C8. Rates Of Reaction (Y11 - Autumn 1)

Question 1

🟢 What are the 2 Equations that can work out the Mean Rate of Reaction (and what unit is this measured in)

Answer 1

Mean Rate of Reaction = Quantity of reactant used up (g or cm^3) / Time
Or
Mean Rate of Reaction = Quantity of product used up (g or cm^3) / Time

Units can be: g/s, or cm^3/s

Question 2

🟢 Rate of Reaction Pyramid

Answer 2

‘Reactant used of product formed’ (g/cm^3) is at the top of the triangle, while the ‘Rate of Reaction’ (g/s or cm^3/s) is in the bottom left, leaving the ‘Time’ (s) in the bottom right.

Question 3

🟢 State the full Chemical Equation (Balanced with state symbols), Half Equation, Ionic Equation, and the appearance of each compound:

Magnesium + Hydrochloric Acid –> Magnesium Chloride + Hydrogen

Answer 3

Full Chemical Equation:

Mg (s) + 2HCl (aq) –> MgCl2 (aq) + H2 (g)

Half Equation:

Mg –> Mg^2+ + 2e-
(Magnesium is Oxidised)

Appearances:

• Magnesium - Grey Solid
• Hydrochloric Acid - Colourless Solution
• Magnesium Chloride - Colourless Solution
• Hydrogen - Colourless Gas

Question 4

🟢 How Does The Follow Affect The Reaction Time of Magnesium ribbon and Hydrochloric Acid:

(Faster or Slower + Explanation)

• Constant Stirring
• Replacing 25cm^3 acid with 25cm^3 water
• The Mg ribbon folded twice
• Half the Volume of Acid
• Acid at 40°C

Answer 4

Constant Stirring:
Prediction - Faster
Explanation - This means the particles will have more energy and have more frequent collisions

Replacing 25cm^3 acid with 25cm^3 water:
Prediction - Slower
Explanation - Fewer particles per cm^3, therefore there are less frequent collisions

The Mg ribbon folded twice:
Prediction - Slower
Explanation - Smaller surface area, therefore there are less frequent collisions

Half the Volume of Acid:
Prediction - Slower
Explanation - Fewer particles, therefore there are less frequent collisions

Acid at 40°C:
Prediction - Faster
Explanation - Particles have more energy, therefore there are more frequent, energetic collisions

Question 5

🟢 What Things Happen in a Reaction after the following:

• Reactants Moving too Slowly
• Reactants Not Facing the Right Way
• Reactants Collide in correct direction and with Enough Energy

Answer 5

In Chemical Reactions, for the reactants to react, they must hit each other in the right way with a good connection. It’s like in tennis, you can hit the ball, but to get it in the righ direction, it has to be a good hit with a good connection.

Reactants Moving too Slowly:
-Molecules bounce (no reaction)

Reactants Not Facing the Right Way:
-Molecules bounce (no reaction)

Reactants Collide in correct direction and with Enough Energy:
-Successful collision, therefore a chemical reaction

Question 6

🟢 What is the Collision Theory (+ What’s Activation Energy)

Answer 6

Chemical reactions can occur only when reacting particles collide with each other in the correct orientation, and with enough energy.

There is a minimum amount if energy that particles must have to react called the Activation Energy

Question 7

🟢 What Equipment do you need when measuring/trying to prevent mass loss when there is a gaseous product?

Answer 7

• Cotton Wool - to stop acid spray + loss of reactants
• Conical Flask
• Reactants
• Mass Balance

(For other gases like Hydrogen you may use:

• Gas Syringe
• Conical Flask
• Reaction Mixture)

Question 8

🟢 (Mass Loss - When there is a gaseous product)

Explain why the reacation is fastest at the beginning and then slows down?

Answer 8

The reaction is fastest in the beginning because there are many reactants that have many opportipunities to collide many times. However, as the reaction continues, there will be less products to react with each other at each moment in time, meaning the reaction will begin to slow down until the limiting reactant is used up.

