module three: chapter ten: reaction rates and equilibrium

Question 1

what is the rate of a chemical reaction?

Answer 1

measures how fast a reactant is being used up or how fast a product is being formed

Question 2

what is the equation for rate of reaction?

Answer 2

rate= change in concentration/ time

Question 3

when is the rate of reaction the fastest?

Answer 3

at the start as each reactant is at it’s highest concentration

Question 4

why does the rate of reaction slow down?

Answer 4

the reactants are being used up and their concentrations decrease

Question 5

how can you alter the rate of reaction?

Answer 5

concentration, temperature, catalyst, surface area

Question 6

when will a collision be effective?

Answer 6

the particles collide with the correct orientation, the particles have sufficient energy to overcome the activation energy barrier

Question 7

how does increasing the concentration affect the rate of reaction?

Answer 7

an increase in concentration increases the number of particles in the same volume, the particles are closer together and collide more frequently.

Question 8

how does increasing the pressure of a gas affect the rate of reaction?

Answer 8

the concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume, the has molecules are closer together and collide more frequently

Question 9

how can you follow the progress of a reaction?

Answer 9

monitor the removal of a reactant, following the formation of a product

Question 10

how can you determine the rate of reaction if it produces a gas?

Answer 10

monitor the volume of gas produced at regular time intervals using gas collection, monitor the loss of mass of reactants using a balance

Question 11

what is a catalyst?

Answer 11

a substance that changes the rate of a chemical reaction without undergoing any permanent change to itself, it is not used up in a reaction, it may react with the reactant to form an intermediate or may provide a surface on which the reaction can take place, the catalyst is regenerated at the end of a reaction

Question 12

what is a homogeneous catalyst?

Answer 12

it has the same physical state as the reactants, it reacts with the reactants to form an intermediate which breaks down to give the product and regenerates.

Question 13

what is a heterogeneous catalyst?

Answer 13

it has a different physical state to the reactants, they are usually solids in contact with gaseous reactants or in a solution. reactant molecules are absorbed on to the surface of the catalyst where the reaction takes place. after the reaction the product molecules leave by desorption.

Question 14

why are catalysts sustainable?

Answer 14

they increase the rate of reactions by lowering the activation energy which reduced the temperature needed and the energy required so less electricity and fossil fuels are used, this can cut costs and increase profitability.

Question 15

what does the boltzmann distribution show?

Answer 15

spread of molecular energies in gases

Question 16

what are the features of a boltzmann distribution?

Answer 16

• no molecules have zero energy- curve starts at origin
• area under curve is equal to total number of molecules
• no maximum energy for a molecule- curve does not meet x axis

Question 17

what is the effect of temperature on the boltzmann distribution?

Answer 17

as temp increases, average energy also increases. the peak of graph is lower on y axis and further along on x axis.

Question 18

what happens at higher temperature on boltzmann distribution?

Answer 18

• more molecules have energy greater than or equal to activation energy
• greater proportion of collisions lead to a reaction- increasing rate.
• collisions more frequent as molecules moving faster- increased energy of molecules is more important than collisions

Question 19

what is the effect of catalysts on the boltzmann distribution?

Answer 19

provides an alternative reaction route with a lower activation energy (Ec). compared to Ea, a greater proportion of molecules have an energy equal to or greater than lower Ec. more molecules react to form products- increase in rate.

Question 20

what is dynamic equilibrium?

Answer 20

both forward and backwards reaction happens at same time- closed system takes place.

Question 21

what is le chateliers principle?

Answer 21

when a system in equilibrium is subjected to external change the system readjusts itself to minimise effect of that change

Question 22

what is the effect of concentration on equilibrium?

Answer 22

it will change the rate of the forward or reverse reaction: if more products formed, equilibrium shifted right, if more reactants formed, equilibrium shifted left.

Question 23

what is the effect of temperature on equilibrium?

Answer 23

it depends on the sign of ∆H, forward and reverse reactions have same value for enthalpy change. an increase in temp shifts equilibrium in endothermic direction. a decrease in temp shifts equilibrium in exothermic direction

Question 24

what is the effect of pressure on equilibrium?

Answer 24

equilibrium only changes if there are more gaseous molecules on ones side of equation. increasing pressure will shift equilibrium to side with fewer molecules

Question 25

what is the effect of catalysts on equilibrium?

Answer 25

does not change position of equilibrium, it speeds up rates of forward and reverse reaction

Question 26

what is the equation for Kc?

Answer 26

Kc= [C]c [D]d / [A]a [B]b

Question 27

what does Kc value tell us?

Answer 27

value of 1: equilibrium is halfway between reactant and product.
value > 1: equilibrium is towards products
value < 1: equilibrium is towards reactants