Kinetics

Question 1

What is the rate of reaction?

Answer 1

The number of moles of reactant is used up or the number of moles of product produced in a set time

Question 2

What is the calculation for rate of reaction?

Answer 2

Rate = change in concentration/ time

Question 3

What are the four factors rate of reaction is affected by and why?

Answer 3

1) concentration and pressure: higher concentrations of reactant leads to faster reaction because with more particles there is a higher chance of succesful collisions taking place. Increasing gas pressure effectively increases the concentration of a gas which leads to an increase in the rate of reaction
2) temperature: most reactions go faster qt higher temperature. At higher temperatures particles have more kinetic energy, so collisions are more frequent. And more particles will have the required activation energy so there is more chance of succesful collisions
3) particle size of a solid: smaller particles have a larger surface area than the same mass of larger particles. Therefore more particles can collide with the solid so there is a greater chance of succesful collisions
4) presence of a catalyst. They provide an alternate pathway with a lower activation energy meaning that more of the particles have the required activation energy so successful collisions are more likely. They are chemically unchanged after the reaction has taken place

Question 4

What is the key idea behind collision theory?

Answer 4

That for a reaction to occur between particles the particles must collide

Question 5

What is the key to whether two colliding particles react?

Answer 5

The amount of kinetic energy they store

Question 6

What does the collision energy depend on and why?

Answer 6

• the speed at which the molecules are moving: affects how much kinetic energy they store
• the angle at which they collide: the energy transfer when a head-on collision happens is greater than the energy of a collision at an angle

Question 7

Apart from activation energy what else much molecules have when they collide and what is this?

Answer 7

• the correct orientation
• this means that the molecules are aligned correctly: they approach eachother at the right angle and right direction. E.g when ethene reacts with hydrogen chloride the hydrogen end of hydrogen chloride must collide with the double bond

Question 8

What is the combined kinetic energy of two molecules often called?

Answer 8

The collision energy

Question 9

What is the activation energy EA

Answer 9

The minimum energy needed by colliding molecules for them to react. The energy transferred enables bonds in the reactants to be broken before new ones can form in the products

For a collision to lead to a reaction the collision energy must be equal or greater than the activation energy

Question 10

Draw the energy profile diagram for an exothermic and endothermic reaction

Answer 10

Check snap camera roll

Question 11

What is the Maxwell-Boltzmann distribution of molecular energies?

Answer 11

At a given temperature the molecules in a gas move at different speeds. They do not all have the same energy. At any instant in time, a tiny fraction of the molecules have very high kinetic energies. A tiny fraction have very low kinetic energies. Most molecules have kinetic between the two extremes. There is a distribution of kinetic energies

Question 12

Draw a maxwell-Boltzman distribution of energies

Answer 12

Snap camera roll

Question 13

At higher temperatures why does the rate of reaction increase (in regard to the maxwell-Boltzman curve) and draw the maxwell-Boltzman curve for a higher temperature

Answer 13

Because at higher temperatures a greater proportion of the molecules have energies equal to or greater than the activation energy. Therefore more effective collisions happen that result in a reaction

Question 14

What does it mean if the area under the curve of a maxwell-Boltzman curve is double that of another?

Answer 14

That it has double the amount of molecules (double the concentration)

Question 15

For reactions in a solution what are the units of rate?

Answer 15

Mol dm-3 s-1

Question 16

Why is the rate of reaction the fastest at the beginning?

Answer 16

Because the concentration of reactants is the greatest

Question 17

At the end of a reaction why is the rate of reaction zero?

Answer 17

Because at least one of the reactants must have been used up and therefore no more collisions between reactant particles can occur and lead to a reaction

Question 18

What are rate curves?

Answer 18

Graphs show the rate of reaction. The rate of reaction at any instant is given by the gradient of the curve at that instant

Question 19

How can the rate of reaction at any time be calculated using a rate curve?

Answer 19

By drawing a tangent to the curve and making the tangent the hypotenuse of a triangle

Question 20

Why does increasing the concentration increase the rate of reaction?

Answer 20

It increases the frequency of collisions because there are more molecules in the same volume. Therefore, the number of collisions where the collision energy is equal to or greater than the activation energy increases.

Question 21

Why does increasing the pressure of gases increase the rate of reaction?

Answer 21

Because it reduces the volume and increases the concentration of gas moleces. The molecules are forced closer together. This increases the rate of collisions and therefore, there will ne more effective collisions and the reaction rate increases

Question 22

What are the two types of water bath that can help maintain temperature?

Answer 22

• a simple water bath (beaker of water heated on a tripod and gauze)
• thermostated water bath

Question 23

When doing an investigation to how the rate of reaction changes with temperature what changes can we measure and what apparatus would we use?

Answer 23

• measure an increase or decrease in mass - use a balance of suitable range and precision
• measure the gas given off - use gas syringe
• colour change - use a colorimeter or spectrophotometer
• ‘invisible’ change in solution- sample withdrawn and analysed immediately before further reaction takes place
• pH - use pH meter and electronic probe

Question 24

What is a catalyst?

Answer 24

A substance that alters the rate of a chemical reaction without undergoing any permanent chemical change itself. It may change physically

Question 25

Why are only small amounts of a catalyst needed?

Answer 25

Because the catalyst can be used over and over again

Question 26

Why do you use a catalyst?

Answer 26

Because it allows milder conditions to be used

Question 27

How do catalysts alter the rate of a reaction?

Answer 27

• they provide an alternative route with at least one intermediate being formed between the catalyst and one or more of the reactants. This is a two step process
• each step has a lower activation energy than the non-catalysed reaction
• therefore more reactants have sufficient energy to react when they collide so the rate of reaction increases

Question 28

What does the profile of two-step catalysed reactions ohave?

Answer 28

Two peaks corresponding to the formation of the two transition states, one for the change from reactants to reactive intermediate and the second for the change from reactive intermediate to products

Question 29

In a two-step process what controls the rate of reaction?

Answer 29

The step with the highest activation energy known as the rate-determining step

Question 30

Draw a maxwell-Boltzman distribution curve for the effect of using a catalyst

Answer 30

Snap camera roll