Question 9

🟢 (Mass Loss - When there is a gaseous product)

Explain why reactions stop?

Answer 9

Eventually, the reactions stops because eventually, some of the products become used up, before the limiting reactant is fully used up, meaning the reaction cannot continue.

Question 10

🟢 (Mass Loss - When there is a gaseous product)

Why is the cotton wool placed in the conical flask certain experiments?

Answer 10

The cotton wool is placed in the conical flask to ensure that there is no acid spray from the reaction, and to ensure that none of the products are lost

Question 11

🟢 How do you calculate the rate of reaction in a substance? (When you have a graph)

Answer 11

Rate of Reaction = Change in Y-Axis / Change in X-Axis (Rise/Run)

Question 12

🟢 Using ideas about particles and successful collisions per second, suggest why lines in a graph would curve rather than go in a straight line when showing a reaction

Answer 12

The curves , because as the reactants react more and more, there are fewer unreacted particles that are left over. Fewer unreacted particles means the chances of successful collisions gets lower and lower.

Question 13

🟢 Why is it planned that the Acid is in ‘excess’ in reaction instead of the metal reacting with that?

Answer 13

The acid is in excess to make the zinc the limiting reactant. This ensures that the zinc is fully reacted and that there is none left over by the end.

Question 14

🟢 Suggest an independant variable that could change a reaction of 1.5g of zinc powder and 50mls of 2.0M hydrochloric acid) go slower, other than using lumps of zinc rather than powder, and explain why it would slow down?

Answer 14

An independant variable that could changed is the temperature of which the reaction takes place in. This means if the temperature is lowered there will be lesser chances of successful collisions between particles in a certain timefrime. This is because the particles will move slower, due to having less kinetic energy.

Question 15

🟢 What are the Three Methods you can use measuring rates of reaction

Answer 15

• You can use a conical flask, a bung, and a mass balance to measure the mass of the gas change
• You can use the conical flask and a gas cyclinder to measure the volume of gas
• You can use the conical flask and a beaker filled with water and a measuring cyclinder in order to get a less accurate volume.

Question 16

🟢 Explain why changing the surface area of rhubarb when reacting it with potassium manganate has an effect on the rate of reaction (using collision theory)

Answer 16

To increase the surface area, you can cut up a solid substance into smaller pieces. Increasing the surface area means that a larger area of the substance is in contact with the solution. This allows more collisions for the rhubarb and potassium manganate to occur, resulting in higher chances of more successful collisions and therefore a faster rate of reaction.

Question 17

🟢 What is the relationship between the surface area of limestone and the time the reaction takes place in (when it’s reacted with hydochloric acid)

Answer 17

The higher the surface area, the more possible collisions can take place between the limestone and the hydorchloric acid, which means there are more possible successful collisions which would lead to a faster the rate of the reaction.

Question 18

🟢 Explain why changing the surface area of limestone (to powder) when reacting it with hydrochloric acid has an effect on the rate of reaction (using collision theory)

Answer 18

As the powder reacted the quickest, the higher surface area means more possible collisions can take place between the limestone and the hydorchloric acid, which means there are more possible successful collisions which would lead to a faster the rate of the reaction.

Question 19

🟢 Calculate the surface area to volume ratio of one cube measuring 3 cm x 3 cm x 3 cm

Answer 19

3 x 3 x 3 =27cm^3

6 x 3 x 3 = 54cm^2

SA:V

54: 27
2: 1

Question 20

🟢 Another cube cut into 9 smaller cubes. Calculate the surface area to volume ratio of one of these smaller cubes with size 1 cm x 1 cm x 1 cm.

Answer 20

1 x 1 x 1 = 1cm^3

6 x 1 x 1 = 6cm^2

SA:V
6:1

Question 21

🟢 What is the Collision Theory, and how do various factors affect the rate of reaction? (+What is Activation Energy)

Answer 21

Collision theroy explains how various factors affect the rate of reaction. According to this theory, chemical reactions can occur only when:

• Reacting particles collide with each other
• With sufficient energy

Sufficient energy is the minimum amount of energy that particles must have to react and is called activation energy.

