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Collision theory - class-XII

Description: collision theory
Number of Questions: 28
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Tags: chemical kinetics chemical reactions and equations chemistry how far? how fast? electrochemistry and chemical kinetics
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Two identical balls A & B having velocity of 0.5 m/s & -0.3 m/s respectively. colloid elastically in 1-0. The velocity of A & B after collision will be?

  1. -0.5 m/s, 0.3 m/s

  2. -0.3 m/s,0.5 m/s

  3. 0.5 m/s, 0.3 m/s

  4. 0.3 m/s,0.5 m/s


Correct Option: B

On increasing the temperature by $10^{0}$C:

  1. number of collisions get doubled

  2. value of rate constant does not change

  3. energy of activation increases

  4. number of fruitful collisions gets doubled


Correct Option: D
Explanation:

Increasing the temperature of the substance, increases the fraction of molecules which collide with energies greater than $Ea$. For every $10^0 C$ rise in temperature, the fraction of molecules having energy equal to or greater than Ea gets doubled leading to doubling the rate of reaction.

Hence, the number of fruitful collisions gets doubled.

What happen with the rate of the reaction when the frequency and the number of effective collisions between reacting particles increases?

  1. Increases

  2. Decreases

  3. Remains the same

  4. Approaches zero

  5. None of the above


Correct Option: A
Explanation:

The atoms, ions and molecules can react to form products when they collide with enough kinetic energy and at favorable orientation. On increasing the frequency and the number of effective collisions between reacting particles increases the chance of reactants to come together to form a product  that results in the increase of rate of the reaction

What causes an increase in effective collisions without increasing average energy?

  1. An increase in the reactant concentration

  2. An increase in the temperature

  3. A decrease in pressure

  4. Catalysts


Correct Option: A
Explanation:

As the concentration of the reactant increases, effective collision between the reactant molecules also increases. And thus, the reaction rate also increases. It is the main principle of the collision theory.

Option  A is the correct answer.

Collision diameter is least in case of:

  1. ${ H } _{ 2 }$

  2. $He$

  3. ${ CO } _{ 2 }$

  4. ${ N } _{ 2 }$


Correct Option: C
Explanation:

Collision diameter will be least for the low molecular weight.


$d\space \alpha\space \dfrac{1}{M}$

In this case, $Co _2$ has the highest molecular weight.

So, Option C is correct

Collision frequency of a gas at $1\ atm$ pressure is $Z$. Its value at $0.5\ atm$ will be:

  1. $0.25Z$

  2. $2Z$

  3. $0.50Z$

  4. $Z$


Correct Option: D
Explanation:

$\rightarrow$ As the pressure increases collision frequency increases.

$\rightarrow$ Vice versa pressure is decreased to half, frequency of collision decreases to half.
Hence option $C$ is correct.

If X is the total number of collisions which a gas molecule registers with other molecules per unit time under particular conditions, then the collision frequency of the gas containing N molecules per unit volume is?

  1. X/N

  2. NX

  3. $2NX$

  4. $NX/2$


Correct Option: D

The number of collisions of Ar atoms with the walls of container per unit time?

  1. Increases when the temperature increases

  2. Remains the same when $CO _2$ is added to the container at constant temperature

  3. Increases when $CO _2$ is added to the container at constant temperature

  4. Decreases, when the average kinetic energy per molecule is decreased


Correct Option: A

In a closed flask of $5$l $1.0$g $H _2$ is heated from $300$ to $600$K. Which statement is not correct?

  1. Pressure of the gas increases

  2. The rate of collisions increases

  3. The number of moles of gas increases

  4. The energy of gas molecules increases


Correct Option: A

One mole of helium and one mole of neon are taken in a vessel. Which of the following statements are correct?

  1. Molecules of helium strike the wall of vessel more frequently

  2. Moles of neon apply more average force per collision on the wall of vessel

  3. Molecules of helium have greater average molecular speed

  4. Helium exerts higher pressure than neon


Correct Option: A

When the temperature of an ideal gas is increased at constant pressure?

  1. Collision number increases

  2. Collision frequency increases

  3. Mean free path increases

  4. Number of molecules per unit volume increases


Correct Option: A

An increase in the rate of a reaction for a rise in temperature is due to:

  1. increase in collision frequency

  2. shortening of mean free path

  3. increase in the number of activated molecules

  4. none of the above


Correct Option: A,B,C
Explanation:

An increase in the rate of a reaction for a rise in temperature is due to  increase in collision frequency, shortening of mean free path and increase in the number of activated molecules.
Increasing the temperature increases reaction rates because of the disproportionately large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reaction.

