ChemCatalyst

1. ChemCatalyst • Imagine that you have a sample of 100 g of water at 85 °C. You place 100 g of ice at 0 °C into the water. • Predict the final temperature from the choices given. Explain your choice. • Describe how you might measure the amount of heat needed to melt the ice. A. 80°C B. 60°C C. 42.5°C D. 20°C E. 5°C

2. You will be able to: • define latent heat • describe the energy exchanges that take place during a phase change • explain the heating curve of water

3. Discussion Notes • The graph showing changes in temperature as you heat water from solid to liquid to gas is called the heating curve of water. During a phase change, the heat transferred to or from a substance does not result in a change in temperature.

4. Discussion Notes (cont.) • When a substance changes phase during heating, energy must be supplied to overcome the attractions between molecules. • Heat involved in phase changes can be exothermic or endothermic depending on the phase change.

5. Discussion Notes • Heat of vaporization: heat required to change 1 gram of a substance from a liquid into a gas • Heat of fusion: heat required to change 1 gram of a substance from a solid to a liquid

6. Discussion Notes: • Examples: • 1. How much energy is required to melt 56 g of ice? The heat of fusion of ice is 79.9 cal/g. • 2. How much energy is needed to boil 75 g of water? The heat of vaporization is 539 cal/g?

7. Answers • 56g x 79.9 cal= 4474.4 cal • 1 g • 2. 75 g x 539 cal= 40425 cal • 1 g

8. Discussion Notes • Examples: • 3. How much energy is needed to melt a 25 g sample of ice and raise the temperature to 40℃? The heat of fusion is 79.9 cal/g.

9. Calculate the energy needed to melt the ice • Calculate the energy needed to heat the water to 30 degrees Celsius • Add the two values together • 25 g x 79.9 cal= 1997.5 cal • 1 g • q = mCpΔT • q = 25g x 1 cal/g°C x 40°C = 1000 cal • Total energy = 2997.5 cal

10. Wrap Up • What happens to the energy that is transferred during a phase change? • The heating curve of water beginning with ice and ending with boiling water shows a sloped line, a plateau, another sloped line, and another plateau. • The two plateaus on the heating curve show that the temperature remains constant as ice melts and as water boils.

11. Wrap Up (cont.) • Heat that is required to change a substance from a liquid to a gas is called the heat of vaporization. Heat required to change a substance from a solid to a liquid is called the heat of fusion.

12. Check-In • If you boil water on a stove, the temperature of the water will remain close to 100 °C. As long as some liquid water remains in the pot, the temperature of the water will stay the same regardless of the outside temperature. • Explain why this is true.