Chemical Reactions
Which observation would best indicate that a gas was produced during a chemical reaction in a closed container?
A. The container’s color changes but its pressure remains constant.
B. The container’s measured pressure increases compared to before the reaction.
C. The container becomes colder with no pressure change.
D. The mass of the container plus contents decreases.
B. The container’s measured pressure increases compared to before the reaction.
An ICE table is used to:
A. Calculate activation energy from experimental data
B. Track initial, change, and equilibrium concentrations for reactions
C. Measure reaction rate directly from collision counts
D. Determine the entropy change of a reaction
B. Track initial, change, and equilibrium concentrations for reactions
A chemist observes that adding a catalyst to a reaction lowers the activation energy. Which is the most accurate model-based explanation?
A. The catalyst increases the concentration of reactants so more collisions occur.
B. The catalyst provides an alternate pathway with a lower-energy transition state, increasing the fraction of effective collisions.
C. The catalyst raises the temperature of the reaction automatically.
D. The catalyst changes reactant identities into different elements.
B. The catalyst provides an alternate pathway with a lower-energy transition state, increasing the fraction of effective collisions.
A reversible reaction at equilibrium is disturbed by adding more reactant. According to Le Châtelier’s principle and molecular models, what happens next?
A. The system shifts to produce more products until a new equilibrium is established.
B. The system produces more reactant to restore the original composition.
C. The temperature of the system must immediately drop to compensate.
D. Nothing changes because equilibrium cannot shift.
A. The system shifts to produce more products until a new equilibrium is established.
A catalyst speeds up a reaction by:
A. Increasing the temperature of the reaction mixture
B. Lowering the activation energy pathway for the reaction
C. Increasing the concentration of reactants
D. Shifting the position of equilibrium toward reactants
B. Lowering the activation energy pathway for the reaction
According to atomic-level models, what explains why heating increases the rate of many chemical reactions?
A. Heating reduces the number of collisions between molecules.
B. Heating increases the average kinetic energy so a greater fraction of collisions exceed the activation energy.
C. Heating decreases molecular motion so reactants stay together longer.
D. Heating increases the mass of reactant molecules, causing faster reactions.
B. Heating increases the average kinetic energy so a greater fraction of collisions exceed the activation energy.
Which experimental design best tests the effect of temperature on the rate of a decomposition reaction?
A. Run the reaction once at room temperature and record how long it takes.
B. Run identical reaction mixtures at several controlled temperatures, measure the concentration of reactant over time, and compare initial rates.
C. Change the concentration of the reactant while keeping temperature constant.
D. Vary the container volume while keeping temperature constant.
B. Run identical reaction mixtures at several controlled temperatures, measure the concentration of reactant over time, and compare initial rates.
For the reaction A + B ⇋ C + D, which change will shift the equilibrium toward products according to Le Châtelier’s principle?
A. Removing product D
B. Decreasing temperature if the forward reaction is endothermic
C. Adding an inert gas at constant volume
D. Decreasing pressure when gases are involved and there are more moles of gas on the product side
A. Removing product D
When constructing a model to explain gas production from a reaction, which piece of evidence would best support a molecular-level explanation instead of a purely macroscopic description?
A. The mass of the system remains constant during the reaction.
B. The rate of pressure increase corresponds to the number of moles of gas produced predicted by a balanced chemical equation.
C. The color of the solution changes from clear to blue.
D. The temperature of the lab increases slightly.
B. The rate of pressure increase corresponds to the number of moles of gas produced predicted by a balanced chemical equation.
If students model a collision between two molecules, which change in the model would most directly show why a higher temperature increases reaction rate?
A. Increase the model molecules’ radii so collisions are more likely.
B. Increase the model molecules’ speeds so a larger fraction of collisions have energy greater than the activation energy.
C. Decrease the model container size without changing speed.
D. Replace molecules with atoms to simplify the model.
B. Increase the model molecules’ speeds so a larger fraction of collisions have energy greater than the activation energy.
Consider the gas-phase equilibrium: N2O4(g) \rightleftharpoons 2NO2(g). Increasing the temperature will:
A. Shift equilibrium to the left if the forward reaction is exothermic
B. Shift equilibrium to the right if the forward reaction is endothermic
C. Always shift equilibrium to the side with fewer moles of gas
D. Have no effect on the equilibrium constant
B. Shift equilibrium to the right if the forward reaction is endothermic
Which observation would most clearly indicate that a chemical change — not just a physical change — occurred during a lab?
A. The sample changed shape but its molecular composition remained the same.
B. The sample changed color, gas formed, and mass (of a closed system) remained constant consistent with rearrangement of atoms to form new substances.
C. The sample was filtered and its appearance stayed identical.
D. The sample melted and then re-solidified with original properties.
B. The sample changed color, gas formed, and mass (of a closed system) remained constant consistent with rearrangement of atoms to form new substances.
Which factor does NOT generally increase the rate of a chemical reaction?
A. Increasing temperature
B. Increasing concentration of reactants
C. Adding a catalyst
D. Decreasing the surface area of a solid reactant
D. Decreasing the surface area of a solid reactant
For the equilibrium H2(g) + I2(g) rightleftharpoons 2HI(g), if the system is compressed (pressure increased), the equilibrium will:
A. Shift toward the side with more moles of gas
B. Shift toward the side with fewer moles of gas
C. Not shift because gases are involved
D. Always shift toward products
B. Shift toward the side with fewer moles of gas
According to collision theory, reaction rate increases when:
A. Collisions become less frequent but more random
B. More collisions occur with energy equal to or greater than the activation energy
C. Activation energy increases
D. Only orientation of molecules changes, not frequency
B. More collisions occur with energy equal to or greater than the activation energy
Which best describes dynamic equilibrium in a closed chemical system?
A. All reactions stop when equilibrium is reached
B. Forward and reverse reactions continue at equal rates, with constant concentrations
C. Only the forward reaction proceeds after equilibrium is reached
D. Concentrations of reactants and products must be equal
B. Forward and reverse reactions continue at equal rates, with constant concentrations