Question 22

🟢 What does Increasing the Concentration of reactants in a solution do?

Answer 22

Increasing the concentration of reactants in solution increases the number of reactant particles per cm^3 of a solution.

This increases the frequency of collisions (but not the energy of the collisions) between reactant particles and so the rate of reaction increases.

Question 23

🟢 What does Doubling the Concentration of reactants in a solution do?

Answer 23

Doubling the concentration of reactants in solution doubles the number of reactant particles per cm^3 of a solution.

This doubles the frequency of collisions (but not the energy of the collisions) between reactant particles and so the rate of reaction doubles.

Question 24

🟢 What does Increasing the Pressure of gaseous reactants in a solution do?

Answer 24

Increasing the pressure of gaseous reactants increases the number of reactant particles per cm^3 of a solution.

This increases the frequency of collisions (but not the energy of the collisions) between reactant particles and so the rate of reaction increases.

Question 25

🟢 What does Doubling the Pressure of gaseous reactants in a solution do?

Answer 25

Doubling the the pressure of gaseous reactants doubles the number of reactant particles per cm^3 of a solution.

This doubles the frequency of collisions (but not the energy of the collisions) between reactant particles and so the rate of reaction doubles.

Question 26

🟢 What does Decreasing the particle size of the reactants do?

Answer 26

Decreasing the particle size of the same mass of solid reactants increases the surface area to volume ratio of the solid reactants. This increases the frequency of collisions (but not the energy of the collisions) between reactant particles and so the rate of reaction increases.

Question 27

🟢 What does Halving the particle size of the reactants do?

Answer 27

Halving the particle size of the same mass of solid reactants doubles the surface area to volume ratio of the solid reactants. This doubles the frequency of collisions (but not the energy of the collisions) between reactant particles and so the rate of reaction doubles.

Question 28

🟢 What does Increasing the Temperature of a reaction do?

Answer 28

Increasing the temperature of a reaction increases the frequency of collisions AND (more importantly) increases the energy of the collisions between reactant particles and so the rate of reaction increases.

Question 29

🟢 What does Doubling the Temperature of a reaction do?

Answer 29

Doubling the temperature of a reaction doubles the frequency of collisions AND increases the energy of the collisions between reactants and so the rate more than doubles

Question 30

🟢 What does an Increased Proportion of Particles exceeding the Activation Energy have a greater affect on than?

Answer 30

An increased proportion of particles exceeding the Activation Energy (the minimum energy required for a reaction to occur) has a greater effect on rate than the increased frequency of collisions

Question 31

🟢 What are Catalysts and What do they do?

Answer 31

Catalysts change the rate of reaction, but are not used up during the reaction.

Different reactions need different catalysts.

Enzymes act as catalysts in biological systems.

Catalysts increase the rate of reaction by providing an alternative route for the reaction that has a lower activation energy.

(A reaction profile to show a catalysed reaction can also be drawn).

Question 32

🟢 What are Reversible Reactions

Answer 32

(Most reactions are irreversible)

In some chemical reactions, the products of the reaction can react to produce the original reactants

Such reactions are called reversible reactions and are respresented:
A + B ⇌ C + D

The direction can be changed by changing the conditions

Question 33

🟢 Give two examples of Reversible Reactions

Answer 33

Hydrated Copper (II) Sulphate (Blue Crystalline Solid) ⇌ Anhydrous Copper (II) Sulphate (White Powedered Solid) + Steam

CuSO4.5H2O (s) ⇌ CuSO4 (s) + H2O (g)

Anhydrous Cobalt (II) Chloride (Blue Powedered Solid)+ Water ⇌ Hydrated Cobalt (II) Chloride (Pink Crystalline Solid)

CoCl2 + H2O ⇌ CoCl2.6H2O

Question 34

🟢 How is Equlibrium reached when a Reversible Reactions happens in a closed System

Answer 34

When a reversible reaction occurs in a closed system (apparatus which prevents the escape of reactants and products) equlibrium is reached when the forward and reverse reactions occur at exactly the same rate

Question 35

🟢 What happens when a system is in Dynamic Equlibrium

Answer 35

The relative amounts of all the reactants and products at equlibrium depend on the conditions of the reaction.

