Chemistry HL
Scientific investigation preparation
- 1.
Plan an initial-rate investigation of the order with respect to A for an aqueous reaction A + B -> products. Concentrations quoted are initial concentrations after mixing. Give a suitable series of three A concentrations, explain how to keep B concentration and total mixed volume fixed while changing A, and state how the measured initial rates could determine the order in A. Keep temperature fixed.
[3 marks] · no calculator - 2.
Hypothetical rate constants for the same reaction are 0.0100 s^-1 at 300 K and 0.0200 s^-1 at 310 K. Assume Arrhenius behaviour with constant A across this interval. Using ln(k2/k1) = Ea/R (1/T1 - 1/T2) and R = 8.314 J mol^-1 K^-1, calculate Ea in kJ/mol. Explain why two points cannot establish that this model holds over a wide temperature range.
[3 marks] - 3.
A hypothetical dilution uses a 10.00 +/-0.02 cm^3 pipette and a 100.0 +/-0.1 cm^3 volumetric flask. Stock concentration uncertainty is negligible. Under a conservative additive relative-uncertainty rule, calculate the percentage uncertainty in diluted concentration. Then predict the direction of concentration bias if the flask is filled above the mark, and propose a prevention step.
[3 marks] - 4.
For written HL investigation preparation, assume rate=k[A][B] with k=0.50 dm^3 mol^-1 s^-1. The reaction consumes A and B in a 1:1 ratio. Initially [A]=0.010 mol dm^-3 and [B]=0.200 mol dm^-3 at constant volume and temperature. Calculate the initial rate and k_obs in the approximation rate=k_obs[A]. If A becomes 0.002 mol dm^-3, calculate B and the resulting k_obs, and evaluate whether the original constant-k_obs approximation is exact.
[5 marks] - 5.
A hypothetical fast-reaction investigation uses a sensor whose displayed signal after a sudden constant true signal S is S_display(t)=S(1-exp(-t/2.0)), with t in seconds. Treat this as a supplied response model, not a reaction law. Calculate the displayed fraction at t=1.0 s and the time to reach 95% of S. Explain why comparing first-second slopes from this sensor may misrepresent reactions with different fast time courses, and propose a targeted improvement.
[4 marks] - 6.
In written HL investigation preparation, hypothetical first-order rate constants are fitted as y=20.0-6000x, where y=ln(k/(1 s^-1)) and x=1/T in K^-1. Use ln(k/(1 s^-1))=ln(A/(1 s^-1))-Ea/(RT), with R=8.314 J mol^-1 K^-1. Identify the slope and intercept, calculate Ea in kJ mol^-1 and A with units, and predict k at 300 K. The fit was obtained at only two distinct temperatures: explain one additional measurement strategy needed before treating the relation as valid over a wider temperature range.
[5 marks] - 7.
In hypothetical initial-rate trials at one temperature, concentrations are quoted after mixing and rates are in mol dm^-3 s^-1. Trial 1: [A]=0.0100, [B]=0.0200, rate=4.00 x 10^-5. Trial 2: [A]=0.0200, [B]=0.0200, rate=1.60 x 10^-4. Trial 3: [A]=0.0100, [B]=0.0400, rate=8.00 x 10^-5. Concentrations have units mol dm^-3. Assume rate=k[A]^m[B]^n over this range. Deduce m and n with evidence, give the rate equation and overall order, calculate k with units, and predict the initial rate for [A]=0.0300 and [B]=0.0100 at the same temperature. This is written investigation analysis, not actual measured data.
[5 marks] - 8.
A hypothetical investigation establishes rate=k_obs[A][B]^2 for overall A+2B -> P. Candidate mechanism M has A+B <=> I (fast pre-equilibrium), then I+B -> P (slow). For this task use [I]=K[A][B] and rate_slow=k2[I][B]. Candidate N has A+B -> I (slow), then I+B -> P (fast), and its slow-step rate is k1[A][B]. Sum the steps for M, identify its intermediate, derive its predicted rate equation, and decide which candidate is consistent with the measured law. Explain why consistency does not prove M uniquely correct. Treat A, B, I and P as abstract species with the stated stoichiometry.
[5 marks] · no calculator - 9.
For written investigation analysis, an ideal-gas mixture undergoes A(g)+B(g) <=> C(g)+D(g) at constant temperature and volume. Kc=9.00 in the concentration convention used here. Initially [A]=[B]=0.600 mol dm^-3 and [C]=[D]=0.200 mol dm^-3. Calculate the initial reaction quotient Qc and predict the net direction. Let x mol dm^-3 react in that direction: write all four equilibrium concentrations, solve for x and give the equilibrium composition. Take the positive square root of Kc to avoid expanding a quadratic. Explain why initial concentrations must not simply be inserted as equilibrium concentrations.
[5 marks] - 10.
For hypothetical written spectroscopy preparation, the short-wavelength convergence limit of the hydrogen series ending at n=1 is measured as 91.2 nm. Use E=hc/lambda, h=6.626 x 10^-34 J s, c=2.998 x 10^8 m s^-1 and NA=6.022 x 10^23 mol^-1. Convert the wavelength to metres, calculate the corresponding photon energy and first ionisation energy in kJ mol^-1, and explain why this limit rather than any one finite transition gives the ground-state ionisation threshold. Propose one specific instrumental check before trusting the inferred value.
[5 marks]