Effective collisions are those in which molecules must:

  1. have energy equal to or greater than the threshold energy

  2. have proper orientation

  3. acquire the energy of activation

  4. all of the above


Correct Option: D
Explanation:

Option (D) is correct. These are the characteristics of effective collisions.
Effective collisions are collision between two reactants with the appropriate orientation & with sufficient energy to overcome the activation energy barrier.
The number of effective collisions increases exponentially with an increase in temperature.

The rate of a chemical reaction generally increases rapidly even for small temperature increase because of a rapid increase in:

  1. Collision frequency

  2. Fraction of molecules with energies in excess of the activation energy

  3. Activation energy

  4. Average kinetic energy of molecules


Correct Option: B
Explanation:

Every chemical reaction whether exothermic or endothermic has an energy barrier which has to be overcome before reactants can be transformed into products. If the reactant molecules have sufficient energy, they can reach the peak of the energy barrier after collision and then they can go to the right side of the slope and consequently change into products. If the activation energy for a reaction is low, the fraction of effective collisions will be large and the reaction will be fast. On the other hand, if the activation energy is high, then fraction of effective collisions will be small and the reaction will be slow. When temperature is increased, the number of active molecules increases, i.e., the number of effective collisions will increase and the rate of reaction will increase.

A chemical reaction occurs as a result of collisions between reacting molecules. Therefore, the reaction rate is given by:

  1. total number of collision occuring in a unit volume per second

  2. fraction of molecules which possess energy less than the threshold energy

  3. total number of effective collisions

  4. none of the above


Correct Option: A,C
Explanation:

According to collision theory, the reaction occurs when molecules collide with each other. The rate is given by:

Rate=${ Z } _{ AB }\times f$

Where $Z _{AB}$=collision frequency of reactants $A$ & $B$.

i.e. total number of collisions occurring in a unit volume per second. &

$f$=fraction of effective collisions.

So, the rate depends on both (A) & (C)

Unit of frequency factor (A) is:

  1. mol/L

  2. mol/L.s

  3. depend upon order of reaction

  4. it does not have any unit


Correct Option: C

How an increase in concentration is related to number of collisions?

  1. Directly

  2. Inversely

  3. Has no effect

  4. None


Correct Option: A
Explanation:

The increase in concentration is directly related to number of collisions.
For higher concentrations, the average distance between molecules is small. Due to this, the number of collisions increases.
For lower concentrations, the average distance between molecules is large. Due to this, the number of collisions decreases.

Assertion: Reactions happen faster at higher temperatures.
Reason: As temperatures increase, there is also an increase in the number of collisions with the required activation energy for a reaction to occur.

  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion

  2. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion

  3. Assertion is true but Reason is false

  4. Assertion is false but Reason is true

  5. Both Assertion and Reason are false


Correct Option: A
Explanation:

When you raise the temperature of a system, the molecules bounce around a lot more. They have more energy. When they bounce around more, they are more likely to collide. 

That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less.

Reason is the correct explanation for assertion.

Which statements describe the condition(s) required for a successful formation of a product in a reaction ?
  1. The collision must involve a sufficient amount of energy, provided from the motion of the particles, to overcome the activation energy

  2. The relative orientation of the particles has little or no effect on the formation of the product

  3. The relative orientation of the particles has an effect only if the kinetic energy of the particles is below some minimum value

  4. The relative orientation of the particles must allow for formation of the new bonds in the product

  5. The energy of the incoming particles must be above a certain minimum value and the relative orientation of the particles must allow for formation of new bonds in the product


Correct Option: E
Explanation:

The statement (E) describe the conditions required for a successful formation of a product in a reaction.
The energy of the incoming particles must be above a certain minimum value (called activation energy) and the relative orientation of the particles must allow for formation of new bonds in the product.

Two molecules collide and a reaction not occur. Which of the following is not a valid explanation for this?

  1. The molecules were not in the proper states of matter.

  2. The molecules did not have enough kinetic energy.

  3. The molecules were not oriented correctly when they struck each other.

  4. The temperature of the reaction mixture was not high enough.


Correct Option: A
Explanation:

For a reaction to occur,

$(i)$ The two molecules must collide with proper orientation.

$(ii)$ They should have high kinetic energy.

$(iii)$ The temperature should be high.

$(iv)$ But the state of matter does not have any effect on the reaction.

The states of matter molecules do not affect the rate of reaction.

What are effective collisions?

  1. Collisions leading to the transformation of reactants to products

  2. formation of activated complex

  3. collison between two reactant to decrease the activation energy

  4. collison between two reactant to overcome activation energy barrier


Correct Option: B,D
Explanation:

 The effective collisions are collisions leading to the transformation of reactants to products.
This will happen when (i) the reactant species collide (ii) the colliding molecules have total kinetic energy equal to or greater than the activation energy of the reaction (iii) the colliding molecules must have proper orientation relative to each other.