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.

The effects of chaning conditions on a system can be predicted using Le Chatelier’s Principle

Question 36

🟢 What are the 3 conditions that a System in Dynamic Equlibrium will counteract

Answer 36

• Concentration of one of the reactants or products is changed
• Temperature of a system in Equilibrium
• Pressure of gasesous reaction at equlibirium

Question 37

🟢 What is the effect of changing the Concentration of one of the reactants or products in a System in Dynamic Equilirbium

Answer 37

If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.

• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
• If the concentration of a product is decreased, more reactants will react until equlibrium is reached again.

Question 38

🟢 (Endothermic in forward direction)

Co(H2P)6^2+ + 4Cl^- ⇌ CoCl4^2- + 6H2O
(Pink) (Blue)

Explain what happens to the colour of the equlibrium mixture when more Hydorchloric Acid is added?

Answer 38

Here, the concentration of the hydrochloric acid has been increased (a reactant). To counteract this change, the system is going to make more products to keep equilibrium. Therefore, as there will be more products (e.g CoCl4^2-) meaning the solution will turn more blue.

Question 39

🟢 (Endothermic in forward direction)

Co(H2P)6^2+ + 4Cl^- ⇌ CoCl4^2- + 6H2O
(Pink) (Blue)

Explain what happens to the colour of the equlibrium mixture when more Water is added?

Answer 39

Here, the concentration of the water has been increased (changing the amount of the product). To counteract this change, the system is going to make more reactants to keep equilibrium. Therefore, as there will be more reactants (e.g Co(H2O)6^2+, meaning the solution will turn more pink.

Question 40

🟢 What happens if the temperature of a system at equlibrium is increased?

Answer 40

If the temperature of a system at equlibrium is increased:

• The relative amount of products (yield) at the equlibrium increases for an endothermic reaction
• The relative amount of products (yield) at equlibrium decreases for an exothermic reaction

Question 41

🟢 What happens if the temperature of a system at equlibrium is decreased?

Answer 41

If the temperature of a system at equlibrium is decreased:

• The relative amount of products (yield) at the equlibrium decreases for an endothermic reaction
• The relative amount of products (yield) at equlibrium increases for an exothermic reaction

Question 42

🟢 (Endothermic in forward direction)

Co(H2P)6^2+ + 4Cl^- ⇌ CoCl4^2- + 6H2O
(Pink) (Blue)

Explain what happens to the colour the equilibrium mixture when the mixture is cooled?

Answer 42

If you decrease the temperature of the surroundings, the reaction is moved in the exothermic direction towards the reactants, therefore heating up the surroundings, meaning the yield of the productss decreases, meaning the solution will become more pink.

Question 43

🟢 (Endothermic in forward direction)

Co(H2P)6^2+ + 4Cl^- ⇌ CoCl4^2- + 6H2O
(Pink) (Blue)

Explain what happens to the yield when the mixture is warmed?

Answer 43

If you increase the temperature of the surroundings, the reaction is moved in the endothermic direction towards the products, therefore cooling down the surroundings, meaning the yield of the amount of products increases, and the solution will become more blue.

Question 44

🟢 What is the effect of an increase in pressure for gaseous reaction at equlibrium?

Answer 44

An increase in pressure causes the equlibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction.

Question 45

🟢 What is the effect of a decrease in pressure for gaseous reaction at equlibrium?

Answer 45

A decrease in pressure causes the equlibrium position to shift towards the side with the larger number of molecules as the symbol equation for that reaction.

Question 46

🟢 The hydrogen used in the Haber process is made in the reaction shown below, which is an equilibrium.

