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CHM 114 Study Guide - Midterm Guide: Reaction Rate, Reaction Rate Constant, Rate Equation

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ASU
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Chemistry
Course
CHM 114
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All

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Document Summary

In contrast to the exams 1, 2 and 3, all having 25 questions each, this exam has 20 questions. Mark your answers and if relevant write your calculations on the questionnaire. Write and bubble your last and first name on scantron of corresponding color code. To understand what the reaction rate is (molarity change per time change; this is the rate of disappearance of a reactant and the rate of appearance of a product). To understand the effect of the concentration of reactants on the reaction rate. What is the reaction order in a certain reactant? (the power of magnitude at the concentration of the reactant). To remember that the reaction order can be fractional, it must be determined experimentally, although for simplicity we consider only 0, 1st and 2nd orders. To remember that unit of the rate constant depends on the reaction order (m/s for the zero, 1/s for the 1st and 1/(m s) for the 2nd order).

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Related Questions


To understand how to use integrated rate laws to solve for concentration.

A car starts at mile marker 145 on a highway and drives at 55 mi/hr in the direction of decreasing marker numbers. What mile marker will the car reach after 2 hours?

This problem can easily be solved by calculating how far the car travels and subtracting that distance from the starting marker of 145.

55 mi/hr×2 hr=110 miles traveled

milemarker 145−110 miles=milemarker 35

If we were to write a formula for this calculation, we might express it as follows:

milemarker=milemarker0−(speed×time)

where milemarker is the current milemarker and milemarker0 is the initial milemarker.

Similarly, the integrated rate law for a zero-order reaction is expressed as follows:

[A]=[A]0−rate×time

or

[A]=[A]0−kt

since

rate=k[A]0=k

A zero-order reaction (Figure 1) proceeds uniformly over time. In other words, the rate does not change as the reactant concentration changes. In contrast, first-order reaction rates (Figure 2) do change over time as the reactant concentration changes.

Because the rate of a first-order reaction is nonuniform, its integrated rate law is slightly more complicated than that of a zero-order reaction.

The integrated rate law for a first-order reaction is expressed as follows:

[A]=[A]0e−kt

where k is the rate constant for this reaction.

The integrated rate law for a second-order reaction is expressed as follows:

1[A]=kt+1[A0]

where k is the rate constant for this reaction.

PART A

The rate constant for a certain reaction is k = 3.50×10−3 s−1 . If the initial reactant concentration was 0.400 M, what will the concentration be after 5.00 minutes?

Express your answer with the appropriate units.

PART B

A zero-order reaction has a constant rate of 5.00×10−4M/s. If after 75.0 seconds the concentration has dropped to 2.50×10−2M, what was the initial concentration?

Express your answer with the appropriate units.