CHAPTER 10: WATER

Question 1

heat capacity

Answer 1

the heat capacity of a substance is a measure of the substance’s capacity to absorb and store heat energy

Question 2

specific heat capacity

Answer 2

• specific heat capacity of a substance measures the energy required to raise 1g of substance by 1 degrees Celsius
• it is reported in units of Jg^-1degreesC^-1
• specific heat capacity of a substance reflects the type of bonding in that substance

Question 3

specific heat capacity of water

Answer 3

• water has a specific heat capacity of 4.18Jg^-1degreesC^-1
• means that 4.18 joules of heat energy are needed to increase the temp of 1g of water by 1 degrees celsius
• specific heat capacity of water is relatively high
  
  • this is because of the presence of hydrogen bonds between water molecules
  • hydrogen bonds are stronger than other intermolecular forces and they are able to absorb and store large amounts of energy before they break

Question 4

density of liquid water

Answer 4

• the density of liquid water is 1gml^-1
  
  • so volumes of water in mL are equal to the mass of water in grams (g)

Question 5

calculating using specific heat capacity

Answer 5

• heat energy = mass x specific heat capacity x temperature change
• q = mcΔT
• q is the amount of heat energy in joules (J)
• c is the specific heat capacity in Jg^-1degreesC^-1
• m is the mass in grams
• ΔT is the temperature change in degrees Celsius

Question 6

latent heat

Answer 6

• is the energy required to change a fixed amount of substance, usually 1 mole, from either a solid to a liquid, or a liquid to a gas
• over a period of time, that latent heat is absorbed → the temperature of a substance will not change - energy is being used to break bonds in the substance rather than increase the temperature
• latent heat values are given the symbol L and have the unit kilojoules per mole, kJmol^-1

Question 7

latent heat of fusion

Answer 7

latent heat of fusion is the heat needed to change 1 mole of a substance from a solid to a liquid as its melting point

Question 8

latent heat of vaporisation

Answer 8

the latent heat of vaporisation is the heat needed to change 1 mole of a substance from a liquid to a gas at its boiling point

Question 9

latent heat of fusion of water

Answer 9

• latent heat of fusion= 6.0kJmol^-1
  
  • the energy needed to disrupt the ice lattice by breaking some of the hydrogen bonds between water molecules

Question 10

latent heat of vaporisation of water

Answer 10

• latent heat of vaporisation: 40.7 kJ mol^-1
  
  • this completely breaks the hydrogen bonds

Question 11

formula to change state of substance

Answer 11

• q= n x L
  
  • q is the heat energy (kJ)
  • n is the amount (mol)
  • L is the latent heat value of fusion or vaporisation (kJmol^-1)
• eg. calculate the heat required to evaporate 200g of water at 100 degrees celsius
  
  • n=200/18 = 11.1 mol
  • L =40.7 kJ mol^-1
  • q=11.1 x 40.7 = 452kJ

Question 12

water properties

Answer 12

Water is unusual in that it is more dense in its liquid form than it is in its solid form. This is because, in the solid state, each water molecule forms a maximum of four hydrogen bonds with four other water molecules. This makes the arrangement of water molecules in ice more open than the arrangement of water molecules in liquid water.