Topic 5.1: Reaction Rates Quiz

Master AP Chemistry Topic 5.1 with this free practice quiz. Test your skills on rate of appearance, rate of disappearance, relative rates, and stoichiometry and rates to prepare for your AP exam.

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Topic 5.1: Reaction Rates Quiz

18 MCQs

Master the reaction rates with these 18 MCQs on AP Chemistry Topic 5.1: Reaction Rates.

Covered Subtopics: Reaction rates, rate measurement, average vs instantaneous

1 / 18

1. A student plots [product] versus time and finds that the curve rises steeply at first and then flattens. What does this show about the instantaneous rate of formation of the product?

It decreases as reactants run out

It stays constant

It drops to zero immediately

It increases with time

As reactant concentration falls, fewer collisions occur per second, so the product forms more slowly and the curve flattens.

As reactant concentration falls, fewer collisions occur per second, so the product forms more slowly and the curve flattens.

2 / 18

2. For the reaction A + 2B -> 3C, the rate of appearance of C is 0.090 M/s. What is the rate of disappearance of B?

0.045 M/s

0.090 M/s

0.030 M/s

0.060 M/s

The rate is one third of the appearance rate of C, which is 0.030 M/s. The disappearance rate of B is twice the rate because its coefficient is 2, giving 0.060 M/s.

The rate is one third of the appearance rate of C, which is 0.030 M/s. The disappearance rate of B is twice the rate because its coefficient is 2, giving 0.060 M/s.

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3. In the reaction N2(g) + 3H2(g) -> 2NH3(g), the concentration of H2 falls from 0.90 M to 0.78 M over 5.0 minutes. What is the average rate of disappearance of H2?

0.024 M/min

0.012 M/min

0.12 M/min

0.048 M/min

The concentration of H2 falls by 0.12 M over 5.0 minutes, so dividing the change by the time gives 0.024 M per minute.

The concentration of H2 falls by 0.12 M over 5.0 minutes, so dividing the change by the time gives 0.024 M per minute.

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4. For the reaction 4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O(g), the concentration of NH3 changes at a rate of -0.12 M/s at one instant. What is the rate of change of [O2] at that instant?

-0.096 M/s

-0.075 M/s

-0.30 M/s

-0.15 M/s

The rate equals one fourth of the NH3 change, or 0.030 M/s, and multiplying by the O2 coefficient of 5 gives a change of negative 0.15 M/s for O2.

The rate equals one fourth of the NH3 change, or 0.030 M/s, and multiplying by the O2 coefficient of 5 gives a change of negative 0.15 M/s for O2.

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5. In the reaction N2(g) + 3H2(g) -> 2NH3(g), NH3 appears at a rate of 0.24 M/min. At what rate does N2 disappear?

0.12 M/min

0.080 M/min

0.36 M/min

0.48 M/min

The rate is half the NH3 appearance rate, or 0.12 M/min, and with a coefficient of 1 the disappearance rate of N2 equals the rate itself.

The rate is half the NH3 appearance rate, or 0.12 M/min, and with a coefficient of 1 the disappearance rate of N2 equals the rate itself.

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6. For the reaction 2N2O5(g) -> 4NO2(g) + O2(g), the rate of appearance of O2 is 0.0080 M/s at one instant. What is the rate of disappearance of N2O5 at that instant?

0.032 M/s

0.0040 M/s

0.016 M/s

0.0080 M/s

Each O2 formed consumes two N2O5, so the disappearance rate of N2O5 is twice the appearance rate of O2, giving 0.016 M/s.

Each O2 formed consumes two N2O5, so the disappearance rate of N2O5 is twice the appearance rate of O2, giving 0.016 M/s.

7 / 18

7. In the rate expression -(Δ[A]/Δt), the negative sign is included because

the concentration of A decreases with time

the products form more slowly

the rate itself is decreasing

the reaction is reversible

Since [A] falls as the reaction proceeds, Δ[A] is negative, and the extra negative sign keeps the reported rate positive.

Since [A] falls as the reaction proceeds, Δ[A] is negative, and the extra negative sign keeps the reported rate positive.

8 / 18

8. For the reaction H2(g) + I2(g) -> 2HI(g), the rate of change of [H2] at one instant is -0.020 M/s. What is the rate of change of [HI] at that instant?

-0.040 M/s

+0.040 M/s

+0.020 M/s

-0.020 M/s

Each H2 consumed makes two HI, so the appearance rate of HI is twice the disappearance rate of H2, giving positive 0.040 M/s.

Each H2 consumed makes two HI, so the appearance rate of HI is twice the disappearance rate of H2, giving positive 0.040 M/s.

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9. The concentration of N2O5 is measured over time: 0.80 M at 0 s, 0.65 M at 10 s, 0.53 M at 20 s, 0.43 M at 30 s, and 0.35 M at 40 s. In which interval is the average rate of disappearance greatest?

