Unit 5 Practice: Easy

Answer: The change in concentration of a reactant or product over time.

Explanation: Reaction rate measures how quickly a reaction occurs.

A common unit is:

mol/L·s, or M/s

Answer: Because reactant concentration decreases over time.

Explanation: For a reactant:

Δ[reactant] < 0

Reaction rates are normally reported as positive values, so a negative sign is placed in front of the expression.

For example:

Rate = −Δ[A]/Δt

Answer:

Rate = k[A]^m[B]^n

Explanation: Here:

k = rate constant

m = reaction order with respect to A

n = reaction order with respect to B

The exponents are determined experimentally.

Answer: A proportionality constant that relates reactant concentrations to reaction rate.

Explanation: The value of k depends mainly on the reaction, temperature, and presence of a catalyst.

A larger k generally means a faster reaction under the same concentration conditions.

Answer: The rate doubles.

Explanation: A is first order.

So:

2¹ = 2

The reaction becomes twice as fast.

Answer: The rate becomes 4 times greater.

Explanation:

2² = 4

A second-order dependence causes concentration changes to have a squared effect.

Answer: The rate does not change.

Explanation: Anything raised to the zero power equals 1.

So:

[A]⁰ = 1

The reaction is zero order in A.

Answer: Third order

Explanation: Add the exponents:

2 + 1 = 3

Answer: No.

Explanation: Rate laws are usually determined experimentally.

The coefficients can be used directly as rate-law exponents only when the reaction is known to be a single elementary step.

Answer: [A] vs. time

Explanation: The zero-order integrated rate law is:

[A]t = [A]₀ − kt

This has the form:

y = b + mx

with slope:

−k

Answer: ln[A] vs. time

Explanation: The first-order integrated rate law is:

ln[A]t = ln[A]₀ − kt

So the slope is:

−k

Answer: 1/[A] vs. time

Explanation: The second-order integrated law is:

1/[A]t = 1/[A]₀ + kt

The slope is:

+k

Answer:

t₁/₂ = 0.693/k

Explanation: A first-order reaction has a constant half-life.

Its half-life does not depend on initial concentration.

Answer: 1/2

Explanation: By definition, one half-life is the amount of time required for half of the reactant to disappear.

Answer: 1/4

Explanation:

After first half-life:

1 → 1/2

After second:

1/2 → 1/4

Answer: A sequence of elementary steps showing how a reaction occurs.

Explanation: The overall balanced reaction often does not describe the actual molecular pathway.

The mechanism shows the individual steps.

Answer: A substance formed in one step and consumed in a later step.

Explanation: An intermediate does not appear in the final overall reaction because it cancels when the elementary steps are added.

Answer: The minimum energy barrier that must be overcome for a reaction to occur.

Explanation: Even if a reaction is energetically favorable overall, particles still need enough energy to reach the transition state.

Answer: Enough energy and proper orientation

Explanation: Particles must collide:

with energy equal to or greater than the activation energy

in an orientation that allows bonds to break and form correctly

Answer: It provides an alternative reaction pathway with lower activation energy.

Explanation: A catalyst does not give particles more kinetic energy.

Instead, it reduces the energy barrier that must be overcome.