chapter 10

• internal energy

  energy store made up of total kinetic energy associated with random motion of particles and total potential energy between particles
• heat capacity and specific heat capacity


  capacity is dependent on the number of particles and the nature and strength of the intermolecular forces

  number of particles because more particles, more energy can be stored

  the nature and strength of the intermolecular forces because stronger force between particles, more energy can be stored

  heat capacity is seen as a property of an object that consists of parts made of different material

  specific heat capacity is seen as a property of a uniform material
• water does not always boil at 100 degrees celsius

  atmospheric pressure is lower at higher altitudes hence this results in lower boiling point

  increased pressure above the hot water surface will make it difficult to form and escape

  this will raise the boiling point of water, food will be cooked faster
• melting


  change from solid state to liquid state when energy is transferred to the substance

  temperature at which solid melts is called melting point
• boiling


  change from liquid state to gaseous state when energy is transferred to the substance

  temperature at which liquid boils is called boiling point
• condensation


  change from gaseous state to liquid state when energy is transferred out of the substance

  temperature at which a gass condenses is called condensation point
• solidification/freezing

  change from liquid state to solid state when energy is transferred out of the substance

  temperature at which a liquid solidifies or freezes is called freezing point
• boiling

  involves vaporisation, change from liquid to gaseous state

  during boiling, bubbles are seen throughout the liquid

  boiling point will remain constant assuming the fact that there is no change in atmospheric pressure
• evaporation

  involves vaporisation, change of state from liquid to gas

  takes place only at the surface of the liquid exposed to air
• how evaporation works


  the particles in the liquid move at different speeds. evaporation occurs when faster particles at the surface of the liquid have enough energy to break away from the other liquid particles to escape from the air. after the faster particles escape into the air, the particles left behind have slightly lower average speed. as the temperature decreases, the average kinetic energy of the particles in the body decreases, thus the liquid is slightly cooler than the surroundings. this temperature difference leads to the transfer of energy from the surroundings to the liquid
• differences between boiling and evaporation

  boiling needs a heat source but evaporation does not

  in boiling, vaporisation takes place throughout the liquid but in evaporation, vaporisation takes place only at the surface of the liquid

  boiling has a faster rate of vaporisation while evaporation has a slower rate of vaporisation

  boiling only happens at the boiling point while evaporation happens at temperatures below boiling point

  in boiling, liquid temperature remains constant while in evaporation, liquid temperature tends to drop
• what are the 6 factors affecting rate of evaporation?

  pressure, humidity of air, surface area of liquid, temperature, wind speed, boiling point of liquid
• how does pressure affect rate of evaporation?

  the higher the pressure, the lower the rate of evaporation

  at higher pressure, liquid molecules escape into the air less quickly
• how does humidity of air affect rate of evaporation?

  the higher the humidity, the lower the rate of evaporation due to the huge amount of water vapour in the air

  humidity is a measure of the amount of water vapour in the air
• how does surface area of liquid affect rate of evaporation?

  the higher the exposed surface area of the liquid, the higher the rate of evaporation

  more molecules can escape from the surface of the liquid
• how does temperature affect rate of evaporation?

  the higher the temperature, the higher the rate of evaporation

  at higher temperature, the average kinetic energy of the liquid molecules is higher hence more liquid particles can escape into the air
• how does wind speed affect rate of evaporation

  the higher the wind speed, the higher the rate of evaporation

  wind removes the molecules that have just escaped into the air hence the air surrounding the liquid is drier
• how does the boiling point of liquid affect rate of evaporation?

  the lower the boiling point of the liquid, the higher the rate of evaporation

  the attractive forces between the particles of liquid with lower boiling point are weaker
• latent heat

  energy released or absorbed to change the state of a substance, at constant temperature

  two types of latent heat

  • for melting and solidification, latent heat of fusion
  • for boiling and condensation, latent heat of vaporisation
• latent heat of fusion

  for melting and solidification/freezing

  change of state from solid to liquid or liquid to solid

  there is an increase/decrease in potential energy but the kinetic energy remains unchanged when the temperature does not change
• latent heat of vaporisation

  for boiling and condensation

  change of state from liquid to gas or gas to liquid

  there is an increase/decrease in potential energy but the kinetic energy remains unchanged when the temperature does not change
• heating curve

  during heating, energy transferred to the substance allows the particles to move faster, there is an increase in kinetic energy. temperature increases with the average kinetic hence the temperature increases. the potential energy of the particles increases with the increase in separation of the particles

  during melting and boiling, energy transferred to the substance results in work done against the attractive intermolecular forces. the average separation of the particles increases hence the potential increases. however, kinetic energy and temperature remains constant
• cooling curve

  during cooling, energy is transferred out of the substance and the particles move slower, leading to a decrease in kinetic energy. the temperature of the substance decreases. the potential energy of the particles decreases as the average separation of the particles decreases.

  during condensation and solidification, energy is transferred out of the substance. the average separation of particles decreases and so does the potential energy of the particles, however, the kinetic energy and temperature remains constant
• internal energy

  internal energy = total kinetic energy of the particles + total potential energy of the particles

  when the substance is heated, internal energy increases

  when substance is cooled, internal energy decreases