Analysis of the Earth and Environment Lecture 11: Acids and Bases

Question 1

Define an Acid

Answer 1

Hydrogen containing solution which releases free hydrogen ions
/
A chemical that gives off hydrogen ions in water and forms salts by combining with certain metals

Question 2

Define and describe a hydrogen ion

Answer 2

A hydrogen ion is an ion with no electrons orbiting the nucleus and no neutrons.

There is only one proton present in nuclei.

All hydrogen ions are cations as they have no electrons.

Question 3

Describe how acid molecules and water molecules interact.

Answer 3

Many acids are molecules which dissociate into their ionic components in water and the cation is H+

HA –> H+ + A-(Any anion)

The acid molecule (HA) interacts with the water molecule by the hydrogen ion in the acid molecule transferring to the water molecule creating a hydronium ion.

HA + H2O → H3O+(aq) + A-(aq)

Question 4

Define a strong acid/ what makes acids stronger

Answer 4

A strong acid is fully ionized in water (ie. fully dissociated)

A strong acid reacts completely with water to produce a high concentration of hydronium ions

Question 5

What are some strong acids?

Answer 5

(Acid rain)
Nitric Acid HNO3
Sulfuric Acid H2SO4

Hydrochloric acid HCl

Question 6

Nitric acid composition?

Answer 6

HNO3

Question 7

Sulfuric acid composition?

Answer 7

H2SO4

Question 8

Hydrochloric acid composition?

Answer 8

HCl

Question 9

Define a Weak acid?

Answer 9

A “weak acid” is only partially ionized in water (partially dissociated)

A “weak acid” reacts incompletely with water to produce few hydronium ions

    CH3COOH + H2O       ---->        CH3COO- + H3O+

Question 10

What is the chemical composition of ethanoic acid

Answer 10

CH3COOH

Question 11

What is ethanoic acid and how does it respond to water

Answer 11

CH3COOH is an example of an “organic acid”
It contains the carboxylic acid group –COOH

–COOH group gives up its proton H+
to leave the anion –COO-

Question 12

What are some weak acids?

Answer 12

Carbonic acid HOCOOH

Ethanoic acid CH3COOH (previously known at acetic acid)

Citric acid C5H7O5COOH

Question 13

Describe the chemical process of carbon dioxide dissolving into water

Answer 13

H2CO3 or HOCOOH

CO2(g) + H2O ⇌ H2CO3 ⇌ HCO3− + H+
⇌
CO32− + H+

Question 14

What is a base?

Answer 14

“Compound that produces hydroxide ions in water”

“Any substance which acts as a proton acceptor”

A substance which can neutralise an acid

use the words BASE and ALKALI interchangeably. (However technically it is only an alkali if it is a molecule)

eg. NH3(g) + H2O(l) → NH4+(aq) + OH-(aq)

NH3(Ammonia) being the base as it receives a proton becoming NH4

Question 15

What are some strong bases/ alkalis

Answer 15

Strong bases dissociate 100% into the cation and the hydroxide anion OH-

KOH - potassium hydroxide
NaOH - sodium hydroxide
Ca(OH)2 - calcium hydroxide

Question 16

How do weak bases behave

Answer 16

Weak bases do not simply dissociate to produce OH- ions. They indirectly generate OH- ions from the water molecules they are dissolved in.

Question 17

How does ammonia interact with water?

Answer 17

Ammonia NH3 is a gas at room temperature.
It is frequently used a household cleaner dissolved in water.

NH3 accepts a proton H+ from the water to form the ammonium ion NH4+ Leaving a free hydroxide ion OH-

Question 18

Describe how chlorine reacts with water

Answer 18

Cl2(g) + H2O(l) ⇌ HOCl(aq) + HCl(aq)

When chlorine gas (Cl2 ) is added to water it reacts to form a equilibrium mixture of hypochlorous acid (HOCl) and hydrochloric acid (HCl)

Chlorine gas Cl2 accepts protons so is acting as a WEAK BASE

Question 19

Describe the erosion of limestone(CaCO3) when in contact with Carbon dioxide and water

Answer 19

CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2 (aq)

The calcium carbonate ACCEPTS hydrogen ions (protons) to make soluble calcium hydrogen carbonate .

Question 20

Define pH

Answer 20

The pH of a solution is a measure of the concentration of hydronium ions in the solution

Question 21

What is the calculation for pH

Answer 21

pH = -log [molar concentration of/molarity of susbtance]

e.g. 1.0 x 10-12 moles/dm or 0.3m

Press -, then log

Question 22

What is an equilibrium reaction

Answer 22

After a time, a steady state is reached where the rates of the forward reaction and the reverse reactions are constant. This state is called equilibrium.

Forward reaction
CH3COOH + H2O → H3O+ + CH3COO-

Reverse reaction
CH3COOH + H2O H3O+ + CH3COO-

Question 23

Define the equilibrium constant/K

Answer 23

At a given temperature the ratio of the concentration of reactants to the concentration of products is always the same at steady rate.

Concentration of reactants on left

This ratio is the equilibrium constant.

The equilibrium constant is referred to as K.

Question 24

What does the magnitude of K signify and what are the three categories

Answer 24

Large values (K>1000) = the equilibrium favours the products on the right

Intermediate values (K 0.001 to 1000) =reactants and products in equal amounts

Small values (<0.001) = the equilibrium favours the reactants on the left

Question 25

What is K referred to as when in an acid

Answer 25

Dissociation constant K