SC14a-d

Question 1

What is the percentage yield of a chemical reaction?

Answer 1

Shows how much product was actually made compared with the amount of product that was expected.

Question 2

What is the theoretical yield?

Answer 2

The maximum mass of product expected from the reaction, using reacting masses.

Question 3

What is actual yield?

Answer 3

The mass of the product that is actually obtained from a chemical reaction.

Question 4

How do we calculate percentage yield?

Answer 4

Percentage yield = actual yield / theoretical yield x 100

Question 5

Why is percentage yield never 100%?

Answer 5

1. Reaction may be incomplete so not all reactants are used up - occurs if not left long enough or the reaction is reversible
2. Might be unwanted side reactions - a reactant might react to make a different product
3. Transfer losses (e.g. left on sides of the container)

Question 6

What is atom economy?

Answer 6

A method of showing how efficiently a particular reaction makes use of the atoms in the reactants.

Question 7

How do we calculate atom economy?

Answer 7

Atom economy = Mᵣ of useful product / sum of Mᵣ of all reactants or products x 100

Question 8

What problems can disposal of waste products cause?

Answer 8

1. Unpleasant smell near the chemical factory/industry
2. We have to treat them (e.g. react with other substances to make them safe) which may be expensive
3. Environmental problems such as: water pollution, burning may release toxic/acidic gases into the atmosphere
4. Social problems such as house prices will drop near the industry

Question 9

How can we improve the atom economy of a reaction?

Answer 9

Finding uses for the by-products of the reaction.

Question 10

Why is it important if the chemical reaction takes place quickly?

Answer 10

Cost: We do not have to keep heating the reaction for a long period of time, saving electricity and energy resources such as fossil fuels.

Impact to the environment: Less fossil fuels are burned (e.g. less CO₂ given out which contributes to the greenhouse effect / less SO₂ when sulfur impurities are present which is an acidic gas that causes acid rain).

Question 11

How do we find the concentration of a solution?

Answer 11

1. Find mass of solute (solid to be dissolved) in grams
2. Dissolve the solute in a known volume of solvent (usually water)
3. Concentration = mass (g) / Volume (dm³)

Question 12

What are the conversion factors for decimetres?

Answer 12

1 dm³ = 1000 cm³ = 1000 ml = 1 litre

Question 13

How do we calculate concentration (mol dm⁻³/ g dm⁻³)?

Answer 13

Concentration (g dm⁻³) = mass of solute (g) / volume of solution (dm³)

Concentration (mol dm⁻³) = number of moles of solute / volume of solution (dm³)

Concentration (mol dm⁻³) = concentration (g dm⁻³) / Mᵣ of solute

Question 14

Why do we carry out titration?

Answer 14

To find the exact amount of alkali needed to react with an acid solution so that the two solutions are mixed in the exact proportions and completely react with each other.

Question 15

What is the required apparatus when performing an acid-alkali titration experiment?

Answer 15

• Hydrochloric acid of unknown concentration
• Sodium hydroxide solution (concentration 0.1 mol dm³)
• Methyl orange indicator
• Bottle of distilled water
• White tile
• Conical flask
• Pipette and filler
• Burette and stand funnel
• Beaker to collect unwanted solutions

Question 16

How do we carry out an acid-alkali titration experiment?

Answer 16

1. Take a burette and wash out using acid to remove any residue from previous experiments
2. Fill burette with the acid so that there is no air in the jet space and the reading is close to zero but not above the line
3. Using a pipette filler and pipette measure out 25cm³ of sodium hydroxide solution and put it into a clean conical flask
4. Add a few drops of methyl orange indicator, enough to see a yellow solution
5. Place this on a white tile under the burette stand
6. With the burette in the stand let out the acid into the conical flask, swirling the flask as you do so
7. When the solution stays orange this is the end point and the reading on the burette should be noted down
8. Repeat this making sure you top up the burette as required 4 more times or until results are concordant
9. If the solution is red, then too much acid has been added and experiment should be repeated

Question 17

What are the uses of titration?

Answer 17

1. Working out concentration of pollutants in water
2. Checking concentration of substances in the blood
3. Verifying the concentration of manufacturer’s products

Question 18

What are the control variables of the acid-alkali titration experiment?

Answer 18

• Always stick to stated values with each titration

Question 19

What are some improvements that can be made for the acid-alkali titration experiment?

Answer 19

• Slow down burette so acid is added drop by drop as end point approaches
• Read burette at eye level and from the bottom of the meniscus
• Make sure funnel is removed during titration to prevent drops from entering the burette
• Ensure jet space on burette is filled with acid

Question 20

What are the safety precautions for the acid-alkali titration experiment?

Answer 20

• Wear eye protection to protect against splashes from sodium hydroxide or hydrochloric acid