Starter: What is a change of state?

Lesson goals… Describe melting, boiling (the difference between boiling and evaporation will come later). Describe condensation & solidification. Explain latent heat of vaporisation & latent heat of fusion including specific latent heat. Describe experiments to measure the latent heats using steam and ice. Know and be able to use the equation E=mL

Each change of state has a name: Adding heat energy Removing heat energy Liquid Melting Boiling Freezing Condensing Sublimation Solid Gas

Changes of state Example water: There are three states or phases, solid (ice), liquid or gas (water vapour). Energy is needed (taken in) or given out when a change of state occurs. The melting point is the temperature at which the solid changes phase to become a liquid. The boiling point is the temperature where the liquid changes phase to a gas.

This flat line shows where energy is being used to push the particles further apart and break the final intermolecular bonds Temp/OC This slope shows how energy is being used make molecules vibrate more and move faster so further increasing the KE of molecules 150 100 50 -50 This slope show how energy is being used make molecules move faster – NO intermolecular bonds left This slope shows how energy is being used make molecules vibrate more and so increase KE of molecules Is this a heating curve for ice? Write a description of what is happening at each stage of this heating curve. You also need to estimate /calculate energy input needed at each stage on graph Time/s This flat line shows where energy is being used to break intermolecular bonds – this has to be done during melting

LATENT HEAT Energy needed to break bonds and melt or boil a material is known as LATENT HEAT Hidden: Additional heat energy doesn’t show as temperature change Specific: Just as for heat capacity, the specific latent heat is the energy needed to change the state of 1kg of the material.

Latent heat of fusion

Latent heat of fusion During the melting time energy is supplied to the ice causing it to change phase. The energy absorbed is called the latent heat of fusion. Ice has a specific latent heat of fusion of 330 000 J/kg

Latent heat of vaporisation At the boiling point, the energy is used to change from a liquid to a gas. The latent heat of vaporisation for water is 2 300 000 J/kg Why do you think more energy is needed to change state from a liquid to a gas?

To calculate the energy needed to change state… 𝐸𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑=𝑚𝑎𝑠𝑠 𝑥 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑙𝑎𝑡𝑒𝑛𝑡 ℎ𝑒𝑎𝑡 𝐸𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑=𝑚𝐿 Example: If 2kg ice is melted (at 0oC): Energy transferred = mL= 2 x 330 000 J/kg =660 000J Question How much energy is needed to change 10kg ice into water at the same temperature? 10k water into water vapour at the same temperature?

Answers 10 kg ice to water E = mL = 10 x 330 000 = 3 300 000 J b. 10 kg water to water vapour E = mL = 10 x 2 300 000 = 23 000 000 J

A jeweller wants to melt 100 g of silver A jeweller wants to melt 100 g of silver. The specific latent heat of fusion for silver is 100 000 J/kg. How much energy will be required? Energy = mass x specific latent heat m = 100g = 0.1 kg L = 100,000J/kg E =mL E =0.1 x 100 000 =10 000J = 10kJ

Extension: What about sulphur? Sulphur sublimes, why do you think that might be?

Experiment to measure the specific latent heat of fusion Method: Heat the ice with the electric heater. Collect at least 30 ml water Weigh the water collected. Knowing the energy supplied to the ice (from the energy meter) use the equation E = mL to calculate L, the specific latent heat of fusion for water.

Results: Mass of dry 100ml beaker = ____________ Mass of 100ml beaker and water = _______ Mass of water = _____________ Energy supplied to ice = ________________ E = mL L (specific latent heat of fusion ice) = _____ Why is this result approximate?

Specific latent heat of vaporisation of water One method to measure the specific latent heat of vaporisation of water Measure the change in mass of boiling water over a fixed period of time (e.g. 500s) and the energy supplied to the boiling water by the electric heater. E = mL L= E/m This result is approximate, why?

Name that key term...

The amount of energy required to raise the temperature of 1kg of a substance by 1˚C

The temperature at which a substance turns from liquid to gas

These are present in solids and liquids but not in gases

A picture that uses colour to represent different temperatures

The point reached when all temperatures are equal

Occurs throughout the liquid   BOILING EVAPORATION A process in which a substance changes its state from the liquid state to the gaseous state A process in which a substance changes its state from the liquid state to the gaseous state without boiling Fast Slow Bubbles are formed No bubbles formed Occurs throughout the liquid Takes place only from the exposed surface of the liquid Occurs at a definite temperature – Boiling point Occurs at all temperature  Source of energy needed  Energy supplied by surroundings