereas, carbon has a trigonal planar electron geometry structure and a trigonal planar molecular geometry structure.

b.) Urea can be made from a direct combination of ammonia and carbon dioxide gasses.
2NH3 (g) + CO2 (g) (H2N)2CO (g) + H2O (g) Enthalpy < 0
Outline what happens to the equilibrium constant when the temperature is increased
Answer:
Since the reaction is exothermic (enthalpy is lesser than 0), the reverse reaction will be favored and the equilibrium constant will decrease.

c.) Urea combustion produces water, carbon dioxide and nitrogen. Write a balanced equation for the reaction with the state symbols.
Answer:
2(H2N)2CO (s) + 3O2 (g) → 4H2O (l) + 2CO2 (g) + 2N2 (g)
Balanced equation means the total number of moles are equal on both sides. The number of moles on the LHS add up to 12 → [(2 x 2) + 2 + (3 x 2)] = 12. For the RHS, it also adds up to 12 → [4 + (2 x 2) + (2 x 2)]

2.) This question is about isotopes

a.) Define isotopes
Answer:
Isotopes are atoms of a specific element that have a definite number of neutrons and consequently a different mass. In other words, they are atoms belonging to the same element, having same atomic number Z, but different mass number A

b.) A silicon sample has three isotopes in it in different abundances. The table below shows the isotope and the restive percentage abundance. Calculate the relative atomic mass of the silicon with the given data.
²⁸Si = 91.87% ²⁹Si = 5.24% ³⁰Si = 2.89%
Answer:
²⁸Si = (0.9187 x 28 [mass number]) = 25.72
²⁹Si = (0.0524 x 29 [mass number]) = 1.519
³⁰Si = (0.0289 x 30 [mass number]) = 0.867
Total relative atomic mass: 25.72 + 1.519 + 0.867 = 28.106 = 28.1

3.) This question is regarding periodicity.

a.) Define first ionization energy
Answer:
N2H4 is a polar molecule with London dispersion forces, dipole-dipole forces, and hydrogen bonding between molecules, whereas C2H6 is nonpolar and only has London dispersion forces between molecules. It takes more energy to overcome the stronger IMFs in hydrazine, resulting in a higher boiling point.

b.) Explain why the first ionization energy of silicon is higher than aluminum

Answer:
It is harder to remove the outer electron from Silicon because silicon has 1 more proton than aluminum so has stronger proton charge. However, the electrons are in 3p level so have the same amount of electron shielding. Therefore, the electrostatic force between the nucleus and the valence electrons is higher in silicon than in aluminum.

c.) Explain why sodium oxide has a higher melting point than sulfur trioxide

Answer:
Na2O is ionic therefore it has a stronger electrostatic attraction between Na+ and O2-. SO3, on the contrary, has weak intermolecular forces/London dispersion forces which tend to be easily broken and are weaker than ionic bonds.
d.) Sodium oxide and sulfur trioxide are added to separate beakers containing water. State the equation for each of the reactions and identify the acidity of each of the oxides.

Answer:
Sodium oxide: Na2O (s) + H2O (l) → 2NaOH (aq) – Sodium hydroxide, so basic
Sulfur trioxide: SO3 (l) + H2O (l) → H2SO4 (aq) – Sulfuric acid, so acidic

4.) Phosphorus tribromide (PBr3) is used to manufacture alprazolam.

a.) Predict the shape and the bond angle for PBr3
Answer:
The shape is trigonal pyramidal and the bond angle is 109.5 degrees.
b.) Outline why PBr3 is a polar molecule
Answer:
The bond between Phosphorus and Bromine is a polar bond – Br is a more electronegative element than P. The bond dipoles do not cancel hence resulting in a net dipole. Therefore, the molecule is considered to be polar.

c.) Consider the following reaction
Cr2O7^2- (aq) + I^– (aq) + 8H⁺ (aq) → 2Cr³⁺ (aq) + IO^3- (aq) + 4H2O (l)
Define oxidizing agent and identify it in the reaction above.
Answer:
An oxidizing agent is a chemical species that tends to oxidize other substances, i.e. cause an increase in the oxidation state of the substance by making it lose electrons. The oxidizing agent is the dichromate ion (Cr2O7^2-)

5.) 0.338 grams of methanol was combusted in a spirit burner. The heat released in the reaction increased the temperature of the 50 cm3 of water by 23.9 degree celsius.

a.) Calculate the enthalpy change of combustion of the methanol
Answer:
First, the energy of the water needs to be calculated: q = mcΔT
Q = 0.0500 x 4.18 (specific heat capacity of water) x 23.9 = 4.9951 kJ
Finding the number of moles of methanol: 0.338/32.05 = 0.010546 = 1.05 x 10^-2 mol
Therefore, enthalpy change = 4.9951/1.05 x 10^-2 = 473.648 kJ/mol = – 4.73 x 10²
It is a minus sign because it is an exothermic reaction (the temperature is released)

b.) Methanol can be manufactured using the following equation reaction:
CO (g) + 2H2 (g) CH3OH (g) Enthalpy < 0
State and explain the effect of the following changes on the equilibrium position of the reaction above
i.) Increase in temperature
Answer:
Since the enthalpy is lesser than 0, it indicates an exothermic reaction. Therefore, the equilibrium will shift to the reactants side or it will favor the backwards reaction

ii.) Increase in pressure
Answer:
Increasing the pressure will shift the equilibrium to the side with fewer gas moles, therefore the equilibrium will shift to the right as it has only 1 mole compared to the products side with 3 total moles.

iii.) Catalyst addition
Answer:
Adding a catalyst has no effect on the equilibrium of the reaction. The rate of the backward and forward reaction will increase equally.