Reaction Rates

Question 1

what is the rate of a chemical reaction determined by

Answer 1

the change in the concentration of a reactant or product per unit of time

Question 2

what are the two simple ways you could practically calculate the rate of a reaction

Answer 2

• measuring how fast one of the reactants is being used up

- or measuring how fast one of the products is formed

Question 3

what would a graph of concentration against time look like

Answer 3

• the graph for half lives
• it starts at the top and the gradient of the curve becomes less steep as time goes by
• until it straightens out at the bottom

Question 4

how would you work out the rate of the reaction at a specific point in time using the graph

Answer 4

• draw a tangent at that point in time

- calculate dy / dx

Question 5

what does the line becoming horizontally straight at the end indicate and why

Answer 5

• that the reaction is complete
• because there is no change in the concentration of the substance as time is passing
• meaning the reaction is no longer taking place

Question 6

what is the unit for the rate of reaction using concentration against time

Answer 6

mol dm-3 s-1 (concentration per second)

Question 7

what would that first graph specifically be measuring the concentration of and why

Answer 7

• the reactant

- because its concentration is decreasing as the reactant is being use up in the reaction

Question 8

what would a graph for the concentration of a product against time look like

Answer 8

• it would start at the origin
• where the line would start with a high gradient but its gradient would begin decreasing exponentially
• until the line is completely horizontal

Question 9

what is the first condition that needs to be met in order for molecules to react with reach other at all

Answer 9

they must collide with each other

Question 10

after they collide, what two requirements need to be met in order for a reaction to actually occur

Answer 10

• the molecules must collide with sufficient energy (activation energy)
• the molecules must collide in the correct orientation

Question 11

what is activation energy

Answer 11

the minimum amount of energy required that colliding particles must possess in order for a reaction to occur

Question 12

what happens if particles collide with less than the activation energy required

Answer 12

they bounce apart and no reaction occurs

Question 13

why does there need to be activation energy in the first place in order for reactions to occur

Answer 13

• so that there is enough energy to break bonds that the particles originally have in order to make new ones
• if there is not enough energy to break the bonds the particles will not be available for reactions

Question 14

what specifically needs to happen in order for ethene and hydrogen bromide to react and why

Answer 14

• the hydrogen atom from the HBr needs to collide with the double C=C bond
• because the partially positively charged H atom and the negative region of the C=C bond need to be in contact in order for any bond breaking to happen

Question 15

what do you call it when the shape of a molecule influences reactions

Answer 15

a steric factor

Question 16

how would a steric hindrance commonly occur

Answer 16

• when an atom or group of them in a molecule hinder the course of a reaction
• if the atom or group is really large it can get in the way of an attacking species