Chem always forget A

Why is the starch indicator added only towards the end of titration when the iodine solution is pale yellow, instead of right at the start?

• Starch forms a blue-black water-soluble complex with iodine in which the iodine is trapped within the starch molecules

• Hence, starch should not be added at the beginning of the titration when there is a high concentration of iodine since some iodine may remain trapped in the spiral starch molecules -> release of iodine from starch molecules is slow -> even when sufficient _ is added, some iodine is not fully released and mixture still appears blue -> by the time blue colouration disappeared, excess solution has been added to the conical flask -> higher tire values -> inaccurate results 

Define subshell

Describes distribution and energy of electrons within each electron shell

Electronic configuration 3 rules

Electronic configuration: Aufbau principle -> Pauli exclusion principle -> Hund’s Rule of Multiplicity

Why is anion larger than parent atom

• Anion has more electrons -> electron-electron repulsion increases -> electrostatic attraction between nucleus and valence electron decrease -> increase in size of electron cloud 

Explain why there is an increase in ionic radius from the cationic to anionic series

One more quantum shell of electrons than the cations -> distance between nucleus and valence electrons increase -> electrostatic attraction decrease -> increase in electron cloud

Principal quantum number (i think you forgot this existed) 

Define lattice

• Lattice (Def.) : Refers to a regular, repeating three-dimensional arrangement of particles (atoms, ions, or molecules) that form a crystalline solid. In a lattice, the particles are arranged in a way that maximises stability by minimising energy, often following a symmetrical and ordered pattern

Define simple molecular structure 

• Simple molecular structure (Def.) : A simple molecular structure consists of simple discrete molecules held together by weak intermolecular forces. These molecules consist of a small number of atoms joined by covalent bonds within the molecule itself

Define polar molecule 

• Polar molecule (Def.) : is a molecule that has an uneven distribution of electron density, resulting in regions with partial positive (δ+) and partial negative (δ−) charges. This occurs when the molecule contains polar bonds (bonds with unequal sharing of electrons) and has a geometry that does not cancel out the bond dipoles

Define dipole

• Dipole (Def.): Def. : A dipole refers to a separation of positive and negative charges within a molecule or atom caused by uneven distribution of electrons resulting in a region with a partial positive charge (δ+) and a region with partial negative charge (δ−) leading to formation of a polar molecule

State factors of covalent bonds 

• Bond length 

• Multiplicity of bond 

• Polarity of bond 

• Proximity of lone pair electrons 

VSEPR 5 0, 5 1, 4 2

Why branched structure has lower density than linear 

• A branched structure usually has bulky side chains that introduce steric hindrance, leading to a less compact packing arrangement 

• Branched structures create more empty spaces in the solid or liquid phase, leading to a larger molecular volume -> lower density

Define bond energy and reaction mechanism 

• Bond energy (Def.): Average energy absorbed when 1 mole of covalent bonds between 2 atoms in a gaseous molecule are broken

• Reaction mechanism (Def.): Is a sequence of elementary steps that adds up to the overall reaction

Exothermic:

Heat is released from the system to the surroundings -> enthalpy of the system decreased -> energetically, the system is stabilised

Solubility:

• Since r– is very small, lattice energy is sensitive to a change in r+

• Thus lattice energy becomes less exothermic with increasing cationic radius down the group

• This effect is largely masked by the large magnitude of the Hhyd (anion), since OH− has a very small r− as compared to the cations

• Thus, the sum [Hhyd (cation) + Hhyd (anion)] is relatively constant

Water present in excess -> the concentration of water is effectively constant, and the rate depends on the concentration of ester

Adsorption theory 

• Reactant molecules diffuse towards the catalyst surface

• Reactant molecules become chemically adsorbed through formation of weak temporary bonds with neighbouring active sites

• This increases the surface concentration of reactants and weakens the covalent bonds in the molecules -> lower Ea

• Eventually, the molecules dissociate, forming highly reactive intermediates which then combine to form the product

• After reaction, the product molecules break free from the surface and diffuse away from the surface

• The vacant active sites are now available for adsorbing other reactant molecules

Autocatalysis concentration of reactant time and rate time graph 

State the 4 gas laws

• Boyles’: V and P 

• Charles’: V and T 

• Gay-Lussac’s: P and T 

• Daltons’: Total pressure is sum of individual partial pressure 

Density + ideal gas equations

Define Le Chateliers and weak bronsted-lawry acid 

• Le Chatelier’s principle (Def.): system will reduce (NOT remove) the disturbance by.. 

• Weak Bronsted-Lawry acid (Def.): A weak Bronsted-Lowry acid undergoes partial dissociation to form ions and is a H+ (proton) donor

Explain why a weak acid, CH3COOH and a strong acid HCI produces the same volume of H2 when reacted with excess Zn 

• The amount of CH3COOH and HCI is the same. HCI is a strong acid, while CH3COOH is a weak acid

• As H+ in (2) reacts with Zn, by Le Chatelier’s Principle, the system will counteract decrease in [H+] by favouring forward reaction in (2) and POE shifts right, until eventually all CH3COOH dissociates and reacts with Zn

• Same amount of H+ from CH3COOH and HCI will react with Zn, hence same volume of H2 is evolved 

1. H2O (I) + H2O (I) ⇌ H3O+ (aq) + OH- (aq), Kw = [H3O+][OH-]

2. If the concentration of acid < 1.0 x 10^-7 mol dm^-3 -> include H+ (aq) from water too! 

A salt will undergo hydrolysis (reaction with water) if: 

• Anion is a conjugate base of the weak acid (Eg. CH3CO2- + H2O ⇌ CH3CO2H + OH-)

• Cation that is a conjugate acid of the weak base (Eg. NH4+) 

• Cation has high charge density (Eg. Ai3+, Cr3+)

1. Identify one conjugate acid-base pair

• OH– (strong base) and H2O (conjugate acid) (Do NOT include Na+ spectator ions!) 

Describe relationship between Kw and temp

H2O (I) + H2O (I) ⇌ H3O+ (aq) + OH- (aq), Kw = [H3O+][OH-]

• Dissociation of water is endothermic

• As temperature increases, POE shift right -> [H+], [OH] and Kw will increase
  

NOTE: At 50°C, pH of water is 6.63 -> water is NOT acidic -> at a higher temperature (above 25°C), the pH for neutrality is not 7 but at a lower pH

• At higher temperatures, water still remains neutral, since [H+] is still equal to [OH-]

• At 50C, pH of water is 6.8, pKw = 6.8 x 2 = 13.6 

Describe relationship between Kw, Ka, Kb