Chapter 07: Reaction Kinetics
Short Questions & Flashcards Study Portal
Short Questions
Rate of Reaction
Q.1
rate of a reaction? Rate of reaction is the change in
Answer
concentration of a reactant or product per unit time. It shows how fast or slow a chemical reaction occurs. between
Q.2
Give the difference enthalpy change of reaction and energy of activation of reaction.
Answer
Activation Enthalpy Property burg! (Ea) Change (4H) Heat Minimum Or absorbed energy required to start the released Definition reaction a during reaction barrier Energy and Initial between Depends final energy on reactants states products Ea AH Symbol
Molecularity
Q.3
Differentiate clearly between order and molecularity of a reaction.
Answer
Molecularity Order of Reaction on Based Determined reaction mechanism experimentally be a whole Can Always zero, fractional, or whole number (1, 2, or 3) number Applies to elementary Applies to overall reaction step only instantaneous rate
Q.4
Why the changes during a reaction? concentration of Because the
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reactants decreases over time, reducing the frequency of collisions and slowing down the reaction rate.
Q.4
How do you find the numerical value of a rate constant by initial and half-life methods?
Answer
See Q.10 of theory
Q.5
Briefly summarize the effects of temperature and surface area on the rates of reactions.
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Activation Energy
Q.5
How does the activation energy profile of an uncatalyzed reaction compare with that of the catalyzed reaction?
Answer
See Q.5 of theory
Q.6
Justify that the radioactive decay is always a first-order reaction.
Answer
In radioactive decay, the rate depends only on the amount of radioactive substance present, not on its concentration with other substances. Hence, it follows first-order kinetics:
Reaction Mechanism
Q.6
The reaction between hydrogen peroxide (H202) and iodide ions (I) in acidic solution is believed to occur via the following mechanism: Step 1: H202 + I → H2 0 + O I (slow) Step 2: OI + H+ → HOI (fast) NUMERICAL PROBLEMS
Answer
Q.6
Calculate the reaction rate if the concentration of A is 0.5 M, the concentration of B is 0.2 and the rate constant k is 4.0M 2s!. Given the rate law for a reaction: Rate = k[A][B]2. Solution: We are given: • Rate law: Rate=k[A][B] [A] = 0.5 mol/L [B] = 0.2 mol/L k= 4.0mol-s ' Calculation Rate = 4.0 x (0.5) × (0.2)2 Rate = 4.0 × 0.5 x 0.04 = 0.08 mol. L-1
Answer
Order of Reaction
Q.7
A reaction is second order with respect to a reactant. How is the rate of reaction affected if the concentration is doubled and reduced to half?
Answer
If a reaction is second order: and • Doubling concentration gives rate increases by 4 times (22) • Concentration reduced to half gives rate decreases to 1/4th
Catalysis
Q.7
A first order reaction is found to have a rate constant, k = x x s1. Find the half-life of the reaction. Rate constant Temp. (K) (em? mol-1 S-') (K) 500 6.814 x 104 Step 3: HOI + I + Ht → H2O + I2 (fast) (i) Write the overall balanced equation for the reaction. intermediates and any (ii) Identify catalysts in this mechanism. (iii) What is the rate-determining step? the (iv) Write the rate equation reaction, expressing it in terms of the reactants in the overall reaction.
Answer
(i) The overall balance equation for the reaction is given below: H202+21 + 2H* → 2H20 + 12 (ii) The intermediate for the reaction is HOI and there is no catalyst in reaction mechanism. (iii) The step 1 is called the rate determining step as it is a slow step. (iv) ) The rate equation is derived from the slow step and it is given below: Rate = k [H02 [ 550 2.64 x 102 •600 0.56 x 10° 650 7.31 × 10° 700 66,67 x 10° Solution: To find the half-life (t1/z) of a first-order reaction, we use the formula: 0.693 $112 =- Given Calculation 0.693 412=5.5x10-14=1.26x10 seconds To convert into years: 1.26x10' 4112= - ~399,365 years 60× 60×24x365 Ans. 1/2=1.26x10' seconds ~ 3.99×10 years Three experiments that have
Q.8
What is meant by half-life and what is it used for?
