C10 - Rates of Reaction

Question 1

What is the rate of reaction?

Answer 1

The change in concentration of a reactant or a product in a given time.

Question 2

How do you calculate rate of reaction and what are the units for rate?

Answer 2

Rate (mol dm-3 / s) = change in concentration / time

Question 3

How do you find the rate of reaction at given for a curved graph?

Answer 3

Draw a tangent to the graph at the specific time point. Then find the gradient of this tangent line.

Question 4

What are the four factors that can effect the rate of reaction?

Answer 4

• Concentration (or pressure when reactants are gases)
• Temperature
• Use of a catalyst
• Surface Area of solid reactants

Question 5

What is collision theory?

Answer 5

A reaction will only take place between two particles when:

1. They collide with the correct orientation (right direction). Facing each other the right way.
2. They collide with at least a certain minimum amount of kinetic (movement) energy, to overcome the activation energy.

Question 6

Why do particles need activation energy?

Answer 6

To break the bonds to start the reaction.

Question 7

How does increasing the concentration or pressure (with gas) speed up reactions?

Answer 7

Increasing concentration/pressure of a reactant, increase the overall collision frequency because there are more particles, so they are closer together. If there are more collisions, they’ll be more effective collisions, increasing the rate of reactions.

Question 8

What are the methods for following the progress of a reaction?

Answer 8

Monitoring - decrease in concentration of reactants
Following - increase in concentration of a product

Question 9

What are the practical methods can you use to measure the rate of reaction?

Answer 9

1. Monitoring the volume of gas produced (products formed) using a gas syringe, at regular time intervals.
2. Monitoring the loss of mass of reactants using a balance, at regular time intervals.

Question 10

What are the advantages and disadvantages for a gas syringe?

Answer 10

Adv - Accurate result.
Disadv - vigorous reactions can blow the plunger out of the syringe,

Question 11

What are the advantages and disadvantages for the balance and beaker test for reaction rate?

Answer 11

Adv - Very Accurate, easy to use
Disadv - Releases gas into the room which could be dangerous if toxic

Question 12

Can the rate of reaction be negative?

Answer 12

No it always has to be positive or 0.

Question 13

What is a catalyst?

Answer 13

A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalysts is chemically unchanged at the end of the reaction (NOT USED UP).

Question 14

What is the difference between a normal enthalpy diagram for an Exothermic/Endothermic reaction and one with a catalyst?

Answer 14

The curve upwards (activation energy) is smaller. Both lines finish and start at the same place though on both graphs.

Question 15

What is an heterogenous catalyst?

Answer 15

A heterogenous catalyst has a different physical state from the reactants. Usually solid reaction with gaseous reactants.

Question 16

Explain how heterogenous catalysts work.

Answer 16

Reactant molecules are absorbed (weakly bonded) onto the surface of the heterogenous catalyst, where the reaction takes place. After reaction, the product molecules leave the surface of the catalyst through desorption.

So increasing the surface area of the catalyst increases the number molecules that can react at the same time, therefore increasing the rate of reaction.

Question 17

What is a homogenous catalyst?

Answer 17

Homogenous catalyst are in the same physical state as the reactants.

Question 18

Explain how a homogenous catalyst works.

Answer 18

The catalyst reacts with the reactants to form an IMMEDIATE. The immediate then breaks down to give the product and regenerates the catalyst.

Question 19

Why are catalysts good economically?

Answer 19

Lower activation energy so less energy required (electricity or fossil fuels), this cuts cost on energy so there is an increase in profitability.
Also makes the product faster so more money can be made.

Question 20

Why are catalysts important for Environmental Sustainability?

Answer 20

Lower activation energy, so lower temperatures and pressure are required. So energy is saved and less CO2 emissions are released from fossil fuels.
Also reduce waste by allowing a different reaction pathway to be used with a better atom economy.

Question 21

How do catalyst speed up reactions?

Answer 21

They lower the activation energy by providing a different way for the bonds to be broken and remade. If the activation energy’s lower, more particles will have enough energy to react.

Question 22

What is dynamic equilibrium?

Answer 22

When the rate of forward reaction is equal to the rate of the reverse reaction and the concentration of products and reactants stay constant, in a closed system.

Question 23

What is a closed system?

Answer 23

A reaction where no chemicals can enter or leave.

