Chemical Kinetics

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If the rate constant of a reaction is 0.03 s^-1, how much time does it take for 7.2 mol L^-1 concentration of the reactant to get reduced to 0.9 mol L^-1?(Given: log 2 = 0.301) Answer: 1. 69.3 s Key idea: the unit mathrms⁻¹ means first order: t=2.303/klog[A]₀/[A] . 7.2/0.9=8 , log8=3×0.301=0.903 . t=2.303×0.903/0.03... Integrated rate equations and half-life (zero and first order) 1 answer
If the half-life (t_1/2) for a first order reaction is 1 minute, then the time required for 99.9% completion of the reaction is closest to : Answer: 4. 10 minutes Key idea: first order: k=frac0.693t_(1/2) and t=2.303/kloga/a-x . k=0.693\ mathrmmin⁻¹ . 99.9% complete: a/a-x=1000 ,... Integrated rate equations and half-life (zero and first order) 1 answer
The rate of a reaction quadruples when temperature changes from 27^°C to 57^°C. Calculate the energy of activation.Given R = 8.314 J K^-1 mol^-1, log 4 = 0.6021 Answer: 2. 38.04 kJ/mol Key idea: logk₂/k₁=E_a/2.303RT₂-T₁/T₁T₂ . T₁=300 K, T₂=330 K, k₂/k₁=4 . E_a=2.303×8.314×0.6021×300×330/30≈38046\ J/m... Effect of temperature, Arrhenius equation, activation energy 1 answer
Activation energy of any chemical reaction can be calculated if one knows the value of Answer: 1. rate constant at two different temperatures. Key idea: from the Arrhenius equation, logk₂/k₁=E_a/2.303R(1/T₁-1/T₂) . Knowing k at... Effect of temperature, Arrhenius equation, activation energy 1 answer
Which plot of ln k vs 1/T is consistent with Arrhenius equation? Answer: 1. [graph option 1, see figure] Key idea: ln k=ln A-E_a/R·1/T , a straight line with negative slope -E_a/R and positive intercept ln... Effect of temperature, Arrhenius equation, activation energy 1 answer
Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R :Assertion A : A reaction can have zero activation energy.Reasons R : The minimum extra amount of energy absorbed by reactant molecules so that thei… Answer: 1. A is false but R is true. Key idea: Activation energy is the extra energy reactants must absorb to reach the threshold energy: E_... Effect of temperature, Arrhenius equation, activation energy 1 answer
For a certain reaction, the rate = k[A]^2[B], when the initial concentration of A is tripled keeping concentration of B constant, the initial rate would Answer: 4. increase by a factor of nine. Key idea: Rate is proportional to [A]² when B is constant. New rate = k(3[A])²[B] = 9k[A]²[B] , so... Rate law, rate constant and units 1 answer
Which one is an example of heterogenous catalysis? Answer: 1. Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron. Key idea: In heterogeneous... Rate of reaction and factors affecting it 1 answer
For a first order reaction A → Products, initial concentration of A is 0.1 M, which becomes 0.001 M after 5 minutes. Rate constant for the reaction in min^-1 is Answer: 3. 0.9212 Key idea: for a first order reaction k=2.303/tlog[A]₀/[A] . k=2.303/5log0.1/0.001=2.303/5×2=0.9212\ mathrmmin⁻¹ . Integrated rate equations and half-life (zero and first order) 1 answer
The given graph is a representation of kinetics of a reaction.[Graph: y versus x at constant temperature T]The y and x axes for zero and first order reactions, respectively are Answer: 4. zero order ( y = rate and x = concentration), first order ( y = t_(1/2) and x = concenration) Key idea: a horizontal line means y... Integrated rate equations and half-life (zero and first order) 1 answer
The slope of Arrhenius Plot ( k v/s 1/T) of first order reaction is -5×10^3 K. The value of E_a of the reaction is. Choose the correct option for your answer.[Given R=8.314 J K^-1mol^-1] Answer: 1. 41.5 kJ mol ⁻¹ Key idea: Arrhenius equation ln k=ln A-E_a/RT ; slope of ln k vs 1/T is -E_a/R . E_a=-slope× R=5×10³×8.314=41570\... Effect of temperature, Arrhenius equation, activation energy 1 answer
