Topic C5- Monitoring and Controlling Chemical Reactions

Question 1

What does concentration mean?

Answer 1

• Concentration is a measure of the abundance of a constituent ÷ by the total volume of a mixture.
• Can be measured in grams per dm3

Question 2

What is the formula for finding the concentration from the mass of solute?

Answer 2

Concentration = mass of solute ÷ volume of solution

triangle:

                     mass (g) --------------------------------------------------------- concentration (g/dm3)    x     volume (dm3)

Question 3

How do you convert g/dm3 to mol/dm3 when calculating the concentration?

Answer 3

You just divide the concentration in g/dm3 by the relative formula mass of the solute.

Question 4

How can titrations be used to find out the concentrations?

Answer 4

• Titrations allow you to find exactly how much acid is needed to neutralise a given quantity of alkali (vice versa).
• Using a pipette, measure out a set volume of the alkali into a flask. Add a few drops of an indicator- usually phenolphthalein or methyl orange,
• Fill a burette with a standard solution (known concentration) of acid.
• Use a burette to add the acid to the alkali a bit at a time. Swirl the flask regularly. Go slowly (drop at a time) when the alkali is almost neutralised.
• The indicator changes colour when all the alkali has been neutralised (endpoint) - phenolphthalein is pink in alkalis but colourless in acids, and methyl orange is yellow in alkalis but red in acids.
• Record the volume of acid used to neutralise the alkali (titre)
• Repeat this process a few times, making sure you get similar results each time. Take the mean of your results.

Question 5

What is the formula for finding concentrations in mol/dm3?

Answer 5

concentration = number of moles ÷ volume of solution

triangle:

                           moles ----------------------------------------------------------------- concentration               x              volume

Question 6

How do you convert a concentration into mol/dm3 into g/dm3?

Answer 6

Multiply the concentration in mol/dm3 by the Mr of the solute.

Question 7

What is the molar volume?

Answer 7

The volume occupied by one mole of a gas is known as the molar volume.

Usually has the units dm3/mol

Question 8

What is the formula for calculating molar volume?

Answer 8

molar volume = gas volume ÷ number of moles.

Question 9

How many moles do gases under the same conditions?

Answer 9

One mole of any gas always occupies 24 dm3 (24000cm3) at room temperature and pressure (Room Temp = 20ºC and 1 atmosphere).

So at RTP, all gases have the same molar volume- 24 dm3/mol.

Question 10

What is the formula for calculating the volume (dm3 ) of gas?

Answer 10

Volume = moles x 24

triangle:

                             volume
            -----------------------------------------
             moles          x               24

Question 11

What is percentage yield?

Answer 11

• The overall success of an experiment.
• It compares what you calculated was going to be the result (theoretical yield) with what happens in practice (actual yield).

Question 12

What is ‘yield’ in terms of chemistry?

Answer 12

• The amount of product you get from a reaction is known as yield.
• The more reactants you start with, the higher the actual yield will be.

Question 13

What is the formula for calculating percentage yield?

Answer 13

Percentage yield = actual yield
———————- x100
theoretical yield

Question 14

What is the theoretical yield of a reaction?

Answer 14

The mass of product you’d make if all the reactants were converted to products.

Question 15

How can you calculate the theoretical yield from a balanced equation?

Answer 15

1) Find the relative formula mass of the compound (e.g. iron oxide) and the relative atomic mass of the element (e.g. iron).
2) Work out the number of moles of the reactant you have.
3) Use the balanced chemical equation to work out how many moles of the desired product (iron) you should end up with.
4) Work out the theoretical yield of your desired product by converting this number of moles into mass
5) Put the numbers into the formula to find the % yield.

Question 16

What does 100% percentage yield mean?

Answer 16

You got all the product you expected to get.

Question 17

What does 0% yield mean?

Answer 17

No reactants were converted into product, so no product was made at all.

Question 18

What is atom economy?

Answer 18

The atom economy of a reaction tells you what percentage of the mass of the reactants has been converted into your desired product when manufacturing a chemical.

Question 19

What is the formula for atom economy?

Answer 19

Atom Economy = total Mr of desired products
—————————————— x100
total Mr of all products

Question 20

What does 100% atom economy mean?

Answer 20

All atoms in the reactants have been turned into useful products.
The higher the atom economy, the ‘greener’ the process.

Question 21

How can you calculate the atom economy of a reaction?

Answer 21

1) Identify the desired product.
2) Work out the Mr of all the products.
3) Then work out the Mr of just the desired products.
4) Use the formula to calculate the atom economy.