(Endothermic in the forwards direction)

CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g) - Energy change = +206 kJ/mol

If the temperature of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen?

Answer 46

The equlibrium yield of the hydrogen would increase.

This is because if the temperature of surroundings is decreased, then the reaction moves in the endothermic direction towards the products, resulting in the increase in the yield of the hydrogen.

Question 47

🟢 The hydrogen used in the Haber process is made in the reaction shown below, which is an equilibrium.

(Endothermic in the forwards direction)

CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g) - Energy change = +206 kJ/mol

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen?

Answer 47

The equlibrium yield of the hydrogen would decrease.

An increase in pressure causes the equlibrium position to shift towards the side with the smaller number of molecules. This means more molecules will be formed on the the reactants side, which means the molecules on the side of the products will react to form more of the reactants in order to keep the system in equilibrium. This results in the equilibrium of the products side to decrease, meaning the yield of hydrogen will decrease.

Question 48

🟢 Hydrogen can also be made in the reaction shown below.

(Exothermic in the forwards direction)

CO(g) + H2O(g) ⇌ CO2(g) + H2(g) - Energy change = –42 kJ/mol

If the temperature of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen?

Answer 48

The yield of the hydrogen would decrease.

This reaction is exothermic and increases the temperature moves the reaction in the endothermic direction, meaning more reactants are produced, therfore using up the the products, decreasing the hydrogen yield.

Question 49

🟢 Hydrogen can also be made in the reaction shown below.

(Exothermic in the forwards direction)

CO(g) + H2O(g) ⇌ CO2(g) + H2(g) - Energy change = –42 kJ/mol

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of hydrogen?

Answer 49

The equlibrium yield of the hydrogen would stay the same.

The equlibrium yield of the hydrogen would stay the same because both sides of the equation both contain the same number of molecules in them, meaning the pressure will not affect the yield in any way.

Question 50

🟢 Sulfur trioxide is made by the following reaction. The forward reaction is exothermic.

2 SO2(g) + O2(g) ⇌ 2 SO3(g)

If the pressure of this equilibrium was increased, what would happen to the equilibrium yield of sulfur trioxide?

Answer 50

The equlibrium yield of the sulfur trioxide would increase.

An increase in pressure causes the equlibrium position to shift towards the side with the smaller number of molecules. This means more molecules will be formed on the the products side, which means the molecules on the side of the reactants will react to form more of the products in order to keep the system in equilibrium. This results in the equilibrium of the products side to increase, meaning the yield of sulfur trioxide increases.

Question 51

🟢 !* Explain what is meant by the term equlibrium *!

Answer 51

The forward and backwards reactions occur at the same rate in a closed system, so the concentration of reactants and products remain the same.

Question 52

🟢 !

A​ ​company manufactures ethanol (C2​H​5​O​H) for sale to other chemical companies as a raw ingredient for many products.

The reaction for the process is as follows:

C2​H​4​ (​g) + H2​O​ (g) ⇌ C2​H​5​O​H (g)
(Exothermic in fowards direction) (Δ​H​=−45kJ/mol)

Explain what effect increasing the temperature would have on the ​yield​ of ethanol at equilibrium.

Answer 52

Increasing the temperature will will decrease the yield of ethanol, because the reaction will move in the endothermic direction, therefore means more products will be formed, and the ethanol yield will decrease as a result.

Question 53

🟢 !

A​ ​company manufactures ethanol (C2​H​5​O​H) for sale to other chemical companies as a raw ingredient for many products.

The reaction for the process is as follows:

C2​H​4​ (​g) + H2​O​ (g) ⇌ C2​H​5​O​H (g)
(Exothermic in fowards direction) (Δ​H​=−45kJ/mol)

Explain what effect increasing the pressure would have on the ​yield​ of ethanol at equilibrium.

Answer 53

The yield of the ethanol would increase as an increase in pressure has taken place. This sends the reaction towards the side with fewer molecules, meaning in this case, more products will be produced.