Answer
Half-life (tr/z) is the time required for half the reactant to decay or react. Used to determine: • Speed of radioactive decay • Stability of substances • Kinetics of first-order reactions
Q.8
identical conditions were performed to measure the initial rate of the reaction. 2HI(g) → H2(g) + I2(g) Rate (M/S) ¡HI| (M) Experiment 1.1 × 10-3 0.015 4.4 × 10-3 0.030 3 9.9 × 10-3 0.045 Write the rate law for the reaction. Find the value and units of the specific rate constant, k. Solution: Given Reaction: 2H1 (g) → 12(g) + 12(g) Step 1: Determine the Order of the Reaction Compare Experiments 1 and 2: fr [HI] doubles: 0.015 → 0.030 Rate increases: 1.1 × 10-3 → 4.4 → 10-3 4.4×10-3 • = 4 Rater = 1.1x101 Rate, (0.030 " [HI] = 2" 0.015 So, the reaction is second order with respect to HI. Step 2: Write the rate law Rate=k[HI]? Step 3: Find the rate constant k Use Experiment 1 Rate = 1.1 × 103 M/s, [HI] = 0.015 M 1.1 × 10-3 = k (0.015)2→ k= 1.1x10-3 1.1x10-3 2.25×104=4.89 M's"1 (0.015)
Answer
Q.9
Why does wood burn more rapidly in pure oxygen than in air?
Answer
Because pure oxygen increases the concentration of Or than air, leading to more frequent and effective collisions and faster combustion.
Q.10
A catalyst lowers the activation energy of a chemical reaction. Illustrate it.
Answer
In an energy profile diagram, a catalyst provides an alternative pathway with lower activation energy, making it easier for reactants to convert into products. the rate constant for a certain
Q.11
reaction is 3.5 × 104 5' at 25°C. What is the order of the reaction? Explain based on the units of the rate constant. Rate constant and order
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Q.12
If the initial concentration of the reactant is 0.50 mol dm3 , calculate the initial rate of the reaction.
Answer
Q.13
How would the rate of first order reaction change if the concentration of the reactant were doubled?
Answer
Effect of doubling concentration on rate Since it's first order, rate is directly proportional to concentration: SLO BASED SHORT QUESTION ANSWERS
Introduction to Reaction Kinetics
Q.14
A certain first-order reaction has a rate constant of 2.5 × 103s!. Calculate the half-life of the reaction in minutes. Half-life calculation
Answer
Given: k = 2.5×10-3 sl t1/2 = 0.693 / k = 0.693 / (2.5x10-3) = 2772.2 seconds Rate constant from half-life Given: t1/2 = 12 hrs = 43200 s tı/2 in minutes = 2772.2/60 = 4.62 minutes
Q.15
A radioactive isotope decays by a first-order process with a half-life of 12 hours. Calculate the rate constant for the decay in s
Answer
To calculate the rate constant (k) for a first-order radioactive decay process, use the formula: 0:693 k = - 41/2 Given: Half-life 41/2 = 12 hours Convert hours to seconds: 12 hours = 12 × 60 x 60 = 43,200 seconds Plug into the formula: 0.693 k = - -1.60x10-s1 43200 Answer k = 160x10-s Formula • Rate =[Product]/At=-[Reactant]/At Example: If HCl forms at 0.02 mol/dm? in 10 seconds, the rate = 0.002 mol/dm'.s.
Q.16
What is reaction kinetics?
Answer
Reaction kinetics is the study of the rates of chemical reactions and the factors affecting those rates. It also explains the mechanism by which a reaction occurs. Example: Decomposition of hydrogen peroxide (H202) into water and oxygen is studied to find its rate and mechanism
Rate of Reaction
Q.17
What is the rate of a chemical reaction?
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Q.18
How is the rate of reaction expressed for a general reaction A + B → C?
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Q.19
What are the units of rate of reaction?
Answer
The units are usually mol dm s-1 representing moles formed or used per liter per second
Average and Instantaneous Rate
Q.20
Differentiate between average rate and instantaneous rate.
Answer
Q.21
rate of reaction?
Answer
Increasing temperature increases the kinetic energy of molecules, causing more effective collisions, thus increasing the rate. Example: Rate of rusting increases in summer due to higher temperature.
Q.22
What is the effect of concentration on rate of reaction?
