C6 - Rates of Reaction

Question 1

How do you calculate mean rate of reaction?

Answer 1

There are different ways to determine the rate of a reaction. The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.

The mean rate of reaction can be calculated using either of these two equations:

Question 2

What might we have to measure if we want the rate of reaction?

How do you prevent product escaping from a flask?

Answer 2

• The quantity to be measured depends on the reaction and may be in grams for mass or cm³ or dm³ for volume if the product is a gas
• The units of the rate of reaction would therefore be g s^-1 or cm³ / dm³ s^-1
• Time is usually in seconds
• If one of the products is a gas which is given off, then the reaction can be performed in an open flask on a balance to measure the loss in mass of reactant
• Cotton wool is usually placed in the mouth of the flask which allows gas out but prevents any materials from being ejected from the flask (if the reaction is vigorous)

Question 3

How do you see the difference of rate of reaction on a graph?

Answer 3

The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.

The gradient of the line is equal to the rate of reaction:

• the steeper the line, the greater the rate of reaction
• fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions

Units for rates of reaction - Higher

The rate of a chemical reaction can also be measured in Mol s^-1.

Question 4

What piece of equipment can you use to measure gas volume?

Answer 4

Gas Syringe

Question 5

How can you use precipitation to measure rate of reaction?

Answer 5

• Precipitation reactions form a solid precipitate when 2 clear solutions are mixed together
• The precipitate clouds the reaction mixture so if the flask is placed over a piece of paper with a cross on it, the time it takes for the cross to disappear from view (due to the formation of the precipitate) can be measured
• This method is susceptible to error though as they are subjective, given that different people may not agree on the exact moment that the cross disappears
• Another disadvantage is that only one data point is produced per experiment, so a rate of reaction graph cannot be plotted

Exam Tip

When answering questions on the effect of concentration/pressure on the rate of reaction, you should mention that there are more particles per unit volume (usually cm³) rather than just more particles.

Question 6

How do you find out the rate of reaction at single point on a graph?

Answer 6

Use a tangent like in maths and measure the gradient.

Question 7

How do you measure rate of reaction using moles and Volume?

Answer 7

• However, it is often more useful tp be able to express rate of reaction in terms of moles, in which case the unit will be mol/s
• There is no direct way to measure moles, so first it is necessary to calculate the rate using the rate formula triangle:

moles per unit time = mass per unit time ÷ molar mass of the substance

• After calculating the rate of reaction it can then be converted into mol s^-1
• If you are working in volume per unit time then you have to use the molar gas volume:

volume per unit time ÷ molar gas volume (24 000 cm³ mol^-1 ) = moles per unit time

Question 8

How do you draw a rate of reaction graph?

Answer 8

• Data recorded in rate studies is used to plot graphs to calculate the rate of a reaction
• Time is normally plotted on the x-axis with the concentration of the reactant or product on the y-axis
• A number of measurements should be taken to provide a complete set of data
• If the relationship between the factor being measured and the amount produced is directly proportional (i.e. if the concentration of a reactant doubles the rate also doubles) then the resulting graph will be a straight line graph going through the origin
• The gradient of the line is equal to the initial rate of reaction and the steeper the gradient of the line then the faster the rate of reaction

Question 9

What are the factors that affect the rate of reaction?

Answer 9

• There are several factors that can affect the rate of a reaction. These are:
  
  • Concentration of the reactants in solution or the pressure of reacting gases
  • Temperature at which the reaction is carried out
  • Surface area of solid reactants
  • The use of a catalyst
• Changes in these factors directly influence the rate of a reaction
• It is of economic interest to have a higher rate of reaction as this implies a higher rate of production and hence a more efficient and sustainable process

Question 10

How does increasing the concentration/pressure of gases affect the rate of reaction?

Answer 10

Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

Question 11

How does the increasing temperature affect the rate of reaction and why?

Answer 11

Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction

• reactant particles move more quickly
• the energy of the particles increases
• the frequency of successful collisions between reactant particles increases
• the proportion of collisions which are successful increases
• the rate of reaction increases

The gradient of the line is equal to the rate of reaction. The faster reaction at the higher temperature:

• gives a steeper line
• finishes sooner

The effect of temperature on the rate of reaction is due to two factors: frequency of collisions and energy of collisions. The increase in energy is usually the more important factor.

Question 12

How does the Surface Area affect the rate of reaction and why?

