Thermochemistry 5 Boon Chemistry February 6, 2013

Catalyst (1) What formula would you use to calculate the heat needed to increase the temperature of the ice from 200 K to 273 K? (2) What formula would you use to calculate the heat needed to melt the ice? (1) What formula would you use to calculate the heat needed to increase the temperature of the ice from 200 K to 273 K? (2) What formula would you use to calculate the heat needed to melt the ice?  Objectives I can solve for the total heat transferred to heat or cool a sample through a phase change.  Objectives I can solve for the total heat transferred to heat or cool a sample through a phase change.  Agenda  Catalyst  Multi-Step Thermochemistry Problems  Practice Calculations  Agenda  Catalyst  Multi-Step Thermochemistry Problems  Practice Calculations

Multi-Step Thermochemistry Problems  Today we are calculating the amount of heat transferred to increase or decrease the temperature of a substance and cause a phase change.  We are combining what we learned last week and this week.  Example: How much heat is absorbed when 100 g of water is heated from 365 K to steam at 400 K?

Multi-Step Thermochemistry Problems  Step 1: Break down the problem into “temperature change” and “phase change” steps.  Use the boiling point and melting point as a guide.  Step 2: Solve the steps separately.  Use q=mc Δ T for temperature change and q = m Δ H t for phase change  Step 3: Add up the answers to get the total heat transferred.

 Example: How much heat is absorbed when 100 g of water is heated from 365 K to steam at 400 K?  Step 1: The boiling point of water is 373 K. So, break up the problem in three parts: (a) 365K to 373K, (b) vaporization and (c) 373K to 400K.  Step 2: Set up and solve the problems. Then step 3: add up the answers.

 Example 2: How much heat is released when 2500 g of water is cooled from 70 °C to ice at -10°C?  Step 1: The melting point of water is 0°C. So, break up the problem in three parts: (a) 70°C to 0°C, (b) freezing and (c) 0°C to -10 °C.  Step 2: Set up and solve the problems. Then step 3: add up the answers.

Think, Pair, Share  How many steps would you split these problem into? Why? (1) How much heat is absorbed when 5000 g of water is heated from 275 K to 370 K? (2) How much heat is absorbed when 10,000 g of ice at 0°C is melted and heated to steam at 150°C?

Work Time  1. Work quietly at your table to complete the practice problems.  Get a stamp when you finish and check your work.  2. Work on any thermochemistry worksheets that you have not yet finished.

Homework  Due Thurs/Fri (Feb. 7 & 8): All Thermochemistry notes and handouts.