For a reaction A→B, enthalpy of reaction is -4.2 kJ mol^-1 and enthalpy of activation is 9.6 kJ mol^-1. The correct potential energy profile for the reaction is shown in option. Answer: 2. [graph option - see figure] Key idea: Δ H<0 means the product B lies below reactant A; activation enthalpy 9.6 kJ/mol means the p... Effect of temperature, Arrhenius equation, activation energy 1 answer
The rate constant for a first order reaction is 4.606×10^-3 s^-1. The time required to reduce 2.0 g of the reactant to 0.2 g is : Answer: 3. 500 s For a first-order reaction t=2.303/klog[A]₀/[A] . t=frac2.3034.606×10⁻³log2.0/0.2=frac2.3034.606×10⁻³×1=500\ s Answer: opti... Integrated rate equations and half-life (zero and first order) 1 answer
An increase in the concentration of the reactants of a reaction leads to change in : Answer: 4. collision frequency Raising reactant concentration puts more molecules in the same volume, so collisions per second (collision fr... Rate of reaction and factors affecting it 1 answer
For the chemical reactionN_2(g) + 3H_2(g) ⇌ 2NH_3(g)the correct option is : Answer: 3. -d[N₂]/dt = 1/2d[NH₃]/dt Rate of reaction: divide the rate of change of each species by its stoichiometric coefficient (negative... Rate of reaction and factors affecting it 1 answer
If the rate constant for a first order reaction is k, the time (t) required for the completion of 99% of the reaction is given by : Answer: 3. t = 4.606/k For a first order reaction t = 2.303/klog[A]₀/[A] . When 99% is complete, [A] = 0.01[A]₀ , so t = 2.303/klog 100 = 2.... Integrated rate equations and half-life (zero and first order) 1 answer
The correct difference between first- and second-order reactions is that Answer: 3. the half-life of a first-order reaction does not depend on [A] ₀ ; the half-life of a second-order reaction does depend on [A] ₀... Integrated rate equations and half-life (zero and first order) 1 answer
When initial concentration of the reactant is doubled, the half-life period of a zero order reaction Answer: 3. is doubled For a zero-order reaction, t_(1/2)=[A]₀/2k , directly proportional to the initial concentration. If [A]₀ is doubled, t... Integrated rate equations and half-life (zero and first order) 1 answer
A first order reaction has a specific reaction rate of 10^-2\ sec^-1. How much time will it take for 20 g of the reactant to reduce to 5 g ? Answer: 1. 138.6 sec Key idea: first-order integrated rate law t=2.303/kloga/a-x . t=dfrac2.30310⁻²log20/5=230.3×0.602=138.6\ s . Check: 20... Integrated rate equations and half-life (zero and first order) 1 answer
Mechanism of a hypothetical reaction X_2 + Y_2 → 2XY is given below :(i) X_2 → X + X (fast)(ii) X + Y_2 ⇌ XY + Y (slow)(iii) X + Y → XY (fast)The overall order of the reaction will be : Answer: 3. 1.5 Key idea: the rate law comes from the slow step; intermediates must be replaced using the fast equilibrium. Rate = k[X][Y₂] (... Elementary and complex reactions, order and molecularity 1 answer
The decomposition of phosphine [obscured] on tungsten at low pressure is a [obscured] reaction. It is because the Answer: 4. rate is proportional to the surface coverage Key idea: on a metal surface at low pressure the surface is not saturated, so the ra... Rate law, rate constant and units 1 answer
The addition of a catalyst during a chemical reaction alters which of the following quantities ? Answer: 4. Activation energy Key idea: a catalyst provides a different path with a lower activation energy. It does not change the state fun... Effect of temperature, Arrhenius equation, activation energy 1 answer
The rate of a first-order reaction is 0.04\ mol\ l^-1\ s^-1 at 10 seconds and 0.03\ mol\ l^-1\ s^-1 at 20 seconds after initiation of the reaction. The half-life period of the reaction is : Answer: 1. 24.1 s Key idea: for a first-order reaction, rate = k[A] , so the ratio of rates equals the ratio of concentrations. [A]₁₀/[A]₂₀=... Integrated rate equations and half-life (zero and first order) 1 answer