Question 24

What does Le Chatelier’s principle tell us?

Answer 24

If there’s a change in concentration, pressure or temperature, the equilibrium will move to help counteract the change.

Question 25

If the position of the equilibrium moves more to the left or right what does that mean?

Answer 25

If the position of equilibrium moves to the right then you’ll get more products.

If the position of equilibrium moves to the left then you’ll get more reactants.

Question 26

What happens if you increase the concentration of a reactant or product?

2 SO2 (g) + O2 (g) —> 2 SO3 (g)

Answer 26

If you increase the concentration of a reactant (SO2 or O2), the equilibrium tries to get rid of the extra reactant. It does this by making more product (SO3).

If you increase the concentration of the product (SO3), the equilibrium tries to remove the extra product. The is makes the reverse reaction go faster, so the equilibrium shifts to the left.

Opposite effect for decreasing the concentrations.

Question 27

What happens if you increase or decrease the temperature for a reversible reaction?

Answer 27

Increasing the temperature means adding heat. The equilibrium shifts in the endothermic direction to absorb the heat. (counteract the change in temperature)

Decreasing the temperature removes the heat. The equilibrium shifts in the Exothermic direction to try and replace the heat

REMEMBER - if the forward reaction’s endothermic, the reverse reaction will be Exothermic.

Question 28

What happens if you increase the pressure for a reversible reactant (gases only)?

Answer 28

Increasing the pressure shifts the equilibrium to the side with fewer gas molecules. This reduces the pressure.

Decreasing the pressure shifts the equilibrium to the side with more gas molecules. This raises the pressure again.

Question 29

Explain the effect of a catalyst on the position of equilibrium?

Answer 29

Catalysts have NO EFFECT on the position of equilibrium. They speed up the forward and reverse reactions by the same amount. They can’t increase the yield but mean equilibrium is reached faster.

Question 30

H2 (g) + CO2 (g) ⇌ H2O (g) + CO (g) ΔH= +40 kJ mol-1

What effect would the following changes have on the rate of reaction and the yield of carbon monoxide?

a) increase in tempreture

b) increase in pressure

Answer 30

a) Increase in rate (higher temp higher rate)
Increase in yield of CO (endothermic forward reaction)

b) Increase in rate (higher pressure higher rate)
No change in yield (same moles on each side 2:2)

Question 31

What happens if you increase the pressure in the reaction of:

2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g)

Answer 31

There are 3 moles on the left (reactants), but only 2 moles on the right. So increasing the pressure shifts the equilibrium to the right, get more products.

Question 32

Explain why that in the chemical industry that they COMPROMISE between chemical equilibrium and reaction rate in deciding the operational conditions, such as for ethanol:

C2H4 (g) + H2O (g) ⇌ C2H5OH (g) ΔH= -46 kJ mol-1

Answer 32

• An Exothermic reaction to produce ethanol, so lower temperature favour the forward reaction, this will get you a better yield. But lower temperatures means a slower rate of reaction, it will take years to get a really high yield of ethanol. So 300°C is a compromise between maximum yield and a faster reaction.
• High pressure favours the forward reaction, you will get more ethanol, as this is the side with fewer gas molecules. Higher pressure also increases the rate of reaction. However you can not have that high of pressure as it is very expensive as you need strong pipes and containers to withstand the high pressure. Also can cause side reactions to occur.
  So 60-70 atmospheres is a compromise between maximum yield and expense.

Question 33

Explain the practical to investigate the equilibrium position with changing temperature for the reversible reaction,
2 NO2 (g) ⇌ N2O4 (g)
brown colourless
<— endothermic
—> exothermic

Answer 33

1. Place two sealed containing the equilibrium mixture in water baths, one in cool and one in warm water and observe the colour change.
2. The tube in the warm water will change to a dark brown colour as the endothermic reaction speeds up, shifting the equilibrium to the left.
3. The tube in the cool water will lose colour as the Exothermic reaction speeds up, shifting the equilibrium to the right.