Answer
Higher concentration means more particles in a given volume, increasing collision frequency, thus increasing the rate. Example: Strong Hcl reacts faster with Zn than dilute Hcl.
Q.23
How does surface area influence the reaction rate?
Answer
A larger surface area (finely divided solids) provides more area for collisions, increasing the rate. Example: Powdered magnesium burns faster than ribbon.
Q.24
What role does a catalyst play in a chemical reaction?
Answer
A catalyst increases the rate by providing an alternative pathway with lower activation energy, without being consumed MnOz speeds up H2O2 Example: decomposition
Collision Theory
Q.25
What is the collision theory of reaction rate?
Answer
It states that for a reaction to occur, collide with proper particles must orientation and sufficient energy activation energy). Example: CH4 + Cl2 reacts in sunlight due to energetic collision
Activation Energy
Q.26
Define activation energy.
Answer
The minimum energy that reacting particles must have to form the activated convert into products. complex and Example: In combustion of wood, heat provides activation energy to start burning. a catalyst lower
Q.27
How does activation energy?
Answer
It provides a new. reaction path that requires less energy for the same reaction to proceed
Reaction Mechanism
Q.28
What is a reaction mechanism?
Answer
It is a step-by-step description of a chemical reaction showing how reactants convert into products through elementary steps. Example For: 2NO2 → 2NO + 02 Mechanism: 1. NO2 → NO + O (slow) 2. NO2 + O → NO + Oż (fast)
Q.29
What is the rate-determining step?
Answer
The slowest step in a reaction mechanism that determines the overall rate. Example: In the above NOz mechanism, step 1 is rate-determining
Molecularity
Q.30
What is molecularity of a reaction?
Answer
It is the number of reactant molecules involved in an elementary step. Types: Unimolecular (1), Bimolecular (2), Termolecular (3) Example H2 → 2H (unimolecular) H2 + Cla → 2HCI (bimolecular)
Order of Reaction
Q.31
Define order of a reaction. of
Answer
Q.32
How is determined experimentally?
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By m how changing observing concentrations affect the rate and comparing using rate laws. Rate Law
Q.33
Write a general rate law and define its components.
Answer
Q.34
What are the units of rate constant (k) for different orders?
Answer
Units of rate constant are: • Zero order: mol dm3s:* f • First order: s-1 • Second order: mol ' dm? s' Experimental Methods Name two methods to determine 035. rate of reaction experimentally. Ans. 1. Volume of gas evolved 2. Color change or turbidity Example 02 Measuring evolved during H202 decomposition. Graphical Analysis
Q.36
How can rate be determined from a concentration vs. time graph?
Answer
By finding the slope (tangent) of the curve at a given time
Catalysis
Q.37
What is the difference between and homogeneous heterogeneous catalysis?
Answer
Q.38
Give an example of enzyme catalysis.
Answer
Amylase catalyzes breakdown of starch to maltose in the mouth (biological catalysis).
Q.39
Why are catalysts important in industry?
Answer
They increase production rate, reduce energy costs, and improve yield. Example: Iron catalyst in Haber process. Reaction Profiles
Q.40
What is a reaction profile diagram?
Answer
It shows energy changes during a reaction, with activation energy and enthalpy difference. How does catalyst change reaction
Q.41
profile?
Answer
It lowers the peak (activation energy) without changing reactants/products' energy. Energy Considerations reaction be fast but a
Q.42
Can thermodynamically unfavorable? Yes. and Kinetics
Answer
(rate) thermodynamics (feasibility) are different. Diamond to graphite Example: thermodynamically favored but very slow.
Q.43
How is the concept of reaction kinetics used in medicine?
Answer
Used in drug design to control the rate of drug reactions or breakdown in the body.
Q.44
What role does the activation energy play in chemical reactions?
Answer
Activation energy (Ea) is the minimum energy required for reactant molecules to form the activated complex and proceed to products. It is like an energy barrier that must be crossed for a reaction to occur. Role: Activation energy controls whether a reaction will happen at a given temperature. Molecules must have at least this much energy to react. DESCRIPTIVE QUESTIONS Relate the order of a reaction to the rate law for the reaction. How do you distinguish between zero order, first order and second order reaction? Ans. See Q.8 of theory
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