Chapter 14 Chemical Kinetics Flashcards
Define Chemical Kinetics
The study of the rate with which a chemical reaction occurs and the factors that affect the reaction
Define Rate
How much a quantity changes in a given period of time
Define Reaction Rate
The speed of a chemical reaction
The reactant concentration ___________ and the product concentration ___________ with time
Decreases ; increases
Reactants are ___________ and products are __________
Consumed ; formed
Define Rate of a Chemical Reaction
A measure of how fast the reaction occurs
Determine the rate of reaction of N2
N2 (g) + 3H2 (g) —> 2NH3 (g)
Rate = - Delta [N2] / Delta t
T/F In the rate of a chemical reaction the value will always be positive
True
For reaction A + 2B –> C under a given set of conditions, the initial rate is 0.1 M / s. What is the rate of change of [B] over time under the same conditions?
-0.2 M /s
Define Average Rate of Reaction
The change in concentrations in any time interval
Define Instantaneous Rate of Reaction
The rate at any given moment and is represented by the slope of the curve at that point divided by the stoi. coefficient
What are the factors affecting Reaction Rates?
- Reactant Concentration (Rate Law)
- Temperature
- Catalysts
Define Rate Law
The rate of the reaction is directly proportional to the concentration of each reactant (raised to a power)
T/F Rate law depends on both reactants and products
False ; ONLY reactants
What is the equation for Rate Law?
Rate = k [A]^n
In the equation for Rate Law?
Rate = k [A]^n
What does K, A, and n represent?
K - rate constant
A - concentration of reactant
n - order (n)
Define Zero Order
n = 0 ; when rate is independent of the concentration of A
In Zero Order does the reactant affect rate?
No, the reactant does not affect rate, the rate remains the same
What is the unit of “K” in Zero Order?
M / S
Define First Order
n = 1 ; rate is directly proportional to the concentration of A
In First Order does the reactant affect rate?
Yes, the reactant does affect rate directly because the rate doubles when doubling concentration X2
What is the unit of “K” in First Order?
S-1
Define Second Order
n = 2 ; rate is proportional to the square of the concentration of A
In Second Order does the reactant affect rate?
Yes, the reactant does affect rate to the power of 2 because the rate quadruples when doubling concentration
What is the unit of “K” in Second Order?
M-1 X S-1
Define Integrated Rate Law
The relationship between the concentrations of the reactants and time
Integrated Rate Law is the dependence of _____________ on _________
Concentration ; time
What is the equation for Zero Order Integrated Rate Law?
[A]t = -kt + [A]0
In the equation [A]t = -kt + [A]0
What does [A]t, [A]0, k, and t represent?
[A]t - A at any time (M)
[A]0 - initial A
k - rate constant zero order (M X S-1), first order (S-1), second order (M-1 X S-1)
t - time (seconds)
What is the equation for First Order Integrated Rate Law?
ln[A]t = -kt + ln[A]0
What is the equation for Second Order Integrated Rate Law?
1 / [A]t = kt + 1 / [A]0
Define Half Life?
t (1/2); the time required for the concentration of a reactant to fall to one half of its initial value
What is the equation for Zero Order Half Life Reaction?
t1/2 = [A]0 / 2k
What is the equation for First Order Half Life Reaction?
t1/2 = 0.693 / k or t1/2 = ln2 / k
What is the equation for Second Order Half Life Reaction?
t1/2 = 1 / k[A]0
Which of the following is always true?
i. Increasing rate constant will decrease half life
ii. Half life of reaction depends on order of reaction
iii. Increasing concentration will decrease half life
i and ii
Which statement is incorrect about Rate Law?
i. Rate law only depends on the concentration(s) of the reactant (s)
ii. Rate law depends on the concentration(s) of reactant(s) and product(s)
iii. For a first order reaction, the rate is directly proportional to the concentration of reactant
iv. For a zero order reaction, the rate is independent of the concentration of reactant
ii ; rate law only depends on the concentration(s) of the reactant(s)
Which of the following statements are always true?
i. Increasing the rate constant will decrease half-life
ii. The half-life of the reaction depends on the order of the reaction
iii. Increasing concentration will decrease half-life
i and ii
Which combination would increase the chances the most for effective collisions in a reaction?
