Problem-Solving Lab: How much energy is needed to heat water from a solid to a vapor? Making and Using Heating Curves Water molecules have a strong attraction for one another because they are polar. The polarity of water accounts for its high specific heat and relatively high enthalpies of fusion and vaporization. Water’s specific heat and the presence of an enormous amount of water on Earth’s surface have a large influence on weather. ??????????

4.184 J/g°C is defined as the specific heat of water. One calorie, or 4.184 J, is the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius. 4.184 J/g°C is defined as the specific heat of water. Specific heat of any substance – the amount of heat required to raise the temperature of 1g of that substance by 1°C Different substances have their own specific heat values see table 16-2

Chemical Energy and the Universe Thermochemistry is the study of heat changes that go along with chemical reactions and also changes of state (solid – liquid – gas, physical changes, phase changes) Study the energy transfers Universe = system + surroundings Exothermic Notes notes Endothermic notes Specific heat of any substance – the amount of heat required to raise the temperature of 1g of that substance by 1°C What is Enthalpy? Need to recognize this term

Enthalpy and Enthalpy Changes Difficult to know the total energy or heat content of any substance (many factors involved, not all fully understood) But changes in energy can be measured - for example by using a calorimeter (earlier in this section, p 497 Figure 16-5) Enthalpy (H) is defined as the heat content of a system at constant pressure and the change in enthalpy for a reaction is called the enthalpy (heat) of reaction ΔHrxn Calor, Latin for heat - Calorie need to know what Enthalpy of fusion and vaporization are

Changes of State (Phase) Section 16.3 Changes of State (Phase) Molar enthalpy (heat) of vaporization ΔHvap is defined as the heat required to vaporize one mole of a liquid (endothermic process, ΔHvap will have a positive value) H2O(l) H2O(g) ΔHvap = 40.7kJ Molar enthalpy (heat) of fusion ΔHfus heat required to melt one mole of a solid H2O(s) H2O(l) ΔHfus = 6.01kJ Thermochemical equation for change of state

Problem-Solving Lab: How much energy is needed to heat water from a solid to a vapor? named “ a heating curve” Temp vs. time y = mx + c Time plot on x axis, this is the independent variable Temp is what we measure to find its relationship to time, y axis Analysis Use the data in the table to plot a heating curve of temperature vs. time for a 180-g sample of water as it is heated at a constant rate from -20°C to 120°C. Draw a best fit line (curve) through the points. (Not dot to dot). Note the time required for water to pass through each segment of the graph.

Segment 1 – 1 minute Segment 2 – 1.5 min Segment 3 – 7 min Heating Curve for 180g Sample of Water Segment 1 – 1 minute Segment 2 – 1.5 min Segment 3 – 7 min Temperature (°C) Time (min) 1

Segment 5 – 1 min (could go on) Heating Curve for 180g Sample of Water Segment 4 – 13.5 mins Segment 5 – 1 min (could go on) Temperature (°C) Time (min) 1

Thinking Critically 1. For each of the 5 regions (segment) of the graph, indicate how the absorption of heat changes the energy (kinetic or potential) of the water molecules. p. 503 Check notes

16.1 Energy The Nature of Energy Find in your Notes The Nature of Energy Energy is the ability to do work or produce heat Potential energy – due to composition or position of an object Kinetic Energy – energy of motion, directly related to constant random motion of molecules and proportional to Temperature Book Bookcase

Thinking Critically For each of the 5 regions (segments) of the graph, indicate how the absorption of heat changes the energy (kinetic or potential) of the water molecules. Region 1. -20°C to 0°C (first 1 minute) solid ice is warming up. The temperature is rising and we can say that the molecules are moving more quickly, or the kinetic energy of the water molecules is increasing. p. 503 Check notes

Region 2. Temperature remains at 0°C for 1.5 minutes as solid ice is melting to the liquid phase. The temperature is not rising so the molecules are not increasing their kinetic energy, rather the potential energy of the water molecules is increasing as they absorb thermal energy and use that energy to make the transition from being held in a crystal lattice to having more freedom to move around in space. p. 503 Potential energy – due to composition or position

Region 3. Temperature increases from 0°C to 100°C over 7 minutes as liquid water heats up. The temperature is rising so the molecules are increasing their kinetic energy in this region of the heating curve. p. 503