10 to 20 s

20 to 30 s

30 to 40 s

0 to 10 s

The largest drop in concentration, 0.15 M, occurs in the first 10 seconds, so the average rate of disappearance is greatest in that interval.

The largest drop in concentration, 0.15 M, occurs in the first 10 seconds, so the average rate of disappearance is greatest in that interval.

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10. The concentration of B is 0.60 M at 20 s and 0.48 M at 40 s. What is the average rate of disappearance of B between 20 s and 40 s?

0.060 M/s

0.012 M/s

0.0030 M/s

0.0060 M/s

The concentration of B drops by 0.12 M over 20 seconds, so dividing the change by the time gives 0.0060 M per second.

The concentration of B drops by 0.12 M over 20 seconds, so dividing the change by the time gives 0.0060 M per second.

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11. For the reaction 2HI(g) -> H2(g) + I2(g), which expression equals the reaction rate?

rate = -(1/2)(Δ[HI]/Δt) = Δ[H2]/Δt = Δ[I2]/Δt

rate = (1/2)(Δ[HI]/Δt) = -Δ[H2]/Δt = -Δ[I2]/Δt

rate = -(Δ[HI]/Δt) = (1/2)(Δ[H2]/Δt) = (1/2)(Δ[I2]/Δt)

rate = -2(Δ[HI]/Δt) = Δ[H2]/Δt = Δ[I2]/Δt

The coefficient 2 inverts to one half before the HI change, with a negative sign because HI disappears, while H2 and I2 each carry a coefficient of 1.

The coefficient 2 inverts to one half before the HI change, with a negative sign because HI disappears, while H2 and I2 each carry a coefficient of 1.

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12. As the time interval used to compute an average rate becomes smaller, the average rate

approaches the instantaneous rate

approaches zero

becomes negative

equals the reverse reaction rate

An average rate over a tiny interval samples the curve at nearly one point, so it approaches the slope of the tangent, which is the instantaneous rate.

An average rate over a tiny interval samples the curve at nearly one point, so it approaches the slope of the tangent, which is the instantaneous rate.

13 / 18

13. The equation 2A -> B is rewritten with all coefficients doubled as 4A -> 2B. If the correct rate expression is used for each version, the calculated numerical value of the rate

is halved

doubles

is unchanged

becomes negative

Doubling all coefficients doubles both the reciprocal factor and the stoichiometric scaling, so the two effects cancel and the rate value stays the same.

Doubling all coefficients doubles both the reciprocal factor and the stoichiometric scaling, so the two effects cancel and the rate value stays the same.

14 / 18

14. Students measure the concentration of product Z forming in the reaction X + Y -> Z and find that the average rate between successive measurements steadily decreases. What is the best conclusion?

The reaction has stopped

The measurements are wrong

The temperature is rising

Reactant concentrations are falling

Falling reactant concentrations slow the reaction, so the amount of product formed per interval shrinks as the run continues.

Falling reactant concentrations slow the reaction, so the amount of product formed per interval shrinks as the run continues.

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15. A student proposes that for 2NO(g) + O2(g) -> 2NO2(g) the rate equals -2(Δ[NO]/Δt). What is the error in this expression?

The sign should be positive

O2 needs a factor of 2

The factor should be 1/2

The factor should be 1

The stoichiometric coefficient 2 inverts to one half in the rate expression, so multiplying by 2 overstates the rate by a factor of four.

The stoichiometric coefficient 2 inverts to one half in the rate expression, so multiplying by 2 overstates the rate by a factor of four.

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16. For the reaction 2NO(g) + O2(g) -> 2NO2(g), which expression correctly gives the reaction rate?

rate = -(Δ[NO]/Δt) = -(Δ[O2]/Δt) = (Δ[NO2]/Δt)

rate = -(Δ[NO]/Δt) = -1/2(Δ[O2]/Δt) = 2(Δ[NO2]/Δt)

rate = -1/2 (Δ[NO]/Δt) = -(Δ[O2]/Δt) = 1/2 (Δ[NO2]/Δt)

rate = -2(Δ[NO]/Δt) = -(Δ[O2]/Δt) = 2(Δ[NO2]/Δt)

The coefficients 2, 1, and 2 invert to one half, one, and one half before the concentration terms, with negative signs for the disappearing reactants.

The coefficients 2, 1, and 2 invert to one half, one, and one half before the concentration terms, with negative signs for the disappearing reactants.

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17. A graph of [A] versus time is plotted for a reaction. The tangent to the curve at t = 10 s passes through the points (0 s, 1.00 M) and (25 s, 0.25 M). What is the instantaneous rate of disappearance of A at 10 s?

