Water

Question 1

What two types of bonds do Water molecules have?

Answer 1

Hydrogen bonds (dipole-dipole) and dispersion forces

Question 2

How are hydrogen bonding responsible for the unique properties of water? (5)

Answer 2

dominant intermolecular force
Such as the ability to dissolve many substances, unusually high melting and boiling points, low vapour pressure, high surface tension and high density in the solid and liquid phases.

Question 3

How does the strength of the hydrogen bonds affect H2O?

Answer 3

it causes water to have relatively high latent heat values.

Question 4

What are latent heat values?

Answer 4

a measure of the energy needed to change the state of a substance at its melting or boiling point

Question 5

What is the latent heat fusion of water?

Answer 5

6.0kJ mol-1

This means that 6.0kJ of energy is needed to change 1 mole of water from a Solid to a Liquid at 0C.

A relatively large quantity of energy is require to break the hydrogen bonds between the molecules and disrupt the ice lattice

Question 6

What is the latent heat of vaporisation of water?

Answer 6

It is 44.0kJ mol

This means that 44.0kJ is needed to change 1 mole of water from a liquid to a gas at 100C

Even a larger amount of energy is needed to break all the hydrogen bonds and seperate the water molecules to form a gas

Question 7

How is latent heat of vaporisation of water effective for cooling down during perspiring?

Answer 7

When you sweat water evaporates by absorbing heat energy from your skin.

Water is able to absorb enough energy as it evaporates for you to be cooled efficiently.

Question 8

What is heat capacity of a substance?

Answer 8

Is a measure of the substances capacity to absorb and store heat energy

Question 9

Why does water have a high heat capacity?

Answer 9

Because the hydrogen bonds between water molecules are able to absorb and store large amounts of energy (water has relatively high specific heat capacity)

Question 10

How is it possible to have a non-polar molecule with polar bonds?

Answer 10

Bonds within CO2 are polar as C and O have different electronegativity

The overall molecules is non-polar as the sum of the 2 dipoles cancel leaving the molecule non-polar

Question 11

What are hydrophilic substances? (and examples)

Answer 11

substances that readily dissolve in water

ionic compounds and polar substances (some polar organic substances)

Question 12

What are hydrophobic substances ? (and examples)

Answer 12

substances that don’t dissolve in water (tend to be gases or non-polar substances)

Question 13

What substances may dissolve in water? And what may it form?

Answer 13

if they are polar or charged

this solute may form an ion-dipole, dipole-dipole or hydrogen bond with water

Question 14

What do happens when a salt dissolve in water?

Answer 14

ion-dipole bonds are formed.

the positive part of the water molecule is attracted to the anion of the salt, water molecules will surround the anion until it is perfectly hydrated.

Question 15

Describe the process of a salt dissolve in water. (4)

Answer 15

1. ion is pulled away from the solid lattice
2. Water molecules above and below the ions
3. H atoms have a build up of negative charge so they orientate towards a positive ion OR the O atoms have a build up of negative charge, so they orientate towards a positive ion
4. This forms a hydrated cation

Question 16

Are most organic molecules polar or non-polar and hydrophobic or hydrophilic?

Answer 16

non-polar and hydrophobic

Question 17

How can some organic molecules that hydrophobic in nature be polar? (and also what are some groups that vie polarity)

Answer 17

as some have polar groups are attached to the carbon chain which decreases the hydrophobic nature
eg. -Oh, C=O and -NH

Question 18

What are some organic molecules that can dissolve in water? (5)

Answer 18

Glucose, amino acids, vitamins, enzymes and hormones

Question 19

Give an example of how organic compounds can dissolve in H2O (ethanol)

Answer 19

Ethanol dissolves in H2O due to its polar ends (OH group)

It also has a non-polar end due to the CH3, ethanol will also dissolve in organic solvents

Question 20

What are other factors influencing solubility of substances in water? (2)

Answer 20

Energy required to separate the particles has to be less than the energy released when the particles are hydrated.

Temperature

Question 21

How does temperate affect solubility of substances

Answer 21

it alters the rate at which a substance will dissolve and the amount that can dissolve

If a substance reacts with H2O, then the solubility is higher

Question 22

What are the 2 factors that solubility depends on?

Answer 22

energy required to separate ions from the lattice

attraction of water molecules to the solute

Question 23

Why is energy required to separate ions from the lattice dependent on?

Answer 23

strength of the ionic bonding

Question 24

What is the attraction of water molecules to the solute dependent on?

Answer 24

strength of the ion-dipole attraction

Question 25

What is the strength of the ionic bonding, (which energy required to separate ions from the lattice dependent on) dependent on? (3)

Answer 25

spatial arrangement of the ions in the lattice

size of the ions

charges on the ions

Question 26

What is the strength of the ion-dipole attraction, (which the attraction of water molecules is dependent on) dependent on? (3)

Answer 26

sizes of the ions
change on the ions
geometry of the ion, if polyatomic

Question 27

How is the energy released from the salt dependent on the solubility of the salt?

Answer 27

the more energy released compared with the energy needed, the more soluble is the salt

Question 28

What happens in a reaction of water and a salt (s)?

Answer 28

the salt (s) dissolves into seperated ions (aq)

eg. NaCl(s) -(H2O)–> Na+(aq) + Cl- (aq)

Question 29

What happens in a reaction with a molecular substances that is a solid with water?

Answer 29

the molecular substances (CO2, HCl or C6H12O6), becomes aqueous

Question 30

What happens in a reaction with water and an acid?

Answer 30

the acid (aq) and dissolves to separate ions (aq)

Question 31

When will a solute dissolve in water?

Answer 31

when the energy of the bonds it forms with water is lower than the energy of the bonds between H2O molecules or H2O and ions of the substances between dissolved.

Question 32

Define the solubility of a solute.

Answer 32

solubility of a solute is the max. amount in grams that can dissolve in 100g of the solvent at a given temp