TAE: Acid/Base Theories

Question 1

What did Lavosier contribute to the historical development of ideas about acids?

Answer 1

Proposed that acids contained oxygen.

He showed that non-metal oxides, when dissolved in water formed acids.
Eg. H2O being added to CO2

Did not explain why oxides of metals were not acidic, or why acidic substances (HCL) contained no oxygen

Question 2

What did Davy contribute to the historical development of ideas about acids?

Answer 2

Proposed that acids contained replaceable hydrogen –> showed that acids did not contain oxygen

Acids reacted with metals to form salts.
Bases reacted with acids to form salts and water.

Did not explain why compounds of hydrogen were not acid (methane)

Question 3

What did Arrhenius contribute to the historical development of ideas about acids?

Answer 3

Propsed that Acids produce hydrogen ions (H+) when in solution
Bases produce hydroxide ions (OH-) when in solution

Acids were strong if they ionised completely and weak if the ionised only slightly

His theory explains the process of neutralisation.

Did not recognise the role of the solvent in ionisation

• Why metallic oxides and carbonates are basic
• Why some solutions of various salts are acidic or based rather than neutral.

Question 4

Outline the Bronsted-Lowry theory of acids and based

Answer 4

An acid is a proton donor.

A base is a proton acceptor.

Question 5

What is the Bronsted-Lowry theory of acids and bases important?

Answer 5

• Recognised the importance of hydrogen ions in explaining acid-base behaviour, which was considered as a proton exchange.
• A substance cannot act as an acid without another acting as a base.

–> Explained how some substances could be both acidic and basic (i.e. amphiprotic).

Question 6

What is a conjugate base?

Answer 6

A conjugate base is formed when an acid donates a proton.

Question 7

What is a conjugate acid?

Answer 7

A conjugate acid is formed when a base accepts a proton.

Question 8

Describe the relationship between an acid and its conjugate base and a base and its conjugate acid

Answer 8

The stronger an acid, the weaker its conjugate base.

The stronger a base, the weaker its conjugate acid.

Question 9

What are amphoteric substances?

Answer 9

Amphoteric substances are capable of behaving as either acids or bases (refers to oxides).

Question 10

How is zinc oxide able to behave as an amphothetic substance?

Answer 10

Base: ZnO + 2HCL –> ZnCl2 + H2O

Acid: ZnO + 2NaOH –> Na2ZnO2 + H2O

Question 11

What are amphiprotic substances?

Answer 11

Amphiprotic substances are capable of behaving as either Brönsted-Lowry acids or Brönsted-
Lowry bases

Water:
Base: H2O + HCl –> H3O+ + Cl-

Acid: H2O + NH3 –> NH4+ + OH-

Question 12

Define what a buffer solution is

Answer 12

A buffer solution contains comparable amounts of a weak acid and its conjugate base, and is able to resist changes in when small quantities of a strong acid or base are added.

Question 13

What is an example of buffers in nature?

Answer 13

H2CO3- HCO2 - buffer solution

Occurs in freshwater lakes and rivers to maintain the pH between 6.5 and 7.5 to allow biological processes to occur favourably

CO2 from the air dissolves in water to form carbonic acid, which establishes the buffer solution

NOTE: Only works if limestone is available

Question 14

What are neutralisation reactions?

Answer 14

Neutralisation reactions are proton transfer reactions.

eg. HCl transfers a protion to the base OH- to form H2O

Neutralisation is an exothermic reaction, so heat energy is liberated.

Question 15

What are the positives of using neutralisation reactions as a safety measure or to minimise damage in accidents or chemical spills

Answer 15

Neutralisation reactions are needed as strong acids or bases are very corrosive. They are also used to ensure effluents from factories/labs are neither acidic nor alkaline.

Neutralisation produces salt and water, which are relatively harmless compared to the high toxicity of strong acids and bases which can cause damage to waterways and aquation life by altering the pH, damage to built structures, and chemical burns to people.

Question 16

What are the negatives of using neutralisation reactions as a safety measure or to minimise damage in accidents or chemical spills

Answer 16

Neutralisation reactions generate large amounts of heat which could potentially damage structures or the environment. However this can be minimised by using a weak acid/base for the neutralisation of spills

In the case of chemical spills, adding sufficient liquid to neutralise the spill may cause it to spread out too much. This can be minimised by using a powder such as for the neutralisation

It is sometimes hard to know whether a spill is acidic or basic, so neutralisation may be difficult. This can be solved by using a weak amphiprotic powder for the neutralisation

Question 17

What is the assessment of using neutralisation reactions as a safety measure or to minimise damage in accidents or chemical spills

Answer 17

Although there are negatives associated with the use of neutralisation reactions as safety measures and in cleaning up spills, most of these negatives can be solved or minimised, while there are very significant benefits to the use of neutralisation reactions. Therefore it is clear that neutralisation reactions are very useful for this purpose

Question 18

Identify an example of a neutralisation reaction for chemical spills

Answer 18

Sodium carbonate (Na2CO3) is suitable for neutralising acidic spills as:
– It is a stable solid that is safe to handle.
– It is the cheapest alkali available.
– If too much of it is used, there is less danger than from excess sodium hydroxide or lime.

Question 19

What are characteristics of a good primary standard solution?

Answer 19

Known and high degree of purity
Definite composition
Stability during storage and exposure to air Soluble
High molecular weight (minimises weighing errors)

Question 20

How is a pipette used in volumetric analysis?

Answer 20

• For delivering an aliquot (exact fixed volume) of solution (25 ).
• It is filled by suction (using a pipette filler) to the graduation mark and drained by gravity.
• It has an accuracy of 0.05 so it is more accurate than a measuring cylinder.
• It is rinsed with the solution to be transferred

Question 21

How is a burette used in volumetric analysis?

Answer 21

• For delivering exact volumes of solutions by turning the stop-cock.
• Used to accurately
  determine volume of solution dispensed.
• It is rinsed with the solution to be transferred.

Question 22

How is a conical flask used in volumetric analysis?

Answer 22

• Holds the solution to be titrated and receives liquid from burette.
• Placed on a white tile to reveal colour change in indicator.
• Much easier to swirl without spilling than a beaker.
• It is rinsed with distilled water.