Answer 12

For a given mass of a solid, large lumps have smaller surface area to volume ratios than smaller lumps or powders. If a large lump is divided or ground into a powder:

• its total volume stays the same
• the area of exposed surface increases
• the SA:V ratio increases

Lumps vs powders

The greater the frequency of successful collisions, the greater the rate of reaction. If the SA:V ratio of a reacting solid is increased:

• more reactant particles are exposed at the surface
• the frequency of collisions between reactant particles increases
• the rate of reaction increases

mean energy of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.

The gradient of the line is equal to the rate of reaction. Faster reaction with the powder:

• gives a steeper line
• finishes sooner

Make sure you answer questions in terms of surface area to volume ratio, rather than just surface area. This is because the surface area also depends on the mass of solid reactant used.

Question 13

How does a Catalyst affect the rate of reaction and why?

Answer 13

A catalyst is a substance that:

• increases the rate of a reaction
• does not alter the products
• is not chemically changed or used up at the end of the reaction

Only a very small mass of catalyst is needed to increase the rate of a reaction. However, not all reactions have suitable catalysts. Different substances catalyse different reactions.

A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because more particles have energy greater than the activation energy, therefore there are more successful collisions.n.

Question 14

Required Practical - Measuring the production of a gas

Method and Hazads

Answer 14

Method

1. Support a gas syringe with a stand, boss and clamp.
2. Using a measuring cylinder, add 50 cm³ of dilute hydrochloric acid to a conical flask.
3. Add 0.4 g of calcium carbonate to the flask. Immediately connect the gas syringe and start a stop clock.
4. Every 10 seconds, record the volume of gas produced.
5. When the reaction is complete, clean the apparatus as directed by a teacher.
6. Repeat steps 1 to 5 with different concentrations of hydrochloric acid.

Hazard

Hydrochloric acid - Causes skin and eye irritation so Wear eye protection

Fizzing in the reaction mixture - Acidic spray or foam escaping, which may damage skin and eyes SO use a large conical flask so there is plenty of space inside; do not look over the top when adding the calcium carbonate

Question 15

Required Practical - investigate the rate of reaction by colour change

Using sodium thiosulfate and hydrochloric acid

Answer 15

1. Using a measuring cylinder, add 50 cm³ of dilute sodium thiosulfate solution to a conical flask.
2. Place the conical flask on a piece of paper with a black cross drawn on it.
3. Using a different measuring cylinder, add 10 cm³ of dilute hydrochloric acid to the conical flask. Immediately swirl the flask to mix its contents, and start a stop clock.
4. Look down through the reaction mixture. When the cross can no longer be seen, record the time on the stop clock.
5. Measure and record the temperature of the reaction mixture, and clean the apparatus as directed by a teacher.
6. Repeat steps 1 to 5 with different starting temperatures of sodium thiosulfate solution.

Record the results in a table. This table gives some example results.

HAZARDS

Hot sodium thiosulfate solution - Burns to the skin so Do not heat above 60°C

Sulphur dioxide - Can cause irritation to the eyes and lungs, particularly to people with asthma. Make sure the room is well ventilated, avoid breathing directly over the top of the flask

Question 16

What is Collision Theory?

Answer 16

chemical reactions occur only when the reactant particles collide with sufficient energy to react

The minimum amount of energy needed is called the activation energy, which is different for each reaction

Particles that collide with insufficient energy have unsuccessful collisions and just bounce off each other

The rate of a reaction is therefore also dependent on the energy of collisions as well as the number of collisions

To increase the rate of a reaction then the number of successful collisions needs to be increased

Question 17

Explain Concentration of a Solution / Pressure of a Gas using Collision Theory

Answer 17

• Increasing the concentration of a solution will increase the rate of reaction
• This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second, increasing the rate of reaction
• For a gaseous reaction, increasing the pressure has the same effect as the same number of particles will occupy a smaller space, increasing the concentration
• If you double the number of particles you will double the number of collisions per second
• The number of collisions is proportional to the number of particles present

Question 18

Explain Temperature using Collision Theory

Answer 18

• Increase in the temperature, the rate of reaction will increase
• This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction
• The effect of temperature on collisions is not so straight forward as concentration or surface area; a small increase in temperature causes a large increase in rate
• For aqueous and gaseous systems, a rough rule of thumb is that for every 10 degree (Kelvin) increase in temperature the rate of reaction approximately doubles

Question 19

Explain Surface area of a solid using Collision Theory

Answer 19

Explanation:

• With an increase in the surface area of a solid reactant, the rate of reaction will increase
• This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second
• If you double the surface area you will double the number of collisions per second

Exam Tip

Temperature affects reaction rate by increasing the number of collisions and the energy of the collisions. Of the two factors, the increase in energy is the more important one.

