Heat and Energy J Deutsch 2003

Energy can exist in different forms, such as chemical, electrical, electromagnetic, thermal, mechanical, and nuclear. Kinetic energy – the energy of motion Potential energy – the energy of position (stored energy) J Deutsch 2003

Heat is a transfer of energy (usually thermal energy) from a body of higher temperature to a body of lower temperature. Thermal energy is the energy associated with the random motion of atoms and molecules. The Law of the Conservation of Energy states that energy can neither be created nor destroyed. J Deutsch 2003

Temperature is a measurement of the average kinetic energy of the particles in a sample of material. Temperature is not a form of energy. Two temperature scales used in chemistry are Celsius and Absolute The unit of temperature in the Celsius scale is the degree (ºC) The unit of temperature on the Absolute scale is the Kelvin (K) J Deutsch 2003

To convert between absolute and Celsius temperature scales use K=ºC+273 J Deutsch 2003

Melting Point The temperature at which a liquid and a solid are in equilibrium The melting point for ice is 0ºC The melting point of a substance is the same as its freezing point J Deutsch 2003

Boiling Point The temperature that a substance will boil Going from liquid to gas For water it is 100 degrees celcius

Melting point is the same as freezing point Regents Question: 06/03 #41 The freezing point of bromine is (1) 539°C (2) –539°C (3) 7°C (4) –7°C See Table S Melting point is the same as freezing point Convert K to C (K=C+273) þ J Deutsch 2003

The concepts of kinetic and potential energy can be used to explain physical processes that include: fusion (melting), solidification (freezing), vaporization (boiling, evaporation), condensation, sublimation, and deposition. Add energy (endothermic) subliming melting boiling SOLID LIQUID GAS Remove energy (exothermic) freezing depositing condensing J Deutsch 2003

Regents Question: 06/03 #17 þ Which change is exothermic? (1) freezing of water (2) melting of iron (3) vaporization of ethanol (4) sublimation of iodine þ J Deutsch 2003

A change in phase is a change in Potential Energy, not Kinetic Energy Boiling Point Potential energy changes so temperature doesn’t Melting Point J Deutsch 2003

Energy and phase changes AB - solid warms up (KE inc/PE constant) BC- solid melts (KE constant/PE inc) CD – liquid warms up (KE inc/PE constant) DE- liquid boils (KE constant/PE inc) EF – gas warms (KE inc/PE constant) J Deutsch 2003

Regents Question: 06/02 #16 Which change in the temperature of a 1-gram sample of water would cause the greatest increase in the average kinetic energy of its molecules? (1) 1°C to 10°C (3) 50°C to 60°C (2) 10°C to 1°C (4) 60°C to 50°C þ J Deutsch 2003

Heat is transferred to different materials at different rates. The specific heat capacity (C) determines the rate at which heat will be absorbed. The specific heat capacity for water is 4.18J/g The quantity of heat absorbed (Q) can be calculated by: Q=mCT m=mass T=change in temperature J Deutsch 2003

Regents Question: 06/02 #28 As ice melts at standard pressure, its temperature remains at 0°C until it has completely melted. Its potential energy (1) decreases (2) increases (3) remains the same þ J Deutsch 2003

Regents Question: 08/02 #54 A sample of water is heated from a liquid at 40°C to a gas at 110°C. The graph of the heating curve is shown in your answer booklet. a On the heating curve diagram provided in your answer booklet, label each of the following regions: Liquid, only Gas, only Phase change Liquid Only Gas Only Phase change J Deutsch 2003

Regents Question: cont’d b For section QR of the graph, state what is happening to the water molecules as heat is added. c For section RS of the graph, state what is happening to the water molecules as heat is added. They move faster, their temperature increases. Their intermolecular bonds are breaking, their potential energy is increasing. J Deutsch 2003

Regents Question: 01/02 #47 What is the melting point of this substance? (1) 30°C (3) 90°C (2) 55°C (4) 120°C þ J Deutsch 2003

The quantity of energy absorbed or released during a phase change can be calculated using the Heat of Fusion or Heat of Vaporization Melting (fusion) or freezing (solidification) Q=mHf where Hf is the heat of fusion (for water: 333.6 J/g) Boiling (vaporization) or condensing Q=mHv where Hv is the heat of vaporization (for water: 2259 J/g) J Deutsch 2003 Hf and Hv are given to Table B – m is the mass

Fusion refers to melting. Regents Question: 08/02 #24 In which equation does the term “heat” represent heat of fusion? (1) NaCl(s) + heat  NaCl(l) (2) NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)+ heat (3) H2O(l)+ heat  H2O(g) (4) H2O(l)+ HCl(g) H3O+(aq) + Cl –(aq) + heat þ Fusion refers to melting. J Deutsch 2003

Melting Point The temperature at which a liquid and a solid are in equilibrium The melting point for ice is 0ºC The melting point of a substance is the same as its freezing point J Deutsch 2003

Regents Question: 08/02 #5 þ K=C + 273 Given the equation: H2O(s) H2O(l) At which temperature will equilibrium exist when the atmospheric pressure is 1 atm? (1) 0 K (3) 273 K (2) 100 K (4) 373 K þ K=C + 273 J Deutsch 2003

