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HEAT

HEAT. Heat , represented by q , is energy that transfers from one object to another because of a temperature difference between them. What is temperature? A measure of the average kinetic energy of the particles of a sample; how fast the particles are moving.

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HEAT

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  1. HEAT Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between them. What is temperature? A measure of the average kinetic energy of the particles of a sample; how fast the particles are moving Heat flows from a warmer object to a cooler object until the temperature of both objects is the same. Consider a glass of ice water.

  2. Heat Units: Calorie, calorie & Joule Calorie = Quantity of heat needed to raise the temperature of 1 g of pure water by 1 degree Celsius. 1 Cal = 1000 cal = 1kcal 1cal = 4.184 Joules (a unit of heat and energy) Example 10.1 Express 60.1 cal of energy in units of Joules.

  3. Phase Change: a change in the form of a substance that affects the speed of the particles, the strength of intermolecular forces*, and proximity of the particles; the composition of the substance is NOT affected. • Endothermic phase changes • Melting • Vaporization • Exothermic phase changes • Condensation • Freezing Phase Changes

  4. Intermolecular Forces Forces of attraction that form between two separate molecules usually due to polarity • Polarity = electronegativity difference leaving partial charges on the atoms • Notice the FIVE water molecules in the picture • The dashed lines show attractions between opposite partial charges on different water molecules.

  5. Endothermic Exothermic Phase Changes

  6. Adding Prior Knowledge • Endothermic changes in enthalpy • Will the measurement be positive or negative? • Will a graph of energy change end higher or lower than it began? • Is this considered heating or cooling a substance? • Exothermic changes in enthalpy • Will the measurement be positive or negative? • Will a graph of energy change end higher or lower than it began? • Is this considered heating or cooling a substance?

  7. Heating Curve of Water Does the graph represent an endo or exo change?

  8. Labeling the Graph Number the line segments starting at the left. Label the states of matter and phase changes. Label the segments that represent temperature changes with ∆T. Label the segments that represent NO temperature changes, but instead show only changes in heat content, with ∆H. * Sometimes heat enters a system and changes the KE (temp), but other times heat enters a system to make other changes (enthalpy).

  9. Calculating the Graph q = heat Label the graph with equations that can be used to calculate the heat involved at EACH segment. Select segments to label with q = m c ∆T Select segments to label with q = mol ∆H • Use the graph to substitute values for ∆T. • Use constant values for c. (c differs with substance.) • Use constant values for ∆H. (∆H differs with substance.)

  10. Heat of Fusion & Heat of Solidification Quantities will be given in the problem. Heat of fusion= heat absorbed by one mole of a substance in melting from a solid to a liquid at STP Heat of solidification=heat released by one mole of a liquid as it solidifies at STP ΔHfus = - ΔHsolid

  11. Heat of Vaporization & Heat of Condensation Quantities will be given in the problem. Heat of vaporization= heat absorbed by one mole of a substance vaporizing at STP Heat of condensation=heat released by one mole of a gas as it condenses at STP ΔHvap = - ΔHcond

  12. Water c = 4.184 J/g°C ∆Hfusion = 6.02 kJ/mol ∆Hvaporization = 40.6 kJ/mol Example 14.1 Calculate the energy required to melt 8.5 g of ice at 0°C. Example 14.2 Calculate the energy (in kJ) required to heat 25 g of liquid water from 25°C to 100°C and change it to steam at 100°C. Section Review Question 7 Calculate the energy required to change 1.00 mol of ice at -10°C to water at 15°C.

  13. Phase Diagram A phase diagram gives the conditions of temperature and pressure at which a substance exists as solid, liquid, and gas. • Each of the three regions represents a pure phase (not a mix). • Each line represents the temp & pressure conditions where the phases exist in equilibrium. • Triple point: set of conditions in which all phases exist in equilibrium

  14. Specific Heat Amount of heat required to raise 1 g of the substance by 1 degree Celsius. The units of specific heat are J/gºC or cal/g ºC. These numbers can be found on a table on pg. 329. The numbers are calculated by using q = m c ΔT Example 10.4 A 1.6g sample of a metal that has the appearance of gold requires 5.8 J of energy to change its temperature from 23°C to 41°C. Is the metal pure gold? Specific Heat WS (Practice Packet) 1. A 15.75-g piece of iron absorbs 1086.75 J of heat energy, and its temperature changes from 25°C to 175°C. Calculate the heat capacity of iron.