The fraction of collisions that posses the energy $E _a$ is given by:

  1. $f = e^{\frac{-Ea}{RT}}$

  2. $f = e^{\frac{Ea}{RT}}$

  3. $f = e^{- Ea.RT}$

  4. $f = e^{Ea.RT}$


Correct Option: A
Explanation:
$k=Pze^{\cfrac {-Ea}{RT}}$
$\cfrac kA$ or $\cfrac k{Pz}= $ Fraction of collision possessing $E _a$
$\therefore$ Fraction of collision possessing energy $E _a= \cfrac kA$
$=\cfrac {Ae^{-E _a/RT}}{A}$
$f=e^{-E _a/RT}$

For a chemical reaction to occur, all of the following must happen except.

  1. A large enough number of collisions must occur

  2. Chemical bonds in the reactants must break

  3. Reactant particles must collide which enough energy for change to occur

  4. Reactant particles must collide with correct orientation


Correct Option: A
Explanation:
For a chemical reaction to occur, bonds must break in reactant so that new bonds are formed. Moreover, reactants must collide with energy greater than activation energy for change to ocur.
Also, reactant must have proper orientation for reaction.
However, it is not necessary that enough number of collisions must occur.

According to the collision theory, most molecular collisions do not lead to a reaction. Which of the following is(are) necessary for collisions to successfully lead to the reaction?

  1. The total kinetic energy of the collision must be greater than some minimum value.

  2. A catalyst must be present at the collision.

  3. The colliding particles must be properly oriented in space when they collide.

  4. None of the above.


Correct Option: A,C
Explanation:

The conditions (A) and (C) are necessary for the collisions to successfully lead to reaction .
For an effective collision, molecules must possess sufficient energy (called activation orientation) and proper orientation. The presence of catalyst is not essential for collision as reaction can occur even in absence of a catalyst.

What does it mean when a collision is elastic?

  1. No energy is gained or lost.

  2. Energy is gained.

  3. Energy is lost.

  4. The particles can stretch out.

  5. The particles slow down.


Correct Option: A
Explanation:

(A) , No energy is gained or lost.
An elastic collision is an encounter between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter. Elastic collisions occur only if there is no net conversion of kinetic energy into other forms

Among the following which will decrease the rate of the reaction?
i. Using highly concentrated reactants
ii. Decreasing the temperature by $25\ K$
iii. Stirring the reactants

  1. i only

  2. ii only

  3. i and iii only

  4. ii and iii only

  5. i, ii, and iii


Correct Option: B
Explanation:

$\bullet $  Using highly concentrated reactants will increase the rate of the reaction as rate is directly proportional to the 

    concentration of reactants.
$\bullet $  Decreasing the temperature by 25 K will decrease the rate constant and hence the rate of reaction will decrease.
$\bullet $  Stirring the reactants increases the rate of interaction between the reactants and hence the rate of reaction increases.
$\therefore $ The correct answer is (ii) only.

_________ increases effective collisions without increasing average energy.

  1. An increase in the reactant concentration

  2. An increase in the temperature

  3. A decrease in pressure

  4. Catalysts

  5. $\displaystyle pH$


Correct Option: A
Explanation:

An increase in the reactant concentration increase effective collisions without increasing average energy.

This is due to the fact that molecules comes more and more closer, hence, they tends to collide more easily.

In a reaction carried out at 400 k, $0.0001\%$ of the total number of collisions are effective. The energy of activation of the reaction is:

  1. zero

  2. 7.37 k cal/mol

  3. 9.212 k cal/mol

  4. 11.05 k cal/mol


Correct Option: D
Explanation:

We know that, Arrhenius equation for calculation of energy of activation of reaction with rate constant $K$ and temeperature $T$ is 

$K=A$ $e^{-Ea/RT}$
where, $Ea$= Arrhenius activation energy
$A$= pre exponential factor (frequency factor)

Now, $e^{-Ea/K _BT}$= Fraction of collision having more than activation energy
where, $K _B=$ Boltzmann constant
Given, $T=400$ $K$       and effective collision= $0.0001$%

$\Rightarrow$ Effective Collision= $e^{-Ea/K _BT}$
$\Rightarrow$ $0.0001$%= $e^{-Ea/1.3\times 10^{-23}\times 400}$
$\Rightarrow$ $10^{-6}$= $e^{-Ea/1.3\times 10^{-23}\times 400}$

$\Rightarrow$ $2.303\times \log 10^{-6}$= $\cfrac {-Ea}{1.3\times 10^{-23}\times 400}$
$\Rightarrow$ $2.303\times (-6)$= $\cfrac {-Ea}{1.3\times 10^{-23}\times 400}$

$\Rightarrow$ $Ea$= $1.3\times 10^{-23}\times 400\times6\times 2.303=7.19\times 10^{-20}$ $J/mol$

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