Question 20

Explain How catalysts work?

Answer 20

• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction
• The mass of a catalyst at the beginning and end of a reaction is the same
• An important industrial example is iron, which is used to catalyse the Haber Process for the production of ammonia
• Iron beads are used to increase the surface area available for catalysis
• Normally only small amounts of catalysts are needed to have an effect on a reaction
• Different processes require different types of catalysts but they all work on the same principle of providing an alternate route for the reaction to occur
• They do this by lowering the activation energy required, hence providing a reaction pathway requiring less energy
• they reduce energy costs
• The transition metals are used widely as catalysts as they have variable oxidation states allowing them to readily donate and accept different numbers of electrons. This is key to their catalytic activity

Question 21

What is a reversible reaction and what does it’s symbol look like?

Answer 21

• In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
• It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants)
• The direction a reversible reaction takes can be changed by changing the reaction conditions
• For example heating ammonium chloride produces ammonia and hydrogen chloride gases:

NH₄Cl (s) → NH₃ (g) + HCl (g)

• As the hot gases cool down they recombine to form solid ammonium chloride

NH₃ (g) + HCl (g) → NH₄Cl (s)

• So, the reversible reaction is represented like this:

NH₄Cl (s) ⇌ NH₃ (g) + HCl (g)

Exam Tip

The reverse reaction may also be called the backwards reaction. A generic reversible reaction is shown as

A + B ⇌ C + D

Question 22

What are energy changes in a reversible reaction and give the example of copper sulfate.

Answer 22

• The majority of chemical reactions are exothermic with only a small number being endothermic
• For a reversible reaction, if it is exothermic in one direction then it must be endothermic in the opposite direction
• The amount of energy transferred in either direction is the same

Question 23

What is Dynamic Equilibrium?

Answer 23

• reversible reaction is one that occurs in both directions
• When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium
• Equilibrium is dynamic i.e. the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate
• The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
• It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel

Question 24

Give an example of equilibrium

Answer 24

• An example of a dynamic equilibrium is the reaction between H₂ and N₂ in the Haber process
• When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
• As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease
• However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen)
• Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

Question 25

What is the effect of changing conditions on equilibrium and what is it called?

Answer 25

• (le chaterlier)This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
• The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration
• Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions
• For example, if the pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules

Question 26

What is the effect of changing Concentration on equilibrium

Answer 26

It will shift to wherever there is less concentration

If the concentration of a reactant (on the left) is increased, the equilibrium position moves in the direction away from this reactant, and so more of the products are produced (on the right). If one of the products is removed from a reaction (on the right), then the position of equilibrium moves to the right to make more of that product.

For example, bismuth chloride reacts with water in a reversible reaction:

BiCl3(aq) + H2O(l) ⇌ BiOCl(s) + 2HCl(aq)

The concentration of hydrochloric acid can be increased by adding more hydrochloric acid. When this happens, the equilibrium position moves to the left, away from HCl(aq) in the equation.

Question 27

What would happen if we increase the concentration of:

Answer 27

At equilibrium, the system will counteract any changes that we make so if we change the concentration of the products, then the system is no longer at equilibrium.

The forward and reverse reactions are not taking place at the same rate. The concentrations of all substances will change until equilibrium will change.

Because we have increased the concentration of NO2 more N2O4 will be formed until equilibrium is reached again.

This causes the equilibrium to shift to the right, so more of the N2O4, is formed

Question 28

What would happen if we Decrease the concentration of:

Answer 28

Because we have decreased the concentration of N2O4, the system is no longer at equilibrium so more N02 will react to form N2O4 until equilibrium is reached again.

Question 29

What would happen if we Increase the concentration of:

Answer 29

Because we have increased the concentration the system is no longer at equilibrium so more N2O4 will turn into NO2 until equilibrium is reached again

Question 30

Worked Example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride:

2ICl(l) + Cl₂ (g) ⇌ ICl₃ (s)

dark brown yellow

Predict the effect of a change in the concentration of IClor Cl₂ on the position of equilibrium.

