Phase Changes and Heat Transfer - Video Notes
Phase Changes, Heat, and Enthalpy — Study Notes
Heating curve intuition
- Slanted diagonal lines indicate temperature rise with added heat.
- Plateaus (flat parts) indicate phase changes where temperature remains constant (isothermal).
- Isothermal points occur during phase changes because energy goes into changing the state of the substance, not increasing its temperature.
- The y-axis shows temperature; during phase change, temperature stays at the phase-change temperature (e.g., 0°C for melting/freezing of ice at 1 atm; 100°C for boiling/condensation of water at 1 atm).
Why phase changes appear plateaulike on heating curves
- At the phase-change temperature, all molecules must reach that temperature and reorganize into the new phase.
- Heat is required to overcome intermolecular forces to transform the entire sample from one phase to another.
- Although the temperature reads the same for a period, the system is undergoing a transformation of the phase.
- Once the phase change completes, additional heat causes a new temperature rise (or fall during cooling).
Key constants and terms
- Heat of fusion (enthalpy of fusion): the energy required to melt (or freeze) a given amount of substance.
- Heat of vaporization (enthalpy of vaporization): the energy required to vaporize (or condense) a given amount of substance.
- Enthalpy (H) is a form of heat content; in many introductory chem courses, the symbol ΔH is used to denote the enthalpy change during a process.
- For phase-change quantities, q = m ΔHfus (melting/freezing) or q = m ΔHvap (vaporization/condensation).
Core equations you should memorize
- Sensible heat (temperature change only): q = m c \, \Delta T
- Phase-change heat (no temperature change during the change): q = m \Delta H{fus} for fusion (melting) or q = m \Delta H{vap} for vaporization
- Total heat for a process that includes both heating/cooling and phase changes: sum of the contributions from each step, i.e., q{total} = q{phase\ change} + q{temperature\ change} = m \Delta H{fus\ or\ vap} \ +\ m c \Delta T
Practical notes on units
- For water, common unit for c is c = 4.184\ \text{J g}^{-1}\text{°C}^{-1}.
- Common units for ΔHfus and ΔHvap: per gram (e.g., J g⁻¹) or per mole (e.g., kJ/mol). Typical per-gram values for water are:
- \Delta H_{fus} \approx 333\ \text{J g}^{-1}
- \Delta H_{vap} \approx 2257\ \text{J g}^{-1}
- In many problems, constants are provided by the instructor; use those values for quizzes/exams.
- You may encounter calories as a unit; 1 cal = 4.184 J, so you can convert between units as needed.
Important problem-solving strategy
- Read the problem for keywords:
- Keywords indicating a phase change: melt, freeze, boil, condense, vaporize, sublimation.
- If a single temperature is given (and the word melt/boil/condense is present), the problem likely involves a phase change with no initial/final temperature change apart from the phase-change point.
- If both a phase change and a temperature change are described, you must calculate both contributions and add them to obtain the total heat.
- Always check the mass; if the prompt provides mass, use it; if you notice a discrepancy (e.g., 120 g vs 125 g), note it and follow the given numbers in your calculation or clarify during practice.
Worked examples from the lecture
Example A: How much heat is necessary to melt 55.8 g of ice at 0°C? Answer in calories.
Key idea: The problem involves melting (phase change) at a fixed temperature.
Appropriate equation: q = m \Delta H_{fus}
The problem statement does not provide a value for (\Delta H_{fus}) in the transcript, so you would need the given constant (e.g., 80 cal g⁻¹ or 333 J g⁻¹ per gram) from your course materials or exam constants to compute a numeric answer.
Note: Since the temperature does not change during fusion, you do not use the q = m c \Delta T part for this step.
Final numeric result depends on the provided (\Delta H_{fus}) value (convert to calories as requested).
Example B: Melting ice and raising temperature to 37°C
Prompt (as given): An ice bag filled with either 120 g or 125 g of ice at 0°C; melt the ice and then raise the temperature to 37°C. Find the total heat in kilojoules.
Step 1: Phase-change heat (melting)
- Use q1 = m \Delta H{fus}
- Transcript uses mass = 125 g and (\Delta H_{fus} = 334\ \text{J g}^{-1}) (this is a typical per-gram value used in their worked calculation)
- Calculation (as shown): q_1 = 125\ \text{g} \times 334\ \text{J g}^{-1} = 41{,}750\ \text{J} = 41.8\ \text{kJ}
Step 2: Temperature-change heat after melting
- Use q_2 = m c \Delta T
- Given: mass = 125 g, c=4.184\ \text{J g}^{-1}\text{°C}^{-1}, \Delta T = 37\,\text{°C}
- Calculation: q_2 = 125 \times 4.184 \times 37 \approx 19{,}351\ \text{J} = 19.4\ \text{kJ}
Step 3: Total heat
- q{total} = q1 + q_2 = 41.8\ \text{kJ} + 19.4\ \text{kJ} = 61.2\ \text{kJ}
Note about mass discrepancy: The prompt text mentions 120 g, but the worked steps use 125 g. In exams, use the mass provided; if a mismatch occurs, seek clarification or document the assumption when solving.
Quick recap and lab/exam relevance
- The lab problems will mirror these heat-transfer concepts and require you to combine phase-change heat with sensible heating.
- There were four homework questions in the lecture; answer them on separate paper and submit by the deadline in Brightspace.
- If you need help, attend office hours or email the instructor.
Practical tips for exam readiness
- Always identify whether a segment involves a phase change or a temperature change first.
- Use the appropriate formula for each segment and keep units consistent.
- Convert units as necessary to report the final answer in the requested units (calories, kilojoules, etc.).
- Remember: at the phase-change temperature, temperature does not change; energy goes into changing the phase.
Final takeaway
- To solve problems with both temperature changes and phase changes, separate the problem into two parts: (1) energy to achieve phase change (qphase = m ΔHfus or ΔHvap) and (2) energy to change temperature after the phase change (qtemp = m c ΔT), then sum them for the total energy.
Contact and next steps
- For clarifications, revisit the lecture slides, and use office hours or email to ask about specific constants provided for your course.