Question 22

How is high atom economy better for profits and the environment?

Answer 22

• Reactions with low atom economy use up resources quickly.
• At the same time, they make lots of waste materials that have to be disposed of somehow.
• This makes reactions unsustainable- the raw materials will run out and the waste has to go somewhere.
• Low atom economy reactions are profitable. Raw materials can be expensive to buy and waste products can be expensive to remove and dispose of responsibly.
• However, you could find a use for the waste product rather than disposing of it.
• Often there is more than one way to make the product you want- come up with a reaction that will create useful by-products.
• Atom economy isn’t the only factor to consider in industry.
• You need to consider: percentage yield,
• rate of reaction (fast enough to produce the amount of product you need in a sensible amount of time)
• and whether or not it is reversible. To keep the yield of a reversible reaction high, you might need to alter the reaction conditions which can be expensive.

Question 23

What is the rate of reaction?

Answer 23

• How quickly a reaction happens.

- It can be observed either by measuring how quickly the reactants are consumed or how quickly the products are formed.

Question 24

What is the formula to calculate the rate of reaction?

Answer 24

Rate of reaction =

                                    time

Question 25

How is the precipitation experiment used to measure the rate of reaction?

Answer 25

1) Method works for any reaction where mixing two see-through solutions produces a precipitate, which clouds the solution.
2) You mix the two reactant solutions and put the flask on a piece of paper.
3) Observe the mark through the mixture and measure how long it takes for the mark to be obscured. The faster it disappears, the faster the reaction.
4) The result is subjective- different people might not agree on exactly when the mark ‘disappears’.

Question 26

How can you measure the rate of reaction in a reaction where gas is given off?

Answer 26

1) Measure the rate of reaction that produces gas by using a mass balance.
2) As gas is released, the lost mass is easily measured on the balance. The quicker the reading on the balance drops, the faster the rate of reaction.
3) You can use results to plot a graph of change in mass against time.
4) Method does release gas produced straight into the room, so if gas is harmful, safety precautions are required.

Question 27

Describe the method where, to work out the rate of reaction, you need to measure the volume of gas:

Answer 27

1) This involves the use of a gas syringe to measure the volume of gas given off.
2) The more gas is given off during a set time interval, the faster the reaction.
3) You can use results to plot a graph of gas volume against time elapsed.
4) You need to be careful that you are using the correct size of gas syringe, if the reaction is too vigorous, the plunger can come off of the end.

Question 28

Describe the experiment of the reaction hydrochloric acid and marble chips:

Answer 28

1) You can use this experiment to show how surface area affects the reaction rate.
2) Measure the volume of gas produced using a gs syringe. Take readings at regular time intervals and record your results in a table.
3) You can plot a graph of your results- time on x-axis and volume on y-axis.
4) Repeat the experiment with exactly the same volume and concentration of acid, and exactly the same mass of marble chips, but with the marble chips more crushed.
5) Then repeat with the same mass of powdered chalk.

Question 29

What does using finer particles of a solid in a reaction mean?

Answer 29

A faster rate of reaction- finer particles means that the marble chips have a larger surface area.

Question 30

Describe an experiment that is good for showing how concentration affects the rate of reaction:

Answer 30

Reaction of Magnesium Metal with dilute HCl:

• reaction gives off hydrogen gas, so you can measure the loss of mass as the gas is formed using a mass balance.
• the more concentrated the acid is, the quicker the hydrogen gas is given off/more hydrogen gas is given off.

Question 31

What factors change the rate of reaction?

Answer 31

• Temperature
• Concentration
• Size of the particles.

Question 32

What is the collision theory?

Answer 32

Reaction rates are explained by the collision theory- the rate of a chemical reaction depends on:

• the collision frequency of reacting particles
• the more successful collision there are, the faster the reaction is.
• the energy transferred during a collision. Particles have to collide with enough energy for the collision to be successful.

Question 33

How does more successful collisions increase the rate of reaction?

Answer 33

Reactions happen if particles collide. So, if you increase the number of collisions, the reaction happens more quickly.

Question 34

How does an increase in temperature increase the rate of reaction?

Answer 34

1)When the temperature is increased, the particles move faster (more kinetic energy), so they have more collisions.

2)Higher temperatures increase the energy of the collisions, since the particles are moving faster.
Reactions only happen if particles collide with enough energy.

3)This means that there will be more successful collisions (more particles will collide with enough energy to react).

Question 35

How does increasing the concentration (or pressure) increase the rate of reaction?

