rate and extent of chemical change

Question 1

units of rate of reaction

Answer 1

g/s or cm³/s or mol/s

Question 2

how can you calculate the rate of reaction at a specific point on a curve

Answer 2

draw a tangent to the curve at that point and calculate the gradient

Question 3

the 5 factors affecting rate of chemical reactions

Answer 3

• concentrations of reactants in solution
• pressure of reacting gases
• surface area of solid reactants
• temperature
• presence of catalysts

Question 4

how does increasing concentration of reactants in solution affect the rate of a chemical reaction

Answer 4

• reactions are initially rapid (steep gradient) as there is a high concentration of reactant molecules
• this means there’s a greater number of successful collisions per second because there’s a greater concentration in the same volume
• as reaction progresses, reaction slows down as number of reactant molecules are running out so the frequency of successful collisions is decreasing, decreasing the rate of reaction
• graph then levels off because reactions stop due to all of reactants running out, so no more collisions take place

Question 5

how does increasing pressure of reacting gases affect the rate of a chemical reaction

Answer 5

• increasing pressure results in more reactant particles per given volume
• hence, particles are very close together due to high pressure, meaning there will be more frequent successful collisions between particles
• therefore, as we increase pressure, the rate of reaction increases
• levels off and rate of reaction decreases in the same way as the increasing concentration factor due to reduction in reactant molecules

Question 6

how does increasing surface area of solid reactants affect the rate of a chemical reaction

Answer 6

• as we increase the surface area of a reactant solid, the rate of reaction increases
• increasing surface area means more particles are exposed, meaning more particles can successfully collide and react per second
• smaller sized solid reactants have a greater SA:Vol ratio than larger sized, increasing rate of reaction
• this helps to explain why powdered reactants show a higher rate of reaction than granules as they are smaller and finer, meaning they have a larger SA:Vol

Question 7

what is rate proportional to

Answer 7

the concentration/pressure
- if we double concentration/pressure, we double the rate of reaction
- this results in a faster rate of reaction if doubled, as the gradient is greater

Question 8

how does increasing temperature of reactants affect the rate of a chemical reaction

Answer 8

• particles most collide with sufficient energy, which is the activation energy
• increasing temperature increases kinetic energy of particles, meaning they will have enough energy to meet the activation energy
• this increases frequency of successful collisions because more particles have the ability to collide each second
• increasing kinetic energy also means particles travel faster and collide successfully with other particles much more frequently, which again, increases the rate of reaction

so, increasing temperature increases RoR by
a) increasing frequency of collisions
b) makes collisions more energetic

Question 9

how does presence of catalysts affect the rate of a chemical reaction

Answer 9

• catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy
• lowering activation energy means more particles can meet the activation energy level, meaning more collisions are classed as successful per second, thus rate of reaction increases

Question 10

what are catalysts

Answer 10

substances that alter the rate of reaction without being used up during the reaction

Question 11

define activation energy

Answer 11

the minimum amount of energy that particles must have to react

Question 12

describe collision theory

Answer 12

chemical reactions can occur only when reacting particles collide with eachother and with sufficient energy

Question 13

what are reversible reactions

Answer 13

reactions in which the products of the reaction can react again to produce the original reactants

Question 14

what happens if a reversible reaction is exothermic in one direction? also refer to energy

Answer 14

it will be endothermic in the opposite direction, and the same amount of energy is transferred in each case

Question 15

what happens if a reversible reaction takes place in a closed system

Answer 15

the forward and reverse reactions will eventually reach equilibrium, in which they occur at exactly the same rate

Question 16

what happens when equilibrium is reached

Answer 16

the concentrations of reactants and products remains constant, because the rates of forward and reverse reactions are equal
- in other words, no net change in concentrations of reactants and products

Question 17

what to note about concentrations at equilibrium in reactants and products

Answer 17

although the concentrations remain constant, they may not necessarily be the same - e.g., we might have 60% reactants and 40% products

Question 18

what happens if a change is made to any of the conditions in a system at equilibrium

Answer 18

the system responds to counteract the change
- if there was a high yield of products but a relatively low yield of reactants, the position of equilibrium lies to the left
- if there was a high yield of reactants but a relatively high yield of reactants, the position of equilibrium lies to the right

Question 19

what does le chatelier’s principle state

Answer 19

if we change the conditions of a reversible reaction, the position of equilibrium will shift to try and counteract the change

Question 20

what occurs to the position of equilibrium if we increase the temperature of a system

Answer 20

• the relative amount of products at equilibrium increases for an endothermic reaction
• the relative amount of products at equilibrium decreases for an exothermic reaction

this is because if we increase temp, the reverse reaction would take place at a faster rate so that equilibrium lies to the left - this way, the endothermic reaction will happen at a faster rate, absorbing more heat energy, and this decreases the temperature of the system back down. this results in a higher yield of reactants

Question 21

what occurs to the position of equilibrium if we decrease the temperature of a system

Answer 21

• the relative amount of products at equilibrium decreases for an endothermic reaction
• the relative amount of reactants at equilibrium increases for an exothermic reaction

this is because a decrease in temp means the exothermic reaction will take place at a faster rate to release heat energy. this causes equilibrium to lie to the right, causing an increase in temperature to counteract the change and a higher yield of products can be seen

Question 22

what happens if the concentration of one of the reactants or products is changed in a closed system for a reversible reaction

Answer 22

the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again

Question 23

what happens if the concentration of a reactant is increased

Answer 23

more products will be formed until equilibrium is reached again

Question 24

what happens if the concentration of a product is decreased

Answer 24

more reactants will react until equilibrium is reached again

Question 25

what happens if you increase pressure for gaseous reactions at equilibrium

Answer 25

an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules, causing yield of side with fewer molecules to increase

Question 26

what happens if you decrease pressure for gaseous reactions at equilibrium

Answer 26

a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules, causing yield of side with larger molecules to increase