Mistakes chem
Ionisation Energy & Periodicity
Q: How do you identify the group of an element from its ionisation energy data?
A: Look for a big jump in ionisation energy—this indicates a change in shell. Count backwards to find the group.
Buffer Solutions
Q: What components are required for a buffer solution?
A: A weak acid and its conjugate base, or a weak base and its conjugate acid.
Transition Metals & Colour
Q: Why are transition metal ions coloured?
A: Ligands cause d orbitals to split into two energy levels. Light in the visible region is absorbed to promote electrons between these levels; the remaining light is transmitted, giving the ion its colour.
Ammonia & Acids/Bases
Q: Is ammonia a strong or weak base?
A: A weak base.
Hydrated Salt Calculations (e.g., XH₂O)
Q: How do you find X in a formula like XH₂O?
A:
Find moles of given mass.
Calculate Mr of the whole compound.
Subtract Mr of the compound without XH₂O.
Divide the remainder by 18 to find X (since water has an Mr of 18).
Oxidation State of Vanadium
Q: How do you find the oxidation state of vanadium using gas volumes?
A:
Use gas volumes to find moles of Cl₂.
1 mole of Cl₂ = 2 electrons.
Multiply moles of Cl₂ by 2 for electrons transferred.
Divide by moles of vanadium to find electrons per V.
Subtract from original oxidation state to get new oxidation state.
Covalent Bonding
Q: Why is a C=C bond weaker than a C–C bond?
A: C=C has a π and a σ bond, while C–C has two σ bonds. π bonds are weaker than σ bonds.
Titration Observations
Q: What is the colour change in a potassium manganate(VII) and sulfuric acid titration?
A: Colourless to pale pink.
Q: Why is sulfuric acid added to the conical flask in this titration?
A: To provide H⁺ ions needed for the redox reaction.
Q: What should you always remember when approaching a titration question?
A: Draw the titration diagram and remember that the substance in the conical flask is the one you're finding the amount of first.
Dot and Cross Diagrams
Q: What should you show in the dot-and-cross diagram of magnesium hydroxide?
A: Mg²⁺ with 8 electrons (full outer shell), two OH⁻ ions each with one O⁻ carrying a negative charge.
Periodic Table - d-block Elements
Q: What makes an element a d-block element?
A: The last added electron enters a d subshell.
Equilibria & Kc
Q: What does a large Kc value indicate?
A: The equilibrium lies to the right; products are favoured.
Entropy
Q: What happens to entropy in a system where 7 moles of product become 2 moles of reactant?
A: ΔS system is negative (decrease in entropy).
Forces & Solubility
Q: When can two substances with London forces mix?
A: When they are of similar size and have similar numbers of electrons.
Q: Why are some substances with London forces insoluble in water?
A: They can't form hydrogen bonds.
Q: Are ionic compounds soluble in water? Why?
A: Yes, because hydration enthalpy is greater than lattice energy.
Q: Why are ionic compounds insoluble in non-polar solvents?
A: Because of strong ionic bonds that aren't overcome by interactions with non-polar molecules.
Thermal Decomposition Observation
Q: What is the observation when magnesium nitrate is heated?
A: A white solid remains and it melts
Q: Which bond has the shortest bond length in methanol?
A: O-H bond.
Q: Do you need brackets when drawing 2 repeat units of a polymer?
A: No, brackets aren't needed for 2 repeat units.
Q: Number of isomers: 1 carbon
A: 1 isomer.
Q: Number of isomers: 2 carbons
A: 1 isomer.
Q: Number of isomers: 3 carbons
A: 1 isomer.
Q: Number of isomers: 4 carbons
A: 2 isomers.
Q: Number of isomers: 5 carbons
A: 3 isomers.
Q: Number of isomers: 6 carbons
A: 5 isomers.
Q: Number of isomers: 7 carbons
A: 9 isomers.
Q: How many bonds in cyclohexene?
A: 10 sigma C-H bonds, 6 sigma C-C bonds (16 sigma total), and 1 pi bond.
Q: How to calculate increase in volume from a balanced equation?
A: Find total final volume and subtract the initial volume.
Q: How many bonds does carbon form?
A: Carbon forms 4 bonds.
Q: How to work out mole ratio when not given?
A: Use stoichiometry; if a diatomic molecule reacts with another molecule, use a 1:2 ratio.
Q: What is it called when hydroxide ions react in ethanol?
A: Elimination reaction.
Q: Why might a product not be optically active?
A: It forms a racemic mixture due to a planar carbocation allowing attack from both sides.
Q: How do you know if it's an SN1 mechanism?
A: If there’s 1 species in the rate-determining step.
Q: What must an alcohol be converted into before reacting with KCN in ethanol?
A: A halogenoalkane.
Q: What are the steps in a Grignard reaction?
A:
React Mg with halogenoalkane in dry ether.
Add CO₂, methanal, aldehyde, or ketone.
Add dilute HCl.
Q: How to structure a 6-marker on NMR?
A:
State number of peaks for ¹³C and ¹H NMR
Relative peak areas
Mention splitting
Add together peaks for identical environments
Q: What is a singlet in NMR?
A: A single peak; often seen when O-H does not split.
Q: Where does Eₐₜ go on an energy profile diagram?
A: Before the activation energy peak.
Q: What is on the vertical axis of an energy profile diagram?
A: Number of molecules with activation energy.
Q: What does lowering the temperature do to collision and energy?
A: Reduces the fraction of molecules with energy ≥ activation energy.
Q: If a reaction has a 31% yield and you are asked to find the volume of a different organic compound, what is the calculation?
A: Use:
mass
/
percentage yield
×
100
mass/percentage yield×100
Q: What should you say in bond angle questions about separation?
A: Electron pairs repel to maximise separation and minimise repulsion.
Q: Why can phosphorus form PCl₅ but nitrogen cannot form NCl₅?
A: Phosphorus can expand its octet using the available 3d orbital.
Nitrogen cannot because it has no d orbitals in the n=2 energy level.
Q: Why does fluorine form stronger dipoles compared to chlorine?
A: Fluorine forms larger dipoles due to its higher electronegativity, resulting in stronger dipole–dipole forces.
Q: What is the name of the reference peak in NMR and what is its purpose?
A: Tetramethylsilane (TMS); it is a reference standard.
All chemical shifts are compared to its signal because it is chemically inert and produces a single peak.
Q: In electrophilic substitution, what should you remember about the carbocation?
A: Always place the positive charge on the correct carbon—usually the first carbon if two are shown.
Q: What does retention time in gas chromatography depend on?
A: Retention time depends on the compound's attraction to the stationary phase.
Greater attraction = longer retention time.
Q: How does a catalyst increase the rate constant
A: A catalyst provides an alternative reaction pathway with a lower activation energy.
This increases the proportion of molecules with energy ≥ Ea, speeding up the reaction.
Q: What is the unit for the gradient when calculating the rate constant
k from a graph?
A: The unit is simply
k; it depends on the order of the reaction and rate equation.