Regents Question: 08/02 #18 The solid and liquid phases of water can exist in a state of equilibrium at 1 atmosphere of pressure and a temperature of (1) 0°C (3) 273°C (2) 100°C (4) 373°C þ J Deutsch 2003

Regents Question: 06/02 #13 The strongest forces of attraction occur between molecules of (1) HCl (3) HBr (2) HF (4) HI þ J Deutsch 2003

Table H – the vapor pressure of four liquids at various temperatures. As temp inc, vapor pressure inc. J Deutsch 2003

A liquid will boil when its vapor pressure equals the atmospheric pressure. Raising the temperature will increase the vapor pressure of the liquid Lowering the atmospheric pressure will lower the boiling point On top of a high mountain, water boils at a temperature below 100C J Deutsch 2003

. The stronger the intermolecular forces, the higher the boiling point and melting point. The stronger the intermolecular forces, the lower the vapor pressure. See Table H J Deutsch 2003

Regents Question: 06/03 #40 According to Reference Table H, what is the vapor pressure of propanone at 45°C? 22 kPa 33 kPa 70. kPa (4) 98 kPa þ J Deutsch 2003

Standard pressure is 101.3 kilopascals (kPa) or 1 atmoshpere (atm) The normal boiling point occurs when the atmospheric pressure is 101.3 kPa (standard pressure) The normal boiling point of ethanol is 80ºC. J Deutsch 2003

Regents Question: 08/02 #28 As the pressure on the surface of a liquid decreases, the temperature at which the liquid will boil (1) decreases (2) increases (3) remains the same þ J Deutsch 2003

Regents Question: 08/02 #30 As the temperature of a liquid increases, its vapor pressure (1) decreases (2) increases (3) remains the same þ J Deutsch 2003

Regents Question: 08/02 #44 The vapor pressure of a liquid is 0.92 atm at 60°C. The normal boiling point of the liquid could be (1) 35°C (3) 55°C (2) 45°C (4) 65°C þ The normal boiling point is the temperature at which a liquid boils when the atmospheric pressure is standard pressure (1 atm or 101.3 kPa) J Deutsch 2003

Regents Question: 01/03 #68 What is the vapor pressure of liquid A at 70°C? Your answer must include correct units. 700 mm Hg J Deutsch 2003

Regents Question: 01/03 #69 At what temperature does liquid B have the same vapor pressure as liquid A at 70°C? Your answer must include correct units. 113°C J Deutsch 2003

Regents Question: 01/03 #70 Which liquid will evaporate more rapidly? Explain your answer in terms of intermolecular forces. Liquid A will evaporate more rapidly because, at any temperature, it has the higher vapor pressure. J Deutsch 2003

Kinetic molecular theory (KMT) for an ideal gas states that all gas particles : are in random, constant, straight-line motion. have collisions that may result in the transfer of energy between gas particles, but the total energy of the system remains constant. are separated by great distances relative to their size; the volume of the gas particles is considered negligible. have no attractive forces between them. J Deutsch 2003

Triple Point diagrams (honors only) https://www.youtube.com/watch?v=BLRqpJN9zeA J Deutsch 2003

The amount of thermal energy contained in the molecules depends on how fast they are moving and how many molecules there are. The total kinetic energy of all the molecules combined is called thermal energy Thermal energy is a result of the Kinetic Energy of the molecules’ motion (molecules are always moving.) Which can melt more ice: a small cup of hot water or a swimming pool of cold water? J Deutsch 2003

The concept of an ideal gas is a model to explain the behavior of gases. A real gas is most like an ideal gas when the real gas is at low pressure and high temperature. J Deutsch 2003

Real Gases Molecules do take up space Molecules do attract each other Energy is lost during collisions Under conditions of high temperature and low pressure, real gases behave more like ideal gases Hydrogen and helium are closest to being ideal gases J Deutsch 2003

Collision theory states that a reaction is most likely to occur if reactant particles collide with the proper energy and orientation. Anything that will increase the number of effective collisions will increase the rate at which the reaction will occur: J Deutsch 2003

Kinetic molecular theory describes the relationships of pressure, volume, temperature, velocity, and frequency and force of collisions among gas molecules. P1V1 P2V2 T2 T2 J Deutsch 2003

Gas Laws Boyle’s Law Charles’ law Indirect Relationship between pressure and volume Temperature remains constant PxV = constant Charles’ law Direct relationship between volume and temperature Pressure remains constant V/T = constant Temperature must be Absolute temperature (Kelvins) J Deutsch 2003

Graphing the gas laws As absolute temperature increases, pressure increases at constant volume Pressure Temperature As absolute temperature increases, volume increases at constant pressure Volume Temperature As pressure increases, volume decreases at constant temperature Volume Pressure J Deutsch 2003

Regents Question: 06/02 #14 Which graph shows the pressure-temperature relationship expected for an ideal gas? þ J Deutsch 2003

Regents Question: 06/02 #15 At the same temperature and pressure, which sample contains the same number of moles of particles as 1 liter of O2 (g)? (1) 1 L Ne(g) (3) 0.5 L SO2 (g) (2) 2 L N2 (g) (4) 1 L H2O(l) þ J Deutsch 2003

Avogadro’s Hypothesis Equal volumes of different gases at the same temperature and pressure contain an equal number of particles. Avogadro’s Hypothesis J Deutsch 2003