Unit 14 Flashback: States of Matter & Thermochemistry Characteristic Solids Liquids Gases Shape Volume Density Fluidity Compressibility Picture Phase Diagram Shows the relationship between solid, liquid, and gas. Triple Point: solid, liquid, and gas phases coexist at equilibrium Critical Point: The temp above which the substance cannot be a liquid 1
The triple point of water is 0.01 o C and 0.006 atm. The critical point of water is 374 o C and 225 atm. Normal freezing points and boiling points ALWAYS occur at 1 atm. Water: freezing point is 0.00 o C boiling point is 100 o C Heating Curve/Phase Change Diagram Shows the change in temperature of a substance as heat is added to a substance over time. During any Phase Change: the temperature does not change when heat is added/removed. TEMPERATURE STAYS THE SAME DURING ANY PHASE CHANGE!!! Temperature can only change within a single state of matter, NOT during the phase change. 2
Heating Curve for Water Specific Heat Capacity (Specific Heat): The amount of heat energy needed to raise the temperature of 1 gram of a substance 1 o C. 1 gram 1 o C (g) LOW SPECIFIC HEAT means the substance is EASY to heat up and/or it will heat up FAST. It will also lose that heat just as easily/quickly. HIGH SPECIFIC HEAT means the substance is hard to heat up and/or it will heat up slowly. Once it is heated, it will also lose the heat slowly. Low Specific Heat Heats Up Easily/Fast Cools Off Easily/Fast (CONDUCTORS) High Specific Heat Hard/Slow to Heat Up Hard/Slow to Cool Off (INSULATORS) 3
Substance Copper 0.386 Aluminum 0.900 Water 4.186 Specific Heat (C p ) J/g C Of the three substances listed in this table, which one will heat up the fastest? Substance Copper 0.386 Aluminum 0.900 Water 4.186 Specific Heat (C p ) J/g C Would you rather have your soda in a copper can or an aluminum can? Why? Substance Copper 0.386 Aluminum 0.900 Water 4.186 Specific Heat (C p ) J/g C Which of these substances will heat up more slowly? Do you remember what it is about water that causes it to have a high specific heat??? This is why large bodies of water help to moderate the temperature in that area. Heat of Fusion: the amount of heat needed to make the phase change between solid/liquid for a given substance For Water: H fus = 0.334 kj/g or 6.01 kj/mol Heat of Vaporization: the amount of heat needed to make the phase change between liquid/gas for a given substance For Water: H vap = 2.226 kj/g or 40.7 kj/mol 4
Heat of Fusion Heat of Vaporization ΔH vap There are 2 types of heat calculations we will do: 1. NO Phase Change (On the Slope) Q = mc p T Key Words: "raise/lower the temp" 2. Phase Change (On the Flat Line) Q = mol H Key Words: "melt/freeze/boil/condense" If given mass change to moles using the molar mass of the substance. ΔH fus Q = mc p T Q = heat (calories or Joules) m = mass (grams) c p = specific heat (cal/g o C or J/g o C) T = change in temp ( o C) So... heat (Q) depends on 3 things: mass, specific heat, & change in temp 5
Sample Problems: 1. The temperature of 250.0 g of mercury is raised by 52 o C. How much heat did this require? (c p = 0.14 J/g o C) *Which formula will you use to solve this? 2. 31.5 g of water is being melted at its melting point of 0 o C. How many kj is required? ( H fus = 6.02 kj/mol) *Which formula will you use to solve this? 3. How much heat is required to raise the temperature of 400.0 g of silver from 20.0 o C to 65.0 o C? (c p = 0.24 J/g o C) *Which formula will you use to solve this? 4. Ethanol (C 2 H 5 OH) boils at 351.5 o C. How much heat needs to be added to vaporize 35.7 grams of ethanol? ( H vap = 43.5 kj/mol) *Which formula will you use to solve this? 6
Unit 14 Notes 1516.notebook Combined Heat Calculations: Now we're going to do some really fun heat calculations! Use this as a guide when solving your heat calculations. 1. Solve the heat calculation for each section of the heating curve. 2. Add up all your answers to get the total heat required. **The slopes give their answers in "J". The phase changes give their answers in "kj". Make sure to convert them all to kj before you add them together!!! Temperature vs Heat Sample Problem: How much heat is required to raise the temperature of 32.8 g of water from 15.0oC to 25.0oC? Specific heat (ice) = 2.1 J/goC Specific heat (water) = 4.18 J/goC Hfus = 6.01 kj/mol Temperature: measures the average kinetic energy of the particles in a sample of matter. Thermometers measure temperature. Heat: the SUM of the average kinetic energy that is transferred from one object to another. Which has more heat? A pot of water at 100oC A pool of water at 100oC 7