Answer 35

1) If the solution is made more concentrated, there are more particles of reactant in the same volume.
This makes collisions more likely, so reaction rate increases.

2) In a gas, increasing the pressure means that the particles are more crowded.
This means that the frequency of collisions between particles will increase.

Question 36

How does making solid particles smaller (or creating more surface area to solid particles) increase the rate of reaction?

Answer 36

1) If one reactant is a solid, breaking it into a solid will increase it surface area to volume ratio (more solid will be ‘exposed’, compared to its overall volume)
2) The particles around it will have more area to work on, so frequency of collisions will increase.

Question 37

What are catalysts?

Answer 37

A catalyst is a substance which increases the rate of a reaction, without being chemically changed or consumed in the reaction.

Question 38

How do catalysts work?

Answer 38

They work by decreasing the activation energy needed for a reaction to occur.
The provide an alternative reaction pathway that has a lower activation energy.

Question 39

How can you identify if a substance is a catalyst?

Answer 39

If you have a solid that you think might be a catalyst for a reaction between two solutions:

• you could measure the reaction rate without the solid present, and the again with a known mass of the solid added.
• If the rate increases, but the solid appears to be unchanged, it could be a catalyst.
• To make it a fair test, you need to keep everything else (mass, volume etc.) the same, so that nothing else affects the rate of reaction.
• You can check that none of the solid has been used up by filtering it out at the end of the experiment, drying it and measuring the mass to check if it’s all still there.

Question 40

What are enzymes (i know this is chemistry, but its an important chemical reaction!!)?

Answer 40

Enzymes are biological catalysts.

They catalyse (speed up) the chemical reactions in cells.

Reactions catalysed by enzymes include respiration, photosynthesis and protein synthesis.

Question 41

What is a reversible reaction?

Answer 41

A reversible reaction is one where the products can react with each other to produce the original reactants.

A + B C + D

is the reversible reaction symbol lol.

Question 42

How do reversible reactions reach an equilibrium?

Answer 42

1) During the reaction between the reactants, their concentrations fall- so the forwards reaction will slow down.
The backwards reaction will speed up.

2) After some time, the forwards reaction will be going at exactly the same rate as the backward one- this is equilibrium.

3)At equilibrium, both reactions are still happening, but there is no overall effect- there is a dynamic equilibrium.
So the concentrations of reactants and products have reached a balance and won’t change.

4)Equilibrium can only be reached if the reversible reaction takes place in a close system.
A closed system just means that none of the reactants or products can escape.

Question 43

Does having an equilibrium mean the amounts of reactants and products are equal? Explain your answer:

Answer 43

1) No, sometimes the equilibrium will lie to the right- means that the concentration of products is greater than the concentration of the reactants.
2) Sometimes the equilibrium will lie to the left- this means that the concentration of reactants is greater than the concentration of products.
3) The exact position of the equilibrium depends on the conditions (and reaction itself).

Question 44

What factors can affect the equilibrium?

Answer 44

• Temperature
• Pressure (only affects equilibria involving gas
• Concentration

Question 45

What does le Chatelier’s Principle state?

Answer 45

Le Chatelier’s principle states that if there’s a change in concentration, pressure or temperature in a reversible reaction, the equilibrium position will move to counteract that change.

Question 46

According to Le Chatelier’s Principle, what happens if you change the temperature of a reversible reaction?

Answer 46

All reactions are exothermic in one direction, and endothermic in the other:

• If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat.
• If you increase the temperature, the equilibrium will move in the endothermic direction, to absorb the extra heat.

Question 47

According to Le Chatelier’s Principle, what happens if you change the pressure of a reversible reaction?

Answer 47

This only affects gases:

• If you increase the pressure, the equilibrium will move towards the side that has fewer moles of gas to reduce pressure.
• If you decrease the pressure, the equilibrium will move towards the side that has more moles of gas to increase pressure.

Question 48

According to Le Chatelier’s Principle, what happens if you change the concentration of a reversible reaction?

Answer 48

• If you increase the concentration of the reactants, the equilibrium will move to the right to use up the reactants (making more products).
• If you increase the concentration of the products, the equilibrium will move to the left to use up the products (making more reactants).
• Decreasing the concentration will have the opposite effect.

Question 49

How can you predict how the position of equilibrium will change? How does this benefit industry?

Answer 49

You can apply Le Chatelier’s Principle to any reversible reaction to work out how changing the conditions will affect the equilibrium position.
This has useful applications in industry- you can increase yield by changing the conditions to shift the equilibrium position to the right (towards products).