Reaction Rates

Question 1

Name four factors affecting reaction rate

Answer 1

Concentration, physical state, temperature, and presence of a catalyst

Question 2

Does higher concentration increase or decrease reaction rate? Why?

Answer 2

Increases reaction rate because it causes more molecule collisions

Question 3

If reactants are in different phases, is the reaction rate higher or lower? Why?

Answer 3

Lower. When they are in the same phase, it is easier for them to mix and for molecules to collide

Question 4

Does decreasing temperature increase or decrease reaction rate? Why?

Answer 4

Decreases reaction rate, mostly because it reduces collision energy but also because it reduces collision frequency

Question 5

How is reaction rate defined?

Answer 5

The change in the concentration of a product or reactant over time

Question 6

In what units is reaction rate typically expressed?

Answer 6

M/s, molarity/second

Question 7

When expressing reaction rate in terms of reactant concentration, does the rate increase or decrease over time? Why?

Answer 7

It decreases over time because the reactants get used up

Question 8

When expressing reaction rate in terms of product concentration, does the rate increase or decrease over time? Why?

Answer 8

It increases because the amount of product increases exponentially as more and more reactant is used up.

Question 9

How does the average reaction rate differ from an instantaneous reaction rate?

Answer 9

An average rate is the overall rate from the start until the end of a reaction
An instantaneous rate is the rate at a given instant in a reaction

Question 10

For A –> B, how do you express reaction rate in terms of A, given time t?

Answer 10

-ΔA/Δt (note the negative)
Change in A concentration over change in time

Question 11

For A –> B, how do you express reaction rate in terms of B, given time t?

Answer 11

ΔB/Δt (note it is positive)
Change in B concentration over change in time

Question 12

In what scenario would the product increase three times as fast as the reactant decreases?

Answer 12

If there are 3 moles of product for every 1 mole of reactant

Question 13

For H2 + I2 –> 2HI, why does HI increase twice as fast as H2 decreases?

Answer 13

Because there are 2 moles of HI for every 1 mole of H2

Question 14

For the reaction aA + bB –> cC + dD,
what is the general expression of reaction rate?

Answer 14

Rate = (-1/a)(ΔA/Δt) = (-1/b)(ΔB/Δt) = (1/c)(ΔC/Δt) = (1/d)(ΔD/Δt)