qcaa chem

Why is KHP used as a standard solution rather than NaOH in a titration

KHP does not absorb water from the air; therefore, an accurate concentration can be determined for KHP.

NaOH easily abosrbs water and ionises

difference between alpha glucose and beta glucose, specific wording

α-glucose has its OH group attached on the opposite side of the carbon ring (trans) to the CH2OH group, while β-glucose has its OH group attached on the same side of the ring (cis) as the CH2OH group.

Explain, at an atomic level, why no colour change occurs once the chromate dichromate solution has established equilirbium 

yellow chromate ions and orange dichromate ions are re-formed at the same rate at which they are broken down, so the colour remains constant because the [CrO4 2–] and [Cr2O7 2–] remain constant.

why esterifciation and transesterifcation are optimal processes 

In esterification, free fatty acids (long chain carboxylic acids) react with an alcohol  to produce esters (triglycerides) and water. In transesterification, esters (triglycerides) react with an alcohol to produce fatty acid alkyl esters (FAAEs) (biodiesel) and glycerol. A two-step process maximises the conversion of FFAs to biodiesel (FAAE).

Explain how the biodegradability of polylatic acid is related to its structure

Polylactic acid contains an ester linkage. Microorganisms produce enzymes that can hydrolyse the ester linkage to decompose PLA into CO2 and water.

When calculating Ka

make sure u convert it into a form where its raised to a whole integer

e.g.

10^-7.9= 1.26 × 10^-8  

Explain the relationship between the pH range of colour change for phenol red and its pKa value

identifies that colour change occurs when the [HIn] = [In– ] [1 mark]

• explains that when [HIn] = [In– ], the Ka of phenol red equals [H+], and therefore pH equals the pKa [1 mark]

• identifies that the pH range of colour change is pKa ±1 [1 mark]

• explains that the pH range of colour change occurs either side of pKa because colour change is detected when the [HIn] : [In– ] ratio changes [1 mark]

what does enzymes use to break starch down into glucose

hydrolysis to break glycosidic bonds

Difference between amylose and amylopectin

amylose has 1,4 glycosidic bonds, while amylopectin has 1,4 and 1,6 glycosdic bonds

amylose has a helix shape, while amylopectin has a spherical shape

Comparison of alpha helix and beta pleated sheets in secondary structure of proteins 

a-helix structure and ẞ-pleated sheets both form H-bonds between two peptide bonds on polypeptide chains

Difference: a-helix structure contains only intra-chain H-bonds while B-pleated sheets contain inter-chain (2 polypeptide chains) or intra-chain H bonds

Geometric isomers vs structural isomers

Both keep the same molecular formula

structural isomers change the overall structure, different bonding arrangements with different atoms

geometric isomers have the same bonding of atoms but the atoms are arranged differently in space

Soap and its cleaning related to its structure

Soap contains a non-polar, hydrophobic group and a polar, hydrophilic group.

The ionic salt is attracted to the polar water, allowing the soap to dissolve in water while the non-polar fatty acid chain is attracted to non-polar oils allowing the soap to dissolve in the oils.

Thus, soap can form a bridge between polar water and non-polar oils that water can't dissolve by itself

why cellulose is harder to convert to glucose than starch 

Cellulose is a linear polymer. The β-glucose monomers in cellulose can pack closely together. This increases hydrogen bonding between adjacent chains, which reduces interactions with water (solvents) and makes hydrolysis of cellulose more difficult than starch

if an amino acid

In chromatogrophy, the solvent is the what phase 

mobile phase 

Chromatogrophy rules

“Compounds move with the phase they’re more similar to in polarity.”

