Rates of Reaction and Energy Changes

Question 1

How do measure rate of reaction?

Answer 1

amount of reactant used/product formed

time

Question 2

What are three examples of how rate of reaction can be measured?

Answer 2

1. precipitation
2. change in mass
3. volume of gas given off

Question 3

How can the rate of a reaction be measured using precipitation?

Answer 3

1. mix two transparent solutions in a flask over a piece of paper with a mark on it
2. observe how long it takes for the mark to be obscured (by the precipitate) - the faster it takes the faster the reaction

Question 4

Why is using precipitation a bad way of measuring rate of reaction?

Answer 4

it is subjective - people might not agree on when the mark can no longer be seen

Question 5

How can the rate of a reaction be measured using change in mass?

Answer 5

1. put solution on a mass balance - as it reacts gas is released meaning that mass will be lost - the quicker it is lost, the faster the reaction - the gas released may be harmful so use a fume cupboard

Question 6

How can the rate of a reaction be measured using the volume of gas given off?

Answer 6

1. use gas syringe to measure volume of gas given off
2. the more gas collected during a set time interval - the faster the reaction - be careful the right size gas syringe is used for the experiment

Question 7

How can you measure how surface area affects rate of reaction?

Answer 7

1. set up a conical flask with marble chips in dilute HCl attached to a gas syringe which will measure how much CO2 is produced
2. take readings of how much CO2 is uptaken at regular time intervals
3. repeat experiment with same volume and concentration of acid but with smaller marble chips and then with marble powder to see how surface area affects rate of reaction

Question 8

What affects the rate of reaction? (3)

Answer 8

1. using finer particles gives marble larger surface area
2. more concentrated solutions have higher rates of reaction (or higher pressure for gases)
3. higher temperature means a higher rate of reaction

Question 9

How can you measure how rate of reaction changes with temperature?

Answer 9

(using a precipitation reaction)

1. measure out fixed volumes of Sodium Thiosulfate and Hydrochloric Acid (transparent solutions) in a measuring cylinder
2. use water bath to gently heat the solutions to desired temperature (before you mix them)
3. mix solutions in a conical flask and place over a piece of paper with a mark and watch it disappear through the cloudy, yellow sulfur produced and time this
4. repeat at different temps to see how rate of reaction varies

Question 10

Can you use graphs to calculate rates of reaction?

Answer 10

(pg 131 - incl. tangents to find gradient of a curve)

Question 11

What is the collision theory?

Answer 11

the rate of a reaction depends on the collision frequency of reacting particles (obvious)

Question 12

What is the activation energy?

Answer 12

the minimum energy that particles need for a successful collision

Question 13

Why does increasing the temperature of particles increase rate of reaction? (2 ways)

Answer 13

1. particles move faster at a higher temp so they’re more likely to collide
2. increases energy of collisions - more successful collisions as they’re more likely to reach the activation energy

Question 14

Why does increasing concentration (or pressure in a gas) increase rate of reaction?

Answer 14

more concentrated solution means more particles in the same volume so more collisions will occur

Question 15

Why does using smaller particles in a solid increase rate of reaction?

Answer 15

breaking a large solid into smaller pieces increases its surface area to volume ratio and so frequency of collisions will increase as there is more area to collide with

Question 16

What is a catalyst?

Answer 16

a substance which increases rate of reaction without being chemically changed or used up in the reaction

Question 17

How do catalysts work?

Answer 17

catalysts provide an alternative reaction pathway which has a lower activation energy - so more particles will have at least the minimum amount of energy needed for a collision

Question 18

Do you know what reaction profiles look like?

Answer 18

pg 133

Question 19

What do enzymes do?

Answer 19

biological catalysts - catalyse chemical reactions in living cells (e.g. respiration/photosynthesis/protein synthesis catalysed)

Question 20

What do enzymes in yeast cells do?

Answer 20

used in the fermentation process in alcohol - catalyse the reaction that converts sugars into ethanol and CO2

Question 21

What is an exothermic reaction?

Answer 21

a reaction that gives out energy to the surroundings usually by heat

Question 22

What is an endothermic reaction?

Answer 22

a reaction that takes in energy from the surroundings e.g. by heat

Question 23

What do reaction profiles show?

Answer 23

the amount of energy levels of the reactants and products in a reaction - e.g. exothermic moves down as reactants have more energy than the products (endothermic vice versa)

Question 24

What is activation energy?

Answer 24

the minimum amount of energy needed for bonds to break and a reaction to start

Question 25

How is activation energy shown on reaction profiles?

Answer 25

activation energy is the difference between the reactants and the highest point of energy on the graph