Region 4. Temperature remains at 100°C for 13.5 minutes as liquid water is boiling, and all the molecules are making the transition to the vapor phase. The temperature is not rising so the molecules are not increasing their kinetic energy. The potential energy of the water molecules is increasing. The molecules absorb thermal energy and use that energy to make the transition from being in the liquid phase to having even more freedom to move around in space in the gas phase. Potential energy – due to composition or position

Changes of State (Phase) Go to next section Notes 16.3 Changes of State (Phase) Molar enthalpy (heat) of vaporization ΔHvap is defined as the heat required to vaporize one mole of a liquid (endothermic process, ΔHvap will have a positive value) H2O(l) H2O(g) ΔHvap = 40.7kJ Molar enthalpy (heat) of fusion ΔHfus heat required to melt one mole of a solid H2O(s) H2O(l) ΔHfus = 6.01kJ What do you notice about the size of the heat of vaporization value and the time it took to boil all the water sample vs. the heat of fusion and the time it took for the ice to melt?

Region 5. Temperature increases from 100°C as the molecules are increasing their kinetic energy in this region of the heating curve. p. 503

q = 15,062.4 J = 15kJ Thinking Critically 2. Calculate the amount of heat required to pass through each region of the graph. How does the length of time needed to pass through each region relate to the amount of heat absorbed? Region 1. Use q = cmΔT T final = 0°C T initial = -20°C q = 4.184 J/g°C x 180 g x (0°C - - 20°C) q = 4.184 J/g°C x 180 g x (20°C) q = 15,062.4 J = 15kJ p. 503 Check notes

q = 75, 312 J = 75 kJ Thinking Critically Region 3. T final = 100°C, Tinitial = 0°C Use q = cmΔT q = 4.184 J/g°C x 180 g x (100°C - 0°C) q = 4.184 J/g°C x 180 g x (100°C) q = 75, 312 J = 75 kJ p. 503

q = 15,062 J = 15 kJ Thinking Critically Region 5. T final = 120°C, Tinitial = 100°C Use q = cmΔT q = 4.184 J/g°C x 180 g x (120°C - 100°C) q = 4.184 J/g°C x 180 g x (20°C) q = 15,062 J = 15 kJ p. 503

Thinking Critically cont. Region 2 Thinking Critically cont. Region 2. T final = 0°C, Tinitial = 0°C Can’t use q = cmΔT This is a phase change, need to use the Molar enthalpy (heat) of fusion ΔHfus heat (q) required to melt one mole of a solid H2O(s) H2O(l) ΔHfus = 6.01kJ/mol Molar mass water = 2 + 16 = 18 g/mol How many moles have we got? p. 503

Region 2. cont. Molar enthalpy (heat) of fusion ΔHfus heat required to melt one mole of a solid H2O(s) H2O(l) ΔHfus = 6.01kJ/mol Molar mass water = 2 + 16 = 18 g/mol Mass x mole = 180 g x mole = 10 mol mass1mole 18g To melt all the ice then requires q= number moles x ΔHfus = 10 mol x 6.01kJ/mol = 60 kJ p. 503

Thinking Critically cont. Region 4 Thinking Critically cont. Region 4. T final = 100°C, Tinitial =100°C Can’t use q = cmΔT to find heat required, again this is a phase change need to use the Molar enthalpy (heat) of vaporization ΔHvap heat required to vaporize one mole of a liquid H2O(l) H2O(g) ΔHvap = 40.7 kJ/mol q= number moles x ΔHvap = 10 mol x 40.7kJ/mol = 410 kJ The more heat is required the longer the region will be on the graph. p. 503

Thinking Critically cont. 4 Thinking Critically cont. 4. What would the heating curve of ethanol look like? Make a rough (still neat) sketch of ethanol’s curve from -120°C to 90°C. Ethanol melts at -114°C and boils at 78°C. What factors determine the actual lengths of the flat regions of the graph and the slope of the curve between the flat regions? Hint. See tables 16-2 for Specific heat values and 16-6 for enthalpy changes of fusion and vaporization. p. 503

The length of the plateaus depend on the ΔH of fusion and vaporization values, and the rate of heating. The length of the sloping regions depends on specific heat capacity of ethanol and the rate of heating. 78 Time (min) -114