- Created by: wifd149
- Created on: 11-05-18 03:00
Atoms, molecules and stoichiometry
State the definitions of:
- relative isotopic mass,
Mass of an atom of an isotope compared to 1/12th the mass of a carbon-12 atom, which is 12.00 a.m.u. exactly.
- relative atomic mass (Ar),
Average mass of an atom of an element in a naturally occuring isotopic mixture...
- relative molecular mass (Mr),
Mass of one molecule of an element or a compound...
- relative formula mass,
Mass of one unit of formula of compound...
Atoms, molecules and stoichiometry
What does a mole of a substance means, and what is its relation to Avogadro's constant is?
A mole of a substance is the amount of substance which has the same number of particles in 12g of carbon-12. Avogadro's constant is the number of particles present in 1 mole of a substance.
What is the volume of 1 mole of gases in room temperature and pressure, as well as in standard temperature and pressure?
Room temperature and pressure (298K, 1 atm): 24 dm^3
Standard temperature and pressure (273K, 1 atm): 22.4dm^3
State the formula used for calculating the molecular formula of a hydrocarbon from volumes of combustion products.
CxHy + (x+y/4)O2 --> xCO2 + y/2 H2O
What elements are exempted from the rule of electrons being filled from a lower to higher energy level?
Chromium (Cr) with an electronic configuration of [Ar]4s1 3d5 as half-filled 3d-subshell is more stable, and copper (Cu) with a completely filled 3d-subshell + [Ar]4s1 3d10.
Define what is:
- (first) ionisation energy,
Energy required to remove 1 mol of electrons from 1 mol of gaseous atoms to form 1 mol of gaseous, singly charged cations.
- a shell,
A group of orbitals about the same distance out from the nuclues.
- a subshell,
Orbitals of the same energy level but different arrangement in space.
What are sigma and pi bonds?
Sigma bonds occurs in covalent bonding when two orbitals overlap end-on, with the electron density being concentrated between two neighbouring nuclei to hold atoms together against mutual repulsion.
Pi bonds are formed when orbitals overlap sideways in covalent bonding, with electron density concentrated in electron clouds above and below the sigma bond separated by a nodal plane (a region of zero electron density).
- A pi bond is weaker than a sigma bond, because there is lesser overlapping of the charge cloud in pi bonding.
Basic shapes of molecules to know (8):
Tetrahedral (109; 4BP 0LP), linear (180; 2BP 0LP or 3LP), trigonal planar (120; 3BP 0LP), trigonal bipyramidal (90 & 120; 5BP 0LP), trigonal pyramidal (107; 3BP 1LP), octahedral (90; 6BP 0LP), square planar (90; 4BP 2LP), square pyramidal (90; 5BP 1LP).
What is electronegativity?
Electronegativity is the ability of an atom to pull a lone pair of electrons towards itself in a covalent bond.
- Electronegativity increases across a period, and decreases down the group. The most electronegative atom is flourine (F).
What is meant by the term bond polarity?
Bond polarity is when the difference in electronegativity between atoms form dipoles (delta + and delta -).
...or "unequal sharing of electrons."
- The degree of polarity is measured as dipole moments.
Explain why is ice less dense than water.
In ice, the water molecules are held in a fixed lattice structure by hydrogen bonding. The open lattice structure increases the volume, hence making ice less dense than water.
What does the strength of Van der Waals forces affected by?
- Number of electrons,
- Surface area in contact between molecules.
How does a dative (or coordinate) bond forms?
- The donor must have a lone pair of electrons,
- The acceptor must have a vacant orbital to accept the lone pair of eletrons.
What is a "lone pair of electrons"?
A lone pair of electrons refers to electrons not involved in bonding.
What is meant by "dynamic equilibrium" and state its condition(s)?
Dynamic equilibrium is when the rate of forward reaction equals to the rate of backward reaction in a closed system.
State Le Chatalier's principle.
When any of the conditions affecting the position of dynamic equilibrium are changed, the equilibrium position will shift to minimise/oppose the change so that the system is restored to equilibrium.
What determines whether or not a reaction is reversible?
The activation energy of the reaction must be low for it to be reversible.
Haber process: manufacturing ammonia (NH3)
- 500 degrees Celsius,
- 200 atm,
- Iron catalyst, Fe (s) with metal oxide promoters.
Contact process: manufacturing sulfuric acid (H2SO4)
- 400 ~ 600 degrees Celsius,
- Pressure slightly above atmospheric pressure, 1 atm ~ 5 atm,
- Vanadium (V) oxide, V2O5 catalyst.
# equilibrium arrow is in SO2 + 1/2 O2 -> SO3 (exothermic)
According to the Bronsted-Lowry theory, define what are acids, bases, a strong and weak acid.
- An acid is a proton (H+ or H3O+, hydroxonium ion) donor,
- A base is a proton acceptor,
- A strong acid ionises completely in water (i.e, H2SO4),
- A weak acid only ionises partially in water (i.e, HF and carboxylic acids).
H2SO4 + NaCl -> NaHSO4 + HCl
Bronsted-Lowry base (base-I): Cl- OR HSO4- >>>>> accepts H+
Conjugate acid (acid-II): H+ OR H2SO4 >>> because H2SO4 has one more H+
# does not matter from which side to look at