  15. Heat flows from _______ to ________ until equal _______________________ is reached. In the glass of water, the substance gaining heat is theoretically getting it from the warmer substance. So, if 456 J of heat is lost from the warm substance, how many joules are gained by the cool substance? Heat Exchange

  16. Calorimetry Consider the accuracy of our calculations. What if the system is NOT insulated from other heat sources? q lost + q gained = 0 Calorimeter A calorimeter is an insulated instrument that uses water making heat calculations more accurate.

  17. Calorimetry A 25.0 g sample of pure iron at 85°C is dropped into 75 g of water at 20°C. What is the final temperature of the water-iron mixture? In a calorimeter, we KNOW that heat lost by the warmer object equals heat gained by the cooler object. q lost + q gained = 0 Calorimeter

  18. Calorimetry Chemistry Thermo WS of Practice Problems 16. The specific heat capacities of Hf and ethanol are 0.146J/gC and 2.45J/gC, respectively. A piece of hot Hf weighing 15.6 g at a temperature of 160.0C is dropped into 125 g of ethanol that has an initial temperature of 20.0C. What is the final temperature that is reached, assuming no heat loss to the surroundings?

  19. Stoichiometry & Heat So far, we’ve been analyzing temperature changes and calculating the heat involved in these PHYSICAL changes. Now, we are going to transition back to chemical changes...chemical reactions. Look at the reaction described below: 2S + 3O2 --> 2SO3 ∆H = -791.4 kJ Analyze the reaction: Is heat absorbed or released? What conversion factors could be written to include the heat?

  20. Stoichiometry & Heat The reaction could also be written in this form: Original: 2S + 3O2 --> 2SO3 ∆H = -791.4 kJ 2S + 3O2 --> 2SO3 + 791.4 kJ Calculations: How much heat will be released when 6.44 g of sulfur reacts with excess O2 according to the equation above? Let’s also learn to draw/interpret a graph to represent roughly how the energy has changed during this reaction.

  21. Stoichiometry & Heat Let’s look at the 12-2 Practice Problems in your packet. You don't have to draw a graph! You only need a balanced chemical equation to do stoichiometry. We’ve done #1 together. Begin with question 2 and complete the handout.

  22. Stoichiometry & Heat • Prove your knowledge of Stoichiometry and Heat BEFORE visiting the review stations: • Solve the even questions on 12-2 Practice Problems. • Solve the stoichiometry handout questions. • ½ assignment: 12-2 #2,6,10 & even handout • AFTER CONFIRMING ANSWERS WITH MRS. TARVIN, YOU MAY GO TO THE REVIEW STATIONS.

  23. Heat Exchange A sample of silver with a mass of 63.3 g is heated to a temperature of 111.4ºC and placed in a container of water at 17ºC. The final temperature of the silver and the water is 19.4°C. Assuming no heat loss, what mass of water was in the container? The specific heat of water is 4.184 J/gºC, and the specific heat of silver is 0.24 J/gºC.

  24. Heat Exchange A 133g piece of granite rock is heated to 65.0°C, then placed in 643g of ethanol at 12.7 °C. Assuming no heat loss, what is the final temperature of the granite and ethanol? cgranite = 0.8J/gºC and cethanol = 2.44J/gºC Set up the calculation.

  25. Tips for the Test Work the problems suggested on last week’s calendar. Complete the short answer first. This section is worth 54.5 points. Spend no more than one hour on this section. There are NO calculations in the multiple choice section. The 13 questions are concept-based, and they count 3.5 points each. Check the multiple choice answers...your careless mistakes cost the most.

  26. After the test: Let’s begin our new unit on “Solutions.” Remember, a solution is simply a homogeneous mixture of substances. Notice: The lab area contains two stations for each learning style: Visual, Kinesthetic and Auditory. Instructions: Take a clean piece of paper and a pencil to your first station. Find the HOT PINK sign for your style’s first station. Use the instructions and materials at the station to begin building your basic understanding of graphs called Solubility Curves. When you are done with station one, move on to your style’s second station. Use the instructions and materials to apply your basic understanding.

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