Answer 30

Answer

• An incease in the concentration of IClor Cl₂ causes the equilibrium to shift to the right, so more of the yellow solid, ICl₃, is formed
• An decease in the concentration of IClor Cl₂ causes the equilibrium to shift to the left, so more of the yellow solid, ICl₃, is formed

Question 31

What is the Effect of Temperature Changes on Equilibrium

Answer 31

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction. If the temperature is increased, the equilibrium position moves in the direction of the endothermic process.

For example, in the Haber process:

N2(g) + 3H2(g) ⇌ 2NH3(g) (forward reaction is exothermic)

If the forward reaction is exothermic, the backward reaction must be endothermic.
Therefore, if the temperature is increased, the equilibrium position moves in the endothermic direction (to the left) to reduce the temperature. This means that less ammonia (NH3) will be produced.

Question 32

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride:

2ICl(l) + Cl₂ (g) ⇌ ICl₃ (s)

dark brown yellow

When the equilibrium mixture is heated, it becomes dark brown in colour. Explain whether the backward reaction is exothermic or endothermic.

Answer 32

Answer

• The equilibirum has shifted to the left as the dark brown colour means that more of the IClis produced.Increasingthetemperature moves the equilbriumin theendothermic direction, so thereverse reaction is endothermic

Exam Tip

Remember all reversible reactions are exothermic in one direction and endothermic in the other.

Question 33

What is the Haber Process?

Answer 33

Background to the Haber process

Nitrogen gas is reacted with hydrogen gas to make ammonia gas. The forward reaction is exothermic.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium position is:

• to the left if the concentrations of N₂ and H₂ are greater than the concentration of NH₃
• to the right if the concentration of NH₃ is greater than the concentrations of N₂ and H₂

Question 34

What is the Effect of Pressure Changes on Equilibrium

Answer 34

Changes in pressure only affects gases so firstly you have to identify all gaseous reactants and productsChanging the pressure

If the pressure is increased in a reaction involving gases, the equilibrium position moves in the direction of the fewest molecules of gas, to reduce the pressure.

There are fewer molecules on the right-hand side of the equation for the Haber process:

N2(g)+3H2(g)⇌2NH3(g)

1+3=4 molecules 2 molecules

If the pressure is increased, the equilibrium position moves to the right.

Question 35

What is the effect of increasing the temperature of this system?

Answer 35

If we increase the temperature of this system then the equilibrium shifts to the left to reduce the temperature. This is because the reverse reaction is endothermic, so energy is taken in, causing the temperature to fall.

Therefore, the amount of NO2 would increase and the amount of N2O4 would decrease.

Question 36

What is the effect of decreasing the temperature of this system?

Answer 36

In this case the equilibrium shifts to the right to increase the temperature. That is because the forward reaction is exothermic, so energy is released, causing the temperature to increase. In this case the amount N2O4 would increase and the amount of NO2 would decrease.

Question 37

What is The Effect of Pressure Changes on Equilibrium

Answer 37

• Changes in pressure only affects gases so firstly you have to identify all gaseous reactants and products
• If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilbrium when the pressure is changed
• Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilbrium is unchanged.
• Decreasing the pressure will move to the direction of the larger number of moles to increase pressure again

Question 38

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilbrium reaction:

What is the effect of increasing pressure?

2NO₂ (g) ⇌ N₂O₄ (g)

dark brown colourless

Answer 38

Answer

• • Number of gas molecules on the left side = 2
    
    • Number of gas molecules on the right side = 1
    • An increase in the pressure will cause the equilibrium to shift in the direction that produces the smaller number of molecules of gas so the equilibrium shifts to the right

Question 39

What does the pressure of a gas depend on?

Answer 39

The number of molecules.

Question 40

What happens if we increase the pressure on:

Answer 40

If we increase the pressure on a reversible reaction at equilibrium, the position of the equilibrium shifts to the side with the smaller number of molecules.
If we decrease the pressure, the position of the equilibrium shifts to the side with the larger number of molecules.

So on this reaction the equilibrium will shift to the right hand side as it has fewer molecules.

But if we were to reduce the pressure the equilibrium would shift to the left as it has the larger number of molecules.

Question 41

What is the effect of pressure if the number of molecules on each side are the same?

Answer 41

There is no effect.