When you take an aliquot, what is the significance for the c=n/v 

the concentration stays the same, so that means the moles and volume change by the same proportion 

e.g. if you go from 500ml to a 25ml aliquot, you divided the volume by 20, therefore you also divided the moles by 20.

so to find the amount of moles present in the original solution multiply the moles in the 25ml aliquot by 20

Three common acid + base reactions

Acid + Base

Acid + carbonate

Acid + Metal

1. salt + water

2. salt + water + carbox dioxide gas

3. salt + Hydrogen gas

the pH and pKa equation for indicators

pH = pKa + log (In-/ HIn)

Therefore when the pH < pKa, there exists more Hln and the indicator is in its acidic form

When pH >pKa, there exists more In-, meaning the indicator is in its basic form.

how to differentiate between physical and chemical change 

Does the substance change its chemical identity?

• Yes → Chemical (a new substance is formed)

• No → Physical (same substance, maybe just different form or state)

can it be easily reversed 

Do tert alcohols or primary alcohols have higiher boiling point

Primary alcohols have higher boiling point becuase their OH group is more easily accesible to form hydrogen bonds and keep strong intermolecular forces

tactility for polypropene

X-ray Crystallography

X-ray crystallography involves using x-ray to determine the structure of molecules in a crystal form

What is diffraction 

Waves bending when encountering matter (crystal atoms)

electron density map 

• So the more electrons an atom has (i.e. the higher its atomic number), the more strongly it scatters X-rays.

• For example:

  • Hydrogen (Z = 1) → 1 electron → almost invisible.

  • Carbon (Z = 6) → 6 electrons → clearly visible.

  • Oxygen (Z = 8) → even stronger signal.

Bromine water test 

add Bromine water to test if its an alkene or alkane 

an alkene will react and decolourise while the alkane will stay orange/brown solution colour 

Elimination reaction of Alcohol into alkene

• What’s removed: H₂O

• Reagent: Concentrated acid (H₂SO₄ or H₃PO₄)

• Conditions: Heat (~170 °C)

• Type: Acid-catalysed elimination

Elimination reaction of haloalkane into alkene

• What’s removed: HX (a hydrogen + halogen)

• Reagent: Ethanolic KOH (base)

• Conditions: Heat under reflux

• Type: Base-induced elimination

Alkane into a Haloalkane requires what conditions

UV light, heat energy, X2 (halogen)

Conditions for esterification

H2SO4, heat in reflux

Conditions for haloalkane into alcohol

dilute NaOH

because if it was concentrated NaOH in ethanolic solvent, it would cause an elimination reaction and form an alkene 

conditions to make polyalkene 

catalyst, heat 

alkene into alcohol conditions

concentrated acid (H3PO4), steam

Difference between glycogen and amylopectin

both include 1,4 - glycosidic and 1,6 - glycosodic bonds made from alpha glucosoe monomers 

but glycogen is more branched and compact and found in animals 

while amylopectin is not as frequently branched and found in plants 

Primary Structure of Protein

the specific sequence of amino acids

Secondary Structure of protein

the localised folding of chains due to hydrogen bonding

alpha helicase, and beta pleated sheets

Tertiary structure of protein

The overall 3D shape of proteins, caused by interactions between the R-groups 

can be caused due to hydrogen bonds, hydrophilic and hydrophobic interactions, ionic bonding, disulfide bridges (covalent bonds between SH)

Quaternary structure

Lactose

A disaccharide that consists of one glucose and one galactose molecule together

Maltose

disaccharide of two alpha glucose

sucrose

one alpha glucose and one fructose 

Main goal of green chemistry

reduce waste, save energy (temperature, pressure), use renewable resources, make safer chemicals, use less hazardous reagents

difference between LDPE (low density polyethylene) and HDPE (high density polyethylene)

LDPE has highly branched chains, meaning it can’t pack closely and has weak london dispersion forces

HDPE is a very linear chain with minimal branching, allowing it to pack closer and have strong dispersion forces.

Biodiesel production from FFA-rich oils (the usual “two-step” process)

• We start with waste oils or fats that contain a lot of free fatty acids (FFAs) and triglycerides.

• FFAs can’t directly undergo transesterification efficiently — they form soap instead.

• So, we first convert those FFAs into esters (not triglycerides!) through esterification.

• Then, we perform transesterification on the triglycerides already present in the oil.

During transesterification

saponification