Question 34

Explain the practical to investigate the equilibrium position with changing concentration for the reversible reaction,

Fe 3+ (aq) + 3 SCN - (aq) ⇌ Fe(SCN)3 (aq)
yellow colourless blood red

Answer 34

1. Add equal amount of the equilibrium mixture to four test tubes.
2. Test tube 1- the ‘control’ nothing added, just the equilibrium mixture a reddish colour.
   Test tube 2- add some Iron (III) nitrate, turns a deep red colour.
   Test tube 3- add potassium thiocyanate, turns a deep red colour.
   Test tube 4- add some iron (III) thiocyanate, the mixture turn yellow.
3. By adding more reactant the test tube becomes a deep red colour, equilibrium shifts to the right.
   By adding more products the test tube becomes yellowish, equilibrium shifts to the left.

Question 35

How do you calculate the equilibrium constant, Kc?

aA + bB ⇌ dD + eE

Answer 35

Kc = [D]^d [E]^e / [A]^a [B]^b

Question 36

What do the [ ] brackets mean?

Answer 36

Concentration, in mol dm-3

Question 37

What is the equilibrium constant for the hydrogen iodide at 640K. The concentration are,
[HI] = 0.80
[H2] = 0.10
[I2] = 0.10

H2 (g) + I2 (g) ⇌ 2 HI (g)

Answer 37

Kc = [HI]² / [H2] [I2]
= (0.80²) / (0.10)(0.10)
= 64

Units = (mol dm-3)² / (mol dm-3)(mol dm-3)
= No Units (have to write this)

Question 38

How can you estimate the position of equilibrium with the value of Kc?

Answer 38

1. The larger the value of Kc the further to the right equillibrium lies, and the more product there is relative to reactants.
2. The smaller the value of Kc the further to the left equilibrium lies, and the more reactant there is relative to reactants.

Question 39

Work out the actual number of moles of each species are present at equilibrium.

. CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g)
Initial: 1 1 0 0
Change:
Equilibrium: ? ? 0.4 ?

Answer 39

. CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g)
Initial: 1 1 0 0
Change: (-0.4) (-0.4) (+0.4) (+(3 x 0.4))

Equilibrium: 0.6 0.6 0.4 1.2

Question 40

Some PCl5 was heated in a sealed container resulting in the following equilibrium.
Analysis of the equilibrium mixture showed that it contained 0.04420 moles of PCl5,
0.0400 moles of PCl3 and 0.0400 moles of Cl2. The total volume was 2.00 dm3. 8) Calculate the concentration of each species at equilibrium and then Kc.

PCl5(g) ⇌ Cl2(g) + PCl3(g)

Answer 40

. PCl5(g) ⇌ Cl2(g) + PCl3(g)
I 0.0442 0 0
C -0.04 +0.04 +0.04
E 0.0042 0.04 0.04

Concentration= 0.0042/2 = 2.1 x 10^-3
0.04 / 2 = 0.02

Kc = (0.02) (0.02) / (2.1 x 10^-3) = 0.190 mol dm-3 (3sf)

Question 41

What is the Hager process equation?

Answer 41

N2 (g) + 3 H2 (g) ⇌ 2 NH3 (ammonia)
. Fe (s) - catalyst

Question 42

What are the key features of the Boltzmann distribution graph?

Answer 42

• Curve starts at (0,0) because no molecules have zero energy.
• The area under the graph is equal to the total number of molecules.
• No Maximum energy for a molecule, so the curve does not meet the x-axis.
• Dotted line on the right of the graph is the activation energy, any molecules past this point can react.
• Few particles are moving slowly and most are moving at moderate speed.

Question 43

What is on the x-axis and y-axis on the Boltzmann distribution?

Answer 43

Y - number of molecules
X - kinetic energy

Question 44

Explain the effect of increasing the temperature on the Boltzmann Distribution.

Answer 44

• The activation energy dotted line will stay at the same place on the orignal BD graph.
• The is a lower maximum peak on the new BD graph.
• At a high temperature a greater proportion of molecules have an energy greater than or equal to activation energy.

Question 45

Explain the effect of raising the temperature?

Answer 45

More molecules have an energy equal to or greater than the activation energy.
Therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction.
Collisions will also be more frequent as the molecules are moving faster , energy more important than collision though.

Question 46

Explain the effect of a catalyst on the Boltzmann Distribution.

Answer 46

Same exact graph as the original BD graph.
However the activation energy dotted line has been moved to the left, as the amount of activation energy required has decreased.
A greater proportion of molecules exceeds the new lower activation energy.