Chapter 18 Rates of reactants

Question 1

Define rate of reaction.

Answer 1

Time taken for concentration of reactants to decrease and products to increase

Question 2

Define order.

Answer 2

This is the power in which individual reactants are raised in the rate equation

Question 3

Define overall order.

Answer 3

The sum of individual orders present on the rate equation

Question 4

Define rate constant.

Answer 4

The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.

Question 5

Define half-life

Answer 5

The time taken for the concentration of a reactant to decrease by half.

Question 6

Define rate-determining step.

Answer 6

The slowest step in the reaction mechanism of a multi-step reaction

Question 7

How do you calculate units from a rate equation.

Answer 7

Right down all units present
Cancel out the units that are repeated
Bring the units from the bottom to the top
Change the sign to its opposite

Question 8

Name the order of this concentration-time graph.

Answer 8

0

Question 9

Name the order of this concentration-time graph.

Answer 9

1st

Question 10

Name the order of this concentration-time graph.

Answer 10

2nd

Question 11

Name the order of this rate-concentration graph.

Answer 11

0

Question 12

Name the order of this rate-concentration graph.

Answer 12

1st

Question 13

Name the order of this rate-concentration graph.

Answer 13

2nd

Question 14

what are the units for rate?

Answer 14

mol dm^-3 s^-1

Question 15

Can orders be determined from a balanced chemical equation?

Answer 15

NO

Question 16

How are orders calculated?

Answer 16

From experimental results.

Question 17

A + B + 2C → D + 2E

when calculating order wrt A keep ___ and ___ constant.

Answer 17

B and 2C

Question 18

what do you do when calculating orders and you cannot keep the other concentrations constant?

Answer 18

us the orders you know and substitute like an equation, if rate can be calculated and the order can be deduced as 1 or 2, if not it must be 0.

Question 19

calculate the overall order:

rate = k[X]² [Y]² [Z]⁰

Answer 19

4

Question 20

what does k equal?

Answer 20

rate constant

Question 21

how do you calculate k?

rate = k[NO₂] [O₃]

Answer 21

rearrange:

k = rate /

[NO₂] [O₃]

Question 22

describe the half-life of a first order reaction from a conc-time graph.

Answer 22

CONSTANT

Question 23

what technique can be used to calculate concentrations of solutions?

Answer 23

colorimetry

Question 24

what two methods are used for continuous monitoring of rate?

Answer 24

• gas collection
• mass loss

Question 25

what does exponential decay refer to?

Answer 25

the pattern of 1st order reactions (c-t) having a constant half-life.

Question 26

after a tangent is drawn hoe can you show constant half-life?

Answer 26

the times taken for concentration to half and half should both be equal eg. 0.5 → 0.25 = 100s 0. 25 → 0.125 = 100s

Question 27

how do you calculate k from t_½?

Answer 27

* k* = ln 2 / * t*_1/2

Question 28

how do you calculate k(rate constant) from the rate on a conc-time graph?

Answer 28

- rearrange rate equation - substitute in rate( gradient change in y/x) and concentration (point where tangent is drawn(touching the line))

Question 29

how do you calculate reaction constant of first order from a rate conc graph?

Answer 29

gradient = change in y / x

Question 30

how do you calculate reaction constant of first order from a conc-time graph?

Answer 30

draw tangent at conc wanted and calculate gradient

Question 31

What does a balanced equation show when calculating RDS?

Answer 31

the number of molecules of each species after cancelling the intermediates

Question 32

If Ea is large k is …

Answer 32

SMALL and therefore rate is slow

Question 33

If k is large Ea is …

Answer 33

SMALL

Question 34

what do the two parts in the Arrhenius equation represent? k = *A e^{-E_a /RT}*

Answer 34

* A = pre-exponential factor (FREQUENCY of collision with correct orientation)* * e^{-E_a /RT} *= exponential factor (proportion of molecules that exceed Ea and correct temperature)

Question 35

what do they stand for and units? k = *A e^{-E_a /RT} *

Answer 35

k-rate constant A- Arrhenius constant Ea- activation energy **J/mol** R- gas constant 8.314 **J mol^-1 K^-1** T- temperature **Kelvin**

Question 36

A plot of ____ against ____ .

Answer 36

**ln k** (logarithm) against **1/T**

Question 37

A plot of ln.k again 1/T gives a straight line with …

Answer 37

gradient = -Ea/R y-intercept = ln A on y axis

Question 38

what sign will the gradient ALWAYS be. (hint ; exo)

Answer 38

**NEGATIVE**

Question 39

if y-intercept = ln A how do you work out A?

Answer 39

A = e ^{value at y-intercept}

Question 40

increase in A = ______ in k(rate constant)

Answer 40

INCREASE

Question 41

increase in Ea = ______ in k(rate constant) and why

Answer 41

DECREASE less molecules have sufficient energy to exceed Ea and collide.

Question 42

increase in T = ______ in k(rate constant) and why?

Answer 42

INCREASE more collisions when temperature increase

Question 43

if gradient= -Ea/R how do you work out Ea?

Answer 43

Ea= gradient(negative value) x R(8.314)

Question 43

if gradient= -Ea/R how do you work out Ea?

Answer 43

Ea= gradient(negative value) x R(8.314)