Rate and extent of chemical change

Question 1

how do you calculate the mean rate of a chemical reaction using either the quantity of reactant or product?

Answer 1

• quantity of reactant used/ time taken
  or
• quantity of product formed/ time taken

Question 2

what is rate usually measured in?

Answer 2

• usually g/s or g/min
• or Mol s-1
• could be in cm3/s or mol/s etc.

Question 3

what are on the axis of a rate graph

Answer 3

• x= time from start of reaction
  *y= total mass or volume of product

Question 4

how do you measure the rate of reaction off of a graph showing quantity of product formed or reactant used against time?

Answer 4

by using a tangent to work out the gradient

Question 5

what does a steeper line show on a rate graph

Answer 5

greater rate of reaction

Question 6

what does a horizontal line show on a rate graph

Answer 6

that the reaction has finished

Question 7

describe how concentration of a reacting solution or the pressure of a reacting gas affects the rate of a chemical reaction

Answer 7

• the reactant particles become more crowded (more of them or are more squashed up together)
• the frequency of collisions between reactant particles increases
• the rate of reaction increases

Question 8

describe how surface area (to volume ratio) affects rate

Answer 8

• (when a solid is broken up into smaller pieces)
• more reactant particles are exposed at the surface
• the frequency of collisions between reactant particles increases (particles surrounding it have more area to work on)
• the rate of reaction increases
• however the mean energy of the particles does not change

Question 9

describe how temperature affects rate

Answer 9

• reactant particles move more quickly
• the energy of the particles increases
• the frequency of successful collisions between reactant particles increases
• the proportion of collisions which are successful increases
• the rate of reaction increases

Question 10

describe how the presence of a catalyst affects rate

Answer 10

• it provides an alternative
  reaction pathway that has a lower
  activation energy
  than the uncatalysed reaction
• does not change the frequency of collisions
• does increase the frequency of successful collisions because more particles have energy greater than the activation energy, therefore there are more successful collisions
• does not alter the
  products of the reaction
• is not chemically changed or used up at the end of the reaction

Question 11

what is a successful collision?

Answer 11

A collision between reactant particles with enough energy (the activation energy or more than the activation energy) to produce a reaction.

Question 12

define activation energy

Answer 12

The minimum amount of energy that colliding particles must have for them to react.

Question 13

explain the similarities and differences of a line with and a line without a catalyst on a reaction profile

Answer 13

• reactants and products are the same
• overall change in energy is the same
• activation energy is smaller (curve doesn’t go as high)

Question 14

represent a reversible reaction using symbols

Answer 14

A + B ⇌ C + D

Question 15

what is a reversible reaction?

Answer 15

the products of the reaction can react together to produce the original reactants (chemical reaction that can go both ways)

Question 16

is the same amount of energy transferred in a forwards and reverse reaction?

Answer 16

yes

Question 17

if a reversible reaction is exothermic in the forwards direction, what would it be in the reverse direction?

Answer 17

endothermic

Question 18

what is the state of dynamic equillibrium of a reaction?

Answer 18

the point when the forward and reverse reactions occur at exactly the same rate (and the
concentrations of all the reacting substances remain constant)

Question 19

in what conditions does dynamic equilibrium occur?

Answer 19

when a reversible reaction happens in a closed container

Question 20

what happens when a change in conditions is made to a system at equilibrium?

Answer 20

the position of equilibrium moves to counteract the change that was made

Question 21

what does the position of equillibrium depend on?

Answer 21

the conditions of the reaction: pressure, concentration and temperature

Question 22

explain the effects of a change in concentration of reactants or products on the equillibrium position of a reaction

Answer 22

• if concentration of a reactant (on the left) is increased, the equilibrium position moves in the direction away from this reactant
• so more of the products are produced (on the right)
• if one of the products is removed from a reaction (on the right), then the position of equilibrium moves to the right to make more of that product

Question 23

explain the effects of temperature on the equillibrium position of a reaction

Answer 23

• if the reaction is exothermic in one direction, it is endothermic in the other direction
• if the temperature is increased, the equilibrium position will shift in the direction of the endothermic process to decrease the temperature and minimise the change
• if the temperature is decreased, the equilibrium position will shift in the direction of the exothermic process to increase the temperature and minimise the temperature

Question 24

explain the effects of pressure of gases on the equillibrium position of a reaction

Answer 24

if the pressure is increased in a reaction involving gases, the equilibrium position moves in the direction of the fewest molecules of gas, to reduce the pressure

Question 25

what is le chatelier’s principle?

Answer 25

if a chemical system at equilibrium experiences a change in concentration, temperature or pressure, the equilibrium will shift in order to minimise that change.