2: Water

Question 1

When are covalent bonds formed?

Answer 1

when two atoms share a pair of electrons

Question 2

What happens if the nucleus of one of the atoms in a covalent bond is more attractive to the electrons than the other nucleus?

Answer 2

electrons are not shared equally

= part of the molecule has a slight positive charge and another part has a slight negative charge

Question 3

What is the term for ‘when part of the molecule has a slight positive charge and another part has a slight negative charge’.

Answer 3

polarity

Question 4

Are water molecules polar? Why?

Answer 4

yes:

• hydrogen nuclei are less attractive to electrons than the oxygen nuclei
• so the two hydrogen atoms have a slight positive charge and the oxygen atom has a slight negative charge

Question 5

How many poles do water molecules have? What does this make them? What, therefore, do they show?

Answer 5

2 poles - therefore dipoles

- show dipolarity

Question 6

What sort of bond forms between the positive pole of one water molecule and the negative pole of another?

Answer 6

a hydrogen bond

Question 7

What happens when a hydrogen bond is made? What happens when a hydrogen bond is broken?

Answer 7

• energy is released
• energy is used

(this is true when any bond is made/broken)

Question 8

What sort of energy is used to break hydrogen bonds in water? What does this explain?

Answer 8

• heat energy

- why water is such a good coolant, and therefore why sweat is used as a coolant

Question 9

What does evaporation of sweat remove from the body?

Answer 9

heat

Question 10

How can the significance of hydrogen bonding in water be illustrated?

Answer 10

by comparing water with methane - a substance with similar molecular mass that has weaker intermolecular forces, no hydrogen bonds

Question 11

State the 4 properties for which water and methane can be compared to illustrate the effect of hydrogen bonds.

Answer 11

1. melting point
2. specific heat capacity
3. latent heat of vaporization
4. boiling point

Question 12

Contrast the melting point of water and methane. Explain the difference.

Answer 12

• methane = -182°C
• water = 0°C

ice melts at a much higher temperature: hydrogen bonds restrict the movement of water molecules and heat is needed to overcome this

Question 13

Contrast the specific heat capacity of water and methane. Explain the difference.

Answer 13

• methane = 2.2J per g per °C
• water = 4.2J per g per °C

water’s heat capacity is higher: hydrogen bonds restrict movement so more energy is stored by moving molecules of water than methane

Question 14

Contrast the latent heat of vaporization of water and methane. Explain the difference.

Answer 14

• methane = 760 J/g
• water = 2257 J/g

water has a much higher heat of vaporization: much heat energy is needed to break hydrogen bonds and allow a water molecule to evaporate

Question 15

Contrast boiling point of water and methane. Explain the difference.

Answer 15

• methane = -160°C
• water = 100°C

water’s boiling point is much higher: heat energy is needed to break hydrogen bonds and allow water to change from a liquid to a gas.

Question 16

What is the term used to describe a molecule that is attractive to water? What sort of bonds from between the two molecules?

Answer 16

• hydrophilic

- intermolecular bonds

Question 17

Give general examples of a type of compound and a type of substance that is attractive to water?

Answer 17

• molecule = ionic compounds
• substances with polar molecules

both hydrophilic

Question 18

If a substance is not hydrophilic, what is it?

Answer 18

hydrophobic

Question 19

What does hydrophobic mean?

Answer 19

NOT ‘repelled by water’

but, ‘that water molecules are more strongly attracted to each other than to the non-polar molecules of hydrophobic substances’

Question 20

What sort of molecules are insoluble in water?

Answer 20

hydrophobic molecules

Question 21

Blood transports a variety of substances. How are most of the substances transported? What does the mode of transport of a substance depend on?

Answer 21

• in the blood plasma

- depends on the substances solubility in water

Question 22

How is sodium chloride transported in the blood?

Answer 22

dissolved in the plasma as Na+ and Cl- ions

Question 23

How are glucose and amino acids transported in the blood?

Answer 23

both polar molecules so can be dissolved in the plasma

Question 24

How is oxygen transported in the blood?

Answer 24

red blood cells with haemoglobin (to which oxygen binds) are needed because oxygen is non-polar

Question 25

How are cholesterol and fats transported in the blood?

Answer 25

non-polar and insoluble in water:
- transported in small droplets called lipoproteins

lipoproteins = cholesterol and fat inside, coated by phospholipids and proteins