1.7: Chemical Kinetics

Question 1

what does the rate of reaction show us?

Answer 1

How quickly a chemical reactant is used up or how quickly a chemical product is produced.

Question 2

What is the rate of reaction?

Answer 2

The change in concentration of a reactant or a product in a given time
Rate: conc / time
units: moldm-³ S-¹

Question 3

What must happen for a successful reaction to occur?

Answer 3

Bonds must generally broken. In order to break these bonds, energy must be put into the system to overcome the attractive force of the bonds

Question 4

What is activation energy?

Answer 4

The minimum energy required to start a reaction by the breaking of bonds.

Question 5

What do reaction profile diagrams show?

Answer 5

show the energy level of the chemical substances during the course of reaction

Question 6

How is a reaction profile shown for an exothermic reaction?

Answer 6

Gives out heat energy to the surroundings, so the energy of the chemicals goes down

The reaction needs an initial input of energy to overcome the activation
energy for some particles, but the reaction would then be self-sustaining, as
energy is released as the products are formed.

Question 7

How is a reaction profile shown for an endothermic reaction?

Answer 7

It takes in heat from the surroundings so the energy of the chemicals goes up

If the reaction is endothermic, the profile would be reversed, and the reaction
will require continuous energy input, but the activation energy is still likely
to be higher than the energy of the products.

Question 8

What is activation energy shown as on a reaction profile?

Answer 8

Seen as an energy hump (or peak)

Question 9

What factors can affect the rate of reaction?

Answer 9

• concentration
• pressure
• catalyst
• temperature
• surface area

Question 10

What must particles of in order for a chemical reaction to occur?

Answer 10

• collide
• have energy equal to or greater than activation energy
• collide with correct orientation

Question 11

What is the effect of concentration/pressure on RoR?

Answer 11

As the conc increases, there are more particles in a specific volume. Meaning particles will collide more frequently resulting an increase in RoR

The conc of gases can be increased by applying greater pressure to the system.

Question 12

What is the effect of temperature on RoR?

Answer 12

• more particles have energy greater than the activation energy, E>Ea
• more successful collisions
• increase rate of reaction

Question 13

What is the Maxwell-Boltzmann Distribution of Molecular Energies?

Answer 13

The Maxwell-Boltzmann distribution curve shows the amount of energy that gaseous molecules have in a set amount of gas.

A few particles have little energy, most particles have an average amount of energy and a few particles have a very high amount of energy.
This is the theoretical energy the particles contain within their bonds and as kinetic energy.
The area under the curve represents the number of particles.

Question 14

What are the two main types of catalysts?

Answer 14

heterogeneous and homogeneous.

Question 15

What happens in a heterogeneous reaction?

Answer 15

The catalyst is in a different phase from the reactants

Question 16

What happens in a homogeneous reaction?

Answer 16

The catalyst is in the same phase as the reactants.

Question 17

What is a phase?

Answer 17

Solids, liquids (also liquids which are immiscible), and gases
E.g. Oil and water.

Question 18

What is an example of a homogeneous catalyst?

Answer 18

1. Esters can be formed by the reaction of a carboxylic acid and an alcohol. acts as a catalyst for the reaction.

Question 19

What are enzymes?

Answer 19

• biological catalysts
• Enzymes increase the rate of a specific chemical reaction, as the substrate is reacted in an active site which has a particular
  shape. Enzymes only work at a specific range of temperatures, with the optimum temperature being around 40 ° C, as enzymes are made up of proteins which become denatured at higher temperatures and are also affected by changes in pH

Question 20

What are examples of heterogeneous catalysts?

Answer 20

1. In the Haber process nitrogen and hydrogen gas is passed over a solid catalyst.
2. A catalytic converter platinum, rhodium and palladium.

Question 21

What is a catalytic converter

Answer 21

These metals catalyse a range of reaction which ultimately result in unburnt
hydrocarbons, carbon monoxide and nitrogen oxides being converted into
nitrogen, carbon dioxide and water.

Question 22

How does a heterogeneous catalyst work?

Answer 22

Question 23

How does Adsorption work?

Answer 23

Reactant molecules are adsorbed, bond onto catalyst surface

Question 24

How does reaction work as a heterogeneous catalyst?

Answer 24

Reactant held in right orientation and bonds in the reactants weaken.
Reaction takes place.

Question 25

What occurs in desorption in heterogeneous catalysts?

Answer 25

Products desorb leaving site vacant ready for other reactant molecules.

Question 26

What is rate defined as in Rate of reaction?

Answer 26

Rate is defined as a change in concentration with time.

Question 27

What is an example of a rate of reaction experiment?

Answer 27

Disappearing cross, As the reaction occurs, a solid precipitate of sulfur is formed, so we can see the formation of the solid. We can determine the rate of the reaction by timing how long it takes for an amount of solid to form. Most simply – we look through the solution to a cross and time how long it takes for the cross to disappear

Question 28

What is the symbol equation for the disappearing cross reaction?

Answer 28

Na2S2O3(aq) + HCl(aq)  NaCl(aq) + H2O(l) + SO2(aq) + S(s)

Question 29

What is the equation for the reaction used to test RoR (Magnesium with hydrochloric acid)

Answer 29

Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)

Question 30

How could the magnesium with hydrochloric acid reaction be measured?

Answer 30

Hydrogen gas is produced so the volume of gas could be measured over a period of time. OR the mass of the reaction mixture could be measured over a period of time. The mass will decrease as the hydrogen gas escapes.

Question 31

How could you determine the initial rate of reaction?

Answer 31

Plot a graph of volume or mass against time. Draw a tangent to the curve at the start of the reaction, 0 seconds. Measure the gradient of the tangent. The initial gradient = initial rate of reaction.

Question 32

What is rate proportional to?

Answer 32

The concentration of the reactant raised to some power.

Question 33

What is the power in the concentration of the reactant called?

Answer 33

order of reaction

Question 34

what happens when you change conc of reactant?

Answer 34

Changing the concentration of a reactant can have a profound effect on the rate of reaction

Question 35

What happens when you double the conc of a reactant?

Answer 35

would be expected to produce a doubling in the rate of reaction. This may be the case but not necessarily. The rate of reaction may increase by a factor of four or the rate may not change at all.

Question 36

What does this Rate ∝ [A]x mean?

Answer 36

- where ∝ means proportional to, - [A] means concentration (e.g. of A) in moldm-3 - and x is the order of reaction.

Question 37

what does this 'Rate ∝ [A]x [B]y' mean?

Answer 37

combination of 2 meanings of rate for reactant A and B

Question 38

what can proportionality be converted into?

Answer 38

equality by the introduction of a constant.

Question 39

What can 'Rate ∝ [A]x [B]y' be converted into?

Answer 39

an equality by the addition of k, the specific rate constant.

Question 40

What is the rate equation?

Answer 40

Rate = k [A]x [B]y

Question 41

What does a rate equation tell us?

Answer 41

The particles which are involved in the slowest step of a reaction mechanism. This is called the rate determining step

Question 42

What does the order of reaction do?

Answer 42

tells us how many molecules are involved in the rate determining step.

Question 43

What do we say if one, two, no molecule is involved?

Answer 43

- If one molecule is involved, then we say that the order is first order. - If two molecules are involved, then we say that the order is second order. - If the molecule is not involved, then the order is zero order.

Question 44

How do we determine orders?

Answer 44

Using experimental data, which changes the concentration of individual reactants and looks at the effect on initial rate of reaction.