AP Unit 5 Problems

For 2A → 3B at fixed volume, [A] changes from 0.0500 M to 0.0380 M over 40.0 s. Calculate A’s average disappearance rate, the common reaction rate, and B’s average formation rate. Explain why these data do not establish the instantaneous initial rate.

Supplied observations show the same mass of a powdered solid reacts faster with a solution than a single piece.

1. Suggest a controlled comparison that tests the surface-area explanation. Name a species-related signal to measure over time. Explain why warming only one trial would prevent a clean conclusion.
2. The table gives separate synthetic initial-rate comparisons for A → P in solution. Temperature, solvent, mixing, volume and the rate-measurement method are identical within each pair. No solid is involved. Identify the one changed variable and explain what each pair supports about reaction rate.
3. A student concludes that doubling any reactant concentration must always double a reaction rate. Does the concentration pair establish that claim? Explain what further evidence would be needed.

The catalyst addition has a matched solvent-only control. The illuminated trial is held at the same measured temperature as its dark control, so heating is not the changed variable. These are supplied data for analysis, not a laboratory procedure.

At fixed temperature, initial-rate data are:

Determine the rate law, overall order and k. Predict the rate at [A] =0.150 M and [B] =0.300 M.

A colored reactant is monitored by absorbance at a fixed wavelength. A student doubles both reactant concentrations and sees four times the initial disappearance rate, then claims second order in the colored reactant. Evaluate the inference and describe data that would resolve it. Assume Beer’s law is valid for the colored reactant alone.

For an overall reaction A + 2B → C, doubling [A] leaves the measured initial rate unchanged; tripling [B] triples it. State the empirical law and k units for rate in M s⁻¹. Explain why the coefficient two on B does not control the answer.

For A disappearance at fixed temperature, measurements are t =0, 40.0, 80.0, 120.0 s with [A] =0.160, 0.0800, 0.0400, 0.0200 M. Identify the simplest supported order, the linear graph with slope sign, k, and [A] at 200.0 s.

Data for A: at 0, 50.0, 100.0 s, concentrations are 0.1000, 0.06667, 0.05000 M. The reciprocal plot is linear within measurement precision. Find the order and k, and predict the time to reach 0.02500 M.

A monitored reactant decreases linearly from 0.0600 M at t =0 to 0.0480 M at 30.0 s and 0.0360 M at 60.0 s. State the supported model and k. A line extended to 200 s predicts a negative concentration. Interpret this prediction.

A radioactive isotope has first-order half-life 8.00 d. A supplied record begins with 64.0 μg. Find the mass of undecayed isotope after 24.0 d. Would ordinary chemical catalysis change this half-life? Justify at the level of the process.

A mechanism proposes the elementary step 2NO₂ → NO₃ + NO. Give the elementary rate law and molecularity. Another student models the overall 2NO + O₂ → 2NO₂ as one event and assigns third order without evidence. Evaluate that inference.

At the same concentration, warming a reacting gas greatly increases the observed rate. Draw two particle-energy distributions on the same axes, one for the cooler sample and one for the warmer sample. Label the axes, both curves, and a fixed activation-energy threshold for the same elementary pathway. Make the total curve areas equal and shade the area above the threshold for each curve. Use your drawing and collision orientation to explain the rate increase.

Two collisions between the same reactants have equal energy above the activation requirement. One produces product and the other does not. Give a molecular explanation consistent with collision theory.

A hypothetical elementary rearrangement A–B +C–D →A–C +B–D has reactants at 10, transition state at 75, and products at 40 kJ mol⁻¹ on one energy scale. Draw the labeled energy profile, including reactant/product levels, transition-state peak, forward and reverse activation-energy arrows, and an overall energy-change arrow. Calculate the two barriers and overall change. Explain which bonds change and why breaking bonds itself does not release energy. Predict qualitatively the effect of warming at fixed concentrations.

Proposed steps: NO + O₃ → NO₂ + O₂; NO₂ + O → NO + O₂. Identify the catalyst and intermediate, give the net equation, and explain how supplied detection of NO₂ during the reaction affects confidence in this mechanism.

A proposed mechanism has slow elementary A + A → I, followed by fast I + B → P; labels denote conserved chemical units. The net equation is 2A + B → P. Initial-rate data show that doubling A quadruples rate and doubling B leaves rate unchanged. Assess consistency and predict the change when A triples.

Mechanism: NO + Br₂ ⇌ NOBr₂ (rapid pre-equilibrium); NOBr₂ + NO → 2NOBr (slow). Derive the reactant-only rate law and predict the rate factor if [NO] doubles while [Br₂] halves. State the assumption.

Two mechanisms have the same net reaction A + B → P. M1: slow elementary A + B → P. M2: rapid A ⇌ I followed by slow I + B → P, with pre-equilibrium valid. Explain whether observed first order in A and B distinguishes the mechanisms.

Construct a labeled three-step energy profile using reactants0, TS1 35, I1 10, TS2 60, I2 −25, TS3 20 and products −40 kJ mol⁻¹. Label the energy axis with units, the reaction coordinate, all transition-state peaks and intermediate valleys. Draw each forward activation-energy arrow from its local starting level to the next peak, and draw the overall reactant-to-product energy-change arrow. Calculate the three barriers and overall change, then explain why the plot alone does not prove the slowest step.

An enzyme route is E + S ⇌ ES, ES → EP, EP → E + P.

1. Identify catalyst and intermediates; explain how rate can increase, whether ΔH changes, and whether free [E] must remain constant during binding.
2. For a different supplied elementary-step model, C + A → CA is slow and CA → C + P is fast. Identify the species consumed in the rate-determining step and the step that restores it. Sum the steps to obtain the overall equation.
3. In this second model at constant volume with no catalyst loss or deactivation, write an expression for total catalyst concentration. Explain how that total can remain constant while free [C] changes.

Compare two supplied catalytic models: acid H⁺ is added to a reactant to form a protonated intermediate then returned during product formation; a metal surface binds reactants then releases products. Explain the new steps and intermediates, how catalysts are restored, and why neither must change the net reaction enthalpy.