2: Water Flashcards
When are covalent bonds formed?
when two atoms share a pair of electrons
What happens if the nucleus of one of the atoms in a covalent bond is more attractive to the electrons than the other nucleus?
electrons are not shared equally
= part of the molecule has a slight positive charge and another part has a slight negative charge
What is the term for ‘when part of the molecule has a slight positive charge and another part has a slight negative charge’.
polarity
Are water molecules polar? Why?
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
How many poles do water molecules have? What does this make them? What, therefore, do they show?
2 poles - therefore dipoles
- show dipolarity
What sort of bond forms between the positive pole of one water molecule and the negative pole of another?
a hydrogen bond
What happens when a hydrogen bond is made? What happens when a hydrogen bond is broken?
- energy is released
- energy is used
(this is true when any bond is made/broken)
What sort of energy is used to break hydrogen bonds in water? What does this explain?
- heat energy
- why water is such a good coolant, and therefore why sweat is used as a coolant
What does evaporation of sweat remove from the body?
heat
How can the significance of hydrogen bonding in water be illustrated?
by comparing water with methane - a substance with similar molecular mass that has weaker intermolecular forces, no hydrogen bonds
State the 4 properties for which water and methane can be compared to illustrate the effect of hydrogen bonds.
- melting point
- specific heat capacity
- latent heat of vaporization
- boiling point
Contrast the melting point of water and methane. Explain the difference.
- 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
Contrast the specific heat capacity of water and methane. Explain the difference.
- 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
Contrast the latent heat of vaporization of water and methane. Explain the difference.
- 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
Contrast boiling point of water and methane. Explain the difference.
- 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.
What is the term used to describe a molecule that is attractive to water? What sort of bonds from between the two molecules?
- hydrophilic
- intermolecular bonds
Give general examples of a type of compound and a type of substance that is attractive to water?
- molecule = ionic compounds
- substances with polar molecules
both hydrophilic
If a substance is not hydrophilic, what is it?
hydrophobic
What does hydrophobic mean?
NOT ‘repelled by water’
but, ‘that water molecules are more strongly attracted to each other than to the non-polar molecules of hydrophobic substances’
What sort of molecules are insoluble in water?
hydrophobic molecules
Blood transports a variety of substances. How are most of the substances transported? What does the mode of transport of a substance depend on?
- in the blood plasma
- depends on the substances solubility in water
How is sodium chloride transported in the blood?
dissolved in the plasma as Na+ and Cl- ions
How are glucose and amino acids transported in the blood?
both polar molecules so can be dissolved in the plasma
How is oxygen transported in the blood?
red blood cells with haemoglobin (to which oxygen binds) are needed because oxygen is non-polar
How are cholesterol and fats transported in the blood?
non-polar and insoluble in water:
- transported in small droplets called lipoproteins
lipoproteins = cholesterol and fat inside, coated by phospholipids and proteins