A) Low temperatures and low concentrations
B) Low temperatures and high concentrations
C) High temperatures and high concentrations
D) High temperatures and low concentrations
E) Temperature and concentration do not impact the chances for an effective reaction collision
C
Consider this two-step mechanism for a reaction:
Step 1: NO2(g) + Cl2(g) →ClNO2(g) + Cl(g)
Step 2: NO2(g) + Cl(g) →ClNO2(g)
Which is the intermediate in the mechanism
A) NO2(g)
B) Cl2(g)
C) ClNO2(g)
D) Cl(g)
D
Given the following proposed mechanism, predict the rate law for the overall reaction 2NO2+ Cl2→ 2NO2Cl (overall reaction) Mechanism NO2+ Cl2→ NO2Cl + Cl slow NO2+ Cl → NO2Cl fast
Rate = k[NO2][Cl2]
The mechanism of a reaction is shown below and the overall reaction is HOOH + 2I ̄+ 2H3O+→I2+ 4H2O Predict the rate law and identify the reaction intermediates
HOOH + I –> HOI + OH ̄ (slow)
HOI + I ̄ –> I2+ OH ̄ (fast)
2OH ̄+ 2H3O+ –> 4 H2O (fast)
Rate = k [HOOH][I ̄], intermediates = OH ̄and HOI
Which statement is true regarding the function of a catalyst in a chemical reaction?
A) A catalyst increases the rate of a reaction.
B) A catalyst provides an alternate mechanism for the reaction.
C) A catalyst is not consumed by the reaction.
D) All of the above are true.
D
Given that the chemical reaction: S8(g) + 2 O2(g) --> 4 S2O(g) is found to have a rate law of: Rate = k[S8]. How many of the following actions would increase the rate of this reaction? i. Lowering temperature ii. Increasing concentration of O2 iii. Increasing concentration of S8 iv. Adding a catalyst
iii and iv
What does the Collision Theory of Kinetics state?
- Molecules must collide to react
- Collisions must have the correct orientation
- Collisions must have enough energy
Define Collision Model
When 2 molecules react after a sufficiently energetic collision with the correct orientation brings the reacting groups together
In an energy diagram how can you tell if the energy is exothermic?
When energy of products is lower than energy of reactant; will give you negative E
In an energy diagram how can you tell if the energy is endothermic?
When energy of products is higher than energy of reactant; will give you a positive E
What does the Arrhenius Equation show?
Shows the relationship between the rate constant (k) and the temperature (k) and the activation energy
What is the equation for Arrhenius Equation?
K = Ae ^ -Ea/RT
Define e ^ -Ea/RT in Arrhenius Equation
Exponential factor; the fraction of molecules that have enough energy to overcome the activation barrier
Define z in Arrhenius Equation
Collision frequency; the number of collisions per unit time
Define p in Arrhenius Equation
Orientation factor; the fraction of collisions with the correct orientation that allows the reaction to occur
T/F If “T” increases, overall e ^ -Ea/RT will increasing, giving larger k (faster rate)
True
The activation energy (Ea) _________, overall e ^ -Ea/RT will __________, giving ________k (faster rate)
Decrease; increase; larger
What is the equation for 2 Point Arrhenius Equation?
ln K2/K1 = -Ea/R (1/T2 - 1/T1)
Define Reaction Mechanism
The series of individual chemical steps by which an overall chemical reaction occurs
Define Reaction Intermediate
Something formed first in one step and consumed later in another
What happens to reaction intermediates?
They cancel out and do not show up in the overall reaction
Define Molecularity
The number of reactant particles involved in an elementary step
Define Unimolecular
1 particle
Define Bimolecular
2 particles
Define Termolecular
3 particles
Define Rate Determining Step
The step in a reaction mechanism that occurs much more SLOWLY than the other steps
Define Catalyst
A substance that increases the rate of a chemical reaction but is NOT consumed by the reaction
What do Catalysts provide for reactions?
An alternative mechanism for the reaction with a lower activation energy
What are the elementary steps?
Unimolecular, bimolecular and termolecular
In regards to half life reactions which order is independent?
First order