*You heat 200 grams of water from 10 o C to 90 o C. *You heat 850 grams of water from 10 o C to 90 o C. Which of these has more heat? Liquid water has the same specific heat (4.18 J/g o C) Both samples experience the same change in temp. Q = mc p T The one with more mass has more heat. (A large pot of water has to sit on the burner and take in more heat than a small pot of water would require to see the same change in temp.) Thermochemistry: study of heat changes in chemical reactions. Energy CANNOT be created or destroyed. (It can only change forms. Ex: chemical mechanical) Enthalpy: heat of a chemical reaction ( H) Heat of Reaction Calculations: H = H products H reactants 1 calorie = 4.18 Joules 1000 calories = 1 Calorie (food calorie) Sample Problem: 4Fe (s) + 3O 2(g) 2Fe 2 O 3 (s) Given the balanced equation, what is the enthalpy of the reaction for Fe 2 O 3? Which is bigger a calorie or a Joule? Why? When we eat foods, we are eating Calories. Our body needs a certain amount of Calories in order to function properly. The food contains chemical potential energy that our body will use. The gasoline that you put in your car also has chemical potential energy that your car will use. Substance Fe(s) O 2 (g) Fe 2 O 3 (s) Enthalpy at 298K 0.0 kj/mol 0.0 kj/mol 826 kj/mol 8
Endothermic Reaction Exothermic Reaction Surroundings System System Surroundings Endothermic: Exothermic: The products have more energy than the reactants because it "took in" energy. The products have less energy than the reactants because it "released" energy. 1. Heat the system 2. Q = H = (+) 3. Usually feels 4. Products have energy than the reactants 1. Heat the system 2. Q = H = ( ) 3. Usually feels 4. Products have energy than the reactants During any phase change, temperature STAYS THE SAME. Melting (heat goes in endo) H fus Freezing (heat goes out exo) H solid These are opposite processes. The amount of heat lost by one is gained by the other and vice versa. Vaporizing (heat goes in endo) H vap Condensing (heat goes out exo) H cond These are opposite processes. The amount of heat lost by one is gained by the other and vice versa. Heat always moves from high heat to low heat. So... if you put a hot object into cool water, the heat moves from the object to the water. if you put a cool object into hot water, the heat moves from the water to the object. 9
Endothermic Process: absorbs heat (heat goes in) Ex: A + B + heat C + D Exothermic Process: releases heat (heat exits/goes out) Ex: A + B C + D + heat The energy in the system would go up. The energy in the surroundings would go down. If this reaction takes place inside a beaker, the energy content inside the beaker would go up and the energy content outside the beaker would go down (the beaker "feels" colder because your handis losing heat energy) The reaction in the beaker takes in energy so it ends up with more energy than it had in the beginnig. Q is positive (because the energy in the system increased) The energy in the system would go down. The energy in the surroundings would go up. If the reaction happens inside a beaker, the energy content inside the beaker would down and the energy content outside of the beaker would go up. (the beaker "feels" warmer because your hand is gaining heat energy) The reaction in the beaker releases energy, so it ends up with less than it had in the beginning. Q is negative (because the energy in the system decreased) There are 5 factors that affect the RATE of chemical reactions: 1. Nature of the Reactants Catalyst: speeds up a reaction because it LOWERS the activation energy. Hydrogen reacts violently with chlorine under certain conditions while hydrogen barely reacts with nitrogen under the same conditions. 2. Surface Area A regular piece of chalk reacts slowly with HCl, but powdered chalk reacts much more quickly. 3. Concentration As concentration increases, reactions usually occur more quickly. 4. Temperature As temperature increases, reactions usually occur more quickly. 5. Catalyst Catalysts are used to speed up the rate of reactions. 10