0.075 M/s

0.040 M/s

0.030 M/s

0.020 M/s

The tangent slope is (0.25 minus 1.00) divided by 25, which is negative 0.030 M/s. The negative slope means A disappears, so the rate of disappearance is 0.030 M/s.

The tangent slope is (0.25 minus 1.00) divided by 25, which is negative 0.030 M/s. The negative slope means A disappears, so the rate of disappearance is 0.030 M/s.

18 / 18

18. Which of the following is a valid unit for a reaction rate?

liter per mole per second

mol per second

mol per liter per second squared

mol per liter per second

Reaction rate measures change in concentration per unit time, and concentration is moles per liter, so the correct unit is moles per liter per second.

Reaction rate measures change in concentration per unit time, and concentration is moles per liter, so the correct unit is moles per liter per second.

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What a Rate Is

The rate of a reaction is the change in concentration of a reactant or product per unit time. Its units are molarity per time, written M/s or M/min, since concentration itself is mol/L. A value in mol/s alone is a rate of change of amount, not a reaction rate, because it ignores the volume.

Rates are reported as positive numbers. Reactant concentrations fall, so Δ[reactant] is negative, and a minus sign is written in front of reactant terms to make the rate positive. Product concentrations rise, so product terms need no sign change.

A reaction does not keep one speed. As reactants are consumed, collisions become less frequent and the rate falls, so a concentration against time curve is steep at the start and flattens as the reaction proceeds.

One Reaction, One Rate: The Stoichiometric Factors

Each species in a reaction changes at its own rate, fixed by the balanced equation. For aA + bB → cC + dD, the single reaction rate is

rate = -(1/a) Δ[A]/Δt = -(1/b) Δ[B]/Δt = (1/c) Δ[C]/Δt = (1/d) Δ[D]/Δt

The coefficient of each species becomes a reciprocal factor in front of its concentration change. This makes every term give the same numerical value for the rate, even though the species themselves are disappearing and appearing at different speeds. A species with coefficient 3 changes three times as fast as the reaction rate; a species with coefficient 1 changes exactly at the reaction rate.

Multiplying every coefficient in the equation by the same factor does not change the calculated rate. The reciprocal factors shrink in exactly the proportion that the concentration changes grow, so the value is unchanged.

Average Rate and Instantaneous Rate

The average rate over a time interval is the total concentration change divided by the interval length, Δ[A]/Δt. It smooths over everything that happened inside the interval.

The instantaneous rate is the rate at one moment. On a concentration against time graph it is the slope of the tangent line drawn to the curve at that time. Draw the tangent, read two points on it, and divide the concentration difference by the time difference. For a reactant the slope is negative, and the rate of disappearance is its positive size.

As the interval used for an average rate shrinks toward zero, the average rate approaches the instantaneous rate, because a very short interval samples the curve at effectively one point.

Worked Examples

Example 1: average rate and the related rates

In N₂ + 3H₂ → 2NH₃, [H₂] falls from 0.90 M to 0.78 M over 5.0 min.

Average rate of disappearance of H₂: (0.90 – 0.78) / 5.0 = 0.024 M/min.

The reaction rate divides this by the coefficient 3: 0.024 / 3 = 0.0080 M/min. NH₃ appears at twice the reaction rate: 2 × 0.0080 = 0.016 M/min. N₂ disappears at the reaction rate itself, 0.0080 M/min, because its coefficient is 1.

Example 2: relating two species at one instant

In 4NH₃ + 5O₂ → 4NO + 6H₂O, at one instant Δ[NH₃]/Δt = -0.12 M/s.

The reaction rate is 0.12 / 4 = 0.030 M/s. O₂ disappears at 5 × 0.030 = 0.15 M/s, so Δ[O₂]/Δt = -0.15 M/s. H₂O appears at 6 × 0.030 = +0.18 M/s. The fastest-changing species is the one with the largest coefficient, not the one the question happens to name first.

Traps Specific to This Topic

The reciprocal factor is where marks are lost. The coefficient goes underneath: for 2NO + O₂ → 2NO₂ the NO term is -(1/2) Δ[NO]/Δt, never -2 Δ[NO]/Δt. Multiplying by the coefficient instead of dividing overstates the rate by the square of the coefficient.

A species’ own rate of change is not the reaction rate unless its coefficient is 1. If H₂ disappears at 0.024 M/min in the ammonia synthesis, the reaction rate is 0.0080 M/min, one third as large, and quoting 0.024 as the reaction rate is wrong.

Signs must be handled deliberately. Δ[H₂]/Δt is negative because H₂ is consumed; the rate of disappearance is the same number stated positively. Reporting a negative reaction rate is always an error.

A tangent slope read from a graph uses points on the tangent line, not points on the curve. Reading curve values at two times gives an average rate over that interval, which is a different quantity whenever the curve bends.