Acid-Base Equilibria

Question 0

What were alkali’s originally defined as and why was it changed?

Answer 0

Substances that neutralised acids but this was changed as insoluble metal oxides could neutralise acids. So the term alkali was replaced by the term base.

Question 1

What are some general properties of acids?

Answer 1

They are sour tasting and can produce salts.

Question 2

What were acids originally defined as?

Answer 2

Acids were said to contain hydrogen which could be displaced by a metal.

Question 3

What were the second definitions of acids and bases?

Answer 3

An acid produces hydrogen ions in solution.

A base produces Hydroxide ions and neutralises an acid by the reaction: H⁺+OH⁻ = H₂O

Question 4

What was the first problem with the second definitions of acids and bases?

Answer 4

Ionic compounds conduct electricity in solution and HClⓖ conducts but liquid HCL or HCL dissolved in a non polar solvent does not.

Question 5

What was the second problem with the second definitions of acids and bases?

Answer 5

Bases like ammonia (NH₃) neutralise acids by accepting hydrogen ions rather than giving Hydroxide ions: NH₃+ H⁺ = (NH₄⁺)

Question 6

What was the third problem with the second definitions of acids and bases?

Answer 6

Could the hydrogen cation exist in solution on its own as it is a proton and is 100,000 times smaller than than a lithium cation. This means that it experiences electric fields very intensely so it will be attracted to any molecule with unshared electrons. So unhydrated protons do not exist in solution.

Question 7

What is the hydrated proton (H₃O)⁺ called?

Answer 7

An oxonium ion or hydrogen ion.

Question 8

What are the modern definitions of acids and bases, and who put these ideas forward?

Answer 8

An acid is a substance that can donate a proton to another substance.

A base is a substance that can accept a proton from another substance.

Question 9

What is this relationship called:

Acid⇆Base + H⁺

Answer 9

A conjugate acid - Base pair.

Question 10

Write an equation for Brönsted-Lowry acid-base reaction.

Answer 10

Acid 1 + Base 2 ⇆ Conjugate Base 1 + Conjugate acid 2

Question 11

Write an equation for an aqueous acid.

Answer 11

Acid ⇆ Conjugate Base 1 + H⁺

Question 12

Write an equation for a alkaline solution.

Answer 12

Base 2 + H⁺ ⇆ Conjugate acid 2

Question 13

What has to be present if a acid is in solution and why?

Answer 13

A conjugate base, because hydrogen ions cannot exist on their own so must attach to a base.

Question 14

Give two examples of Brönsted-Lowry acids.

Answer 14

HCL + H₂O ⇆ (H₃O)⁺ + Cl⁻

HSO₄⁻ + H₂O ⇆ SO²⁻ + (H₃O)⁺

Question 15

What type of solvent is water?

Answer 15

Water is an amphoteric solvent as it can act as both an acid and a base.

Question 16

What makes a strong acid or base?

Answer 16

When it fully disassociates into ions in water.

E.G:- HCL + H₂O ⇆(H₃O)⁺ + Cl⁻
NaOH + H₂O

Question 17

What makes a weak acid or base?

Answer 17

When it does not fully dissociate into ions. i.e

NH₄OH ⇆ NH₄⁺ + OH⁻
CH₃COOH ⇆ CH₃COO⁻ + H₃O⁺

Question 18

What is present in all aqueous solutions?

Answer 18

Hydrogen ions.

Question 19

What is the hydrogen ion concentration of a neutral solution?

Answer 19

10⁻⁷mol dm³

Question 20

If the hydrogen ion concentration is less than 10⁻⁷mol dm³ what is it said to be?

Answer 20

Acidic

Question 21

If the hydrogen ion concentration is greater than 10⁻⁷mol dm³ what is it said to be?

Answer 21

Alkaline

Question 22

What did sörensen introduce and why?

Answer 22

He introduced the use of the hydrogen ion exponent or pH value as it was inconvenient to express hydrogen ion concentration in negative powers of 10.

Question 23

What is the hydrogen exponent or pH value defined as?

Answer 23

pH = -log₁₀[H₃O]⁺

Question 24

For [H₃O]⁺ of 10⁻¹³ what does pH equal?

Answer 24

pH=13

Question 25

Where are pH values often found?

Answer 25

On liquid soaps or detergents.

Question 26

What is carbonic acid?

Answer 26

Aqueous carbon dioxide.

Question 27

State the carbonic acid equilibria.

Answer 27

CO₂ + H₂O ⇆ H₂CO₃ ⇆ H⁺ + HCO₃⁻ ⇆ CO₃²⁻ + H⁺

Question 28

What is the problem associated with carbonic acid?

Answer 28

Carbonic acid attacks carbonate rocks such as limestone causing erosion via the equation: H₂CO₃ + CaCO₃ ⇆ Ca(HCO₃)₂

Calcium hydrogen carbonate is soluble and as such is carried away by water leaving caves and potholes in limestone regions. When the water evaporates deposits of calcium carbonate are left in the form of stalagmites and stalactites via - Ca(HCO₃)₂➝ H₂O + CO₂ + CaCO₃

Question 29

What is another problem associated with carbonic acid?

Answer 29

As carbonic acid dissolves in the seawater it causes the pH to change despite the presence of Ca²⁺ and Mg²⁺ ions buffer it to some extent. The increase of carbon dioxide in the atmosphere due to the burning of fossil fuels causes these buffers to be overwhelmed and the sea becomes acidic. This has detrimental effects on marine life.