energy & chemical change
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Energy & Chemical Change. Chapter 16. 16.1 Energy. Start a NEW page in your notebook: What is Energy? What are some types of energy that you are familiar with? Kinetic Potential Thermal Electrical Chemical Nuclear. A. The Nature of Energy. - PowerPoint PPT PresentationTRANSCRIPT
Energy & Chemical Change
Chapter 16
16.1 EnergyStart a NEW page in your notebook:•What is Energy?•What are some types of energy that you are familiar with?
– Kinetic– Potential – Thermal– Electrical– Chemical– Nuclear
A. The Nature of Energy•Energy: ability to do work or produce heat
•Potential Energy: energy due to composition (chemical) or position of an object (gravitational)
•Kinetic Energy: energy of motion
•Kinetic energy of a substance is directly related to the constant random motion of it particles and is proportional to temperature.
•Chemical Potential Energy of a substance depends upon its composition–Type of atoms–# & type of chemical bonds
–How atoms are arranged
Law of conservation of energy•States that in any
chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed
Chemical potential energy•Energy stored in a substance
because of its composition•Ex. Gasoline – when burned
chemical potential energy is converted to useful mechanical energy.
•Heat (q): process of energy flowing from a warmer object to a cooler object
How is Heat Transferred?1. Conduction – transfer of heat
within solid objects by direct contact
2. Convection – transfer of heat within fluids (liquids & gases)
3. Radiation – transfer of heat by electromagnetic radiation (like the sun)
Measuring Heat - Units•calorie: amount of heat
required to raise temperature of one gram of pure water one degree Celsius
• Joule: SI unit of heat & energy– 1 calorie = 4.184 joules
•1000 calories = 1 Kcal or 1 nutritional Calorie
•Practice Problems p.492
Specific Heat• Amount of heat required to raise the
temperature of one gram of that substance by one degree Celsius
• Remember water has a high specific heat – it takes lots of energy to change it’s temperature
Calculating heat evolved & absorbedq = m x c x Tq = heat absorbed or released (the
value is positive if heat is absorbed and negative if heat is released)
m = mass of sample in gramsc = specific heat of substance (can be
determined or looked up in a table)T = difference between final
temperature & initial temperature
Phase Changes – know for your TEST!
Which phase changes require energy? (endothermic)• Melting, evaporation, sublimation
Which phase changes release energy? (exothermic)• Freezing, condensation, deposition
Energy and Phase Change
• Heat of vaporization - energy required to change one gram of a substance from liquid to gas.
• Heat of condensation - energy released when one gram of a substance changes from gas to liquid.
• For water 540 cal/g
Energy and Phase Change
• Heat of fusion - energy required to change one gram of a substance from solid to liquid.
• Heat of solidification - energy released when one gram of a substance changes from liquid to solid.
• For water 80 cal/g
Water
Waterand IceIce
Water and Steam
Steam
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Heating Curve for WaterTe
mpe
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Cel
sius
Pressure in mmHg or torr
Water
Waterand IceIce
Water and
Steam
Steam
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Heating Curve for Water
Heat of Vaporizati
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Water
Waterand IceIce
Water and Steam
Steam
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Heating Curve for Water
Heat of Fusion
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Water
Waterand IceIce
Water and
Steam
Steam
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Heating Curve for Water
Slope = Specific Heat
Steam
Water
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Pressure in mmHg or torr
Water
Waterand IceIce
Water and Steam
Steam
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Heating Curve for WaterBoth
Water and Steam
Tem
pera
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in C
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Pressure in mmHg or torr
Water
Waterand IceIce
Water and Steam
Steam
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0 40 120 220 760 800
Heating Curve for Water
Ice and
Water
Tem
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in C
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Pressure in mmHg or torr
Water
Waterand IceIce
Water and Steam
Steam
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0 40 120 220 760 800
Heating Curve for Water
Tem
pera
ture
in C
elsi
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Pressure in mmHg or torr
Phase Diagram1. Phase diagram – a graph of pressure vs.
temperature that shows in which phase a substance exist under different conditions of temperature and pressure.
2. Triple Point – the point on the phase diagram that represents the temperature and pressure at which all three phases can coexist
3. Critical Point – the point that indicates the critical temperature and pressure. Critical temperature is the temp. above which the sub. can’t exist in the liquid state. Critical pressure is the lowest pressure at which the substance can exist at the critical temperature.
Phase Diagram of Water
16.2 Heat in Chemical Reactions & Processes
Measuring Heat•Calorimeter is an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process
Chemical Energy & Universe• System: specific part of
universe that contains reaction or process
• Surroundings: everything in the universe other than the system
• Universe = system + surroundings
1. Enthalpy & Enthalpy changes• Enthalpy: (H) heat content of
a system at constant pressure• Can’t measure actual energy
or enthalpy of a substance, you can measure change in enthalpy, which is heat absorbed or released in a chemical reaction
Enthalpy• ∆Hrxn = Hproducts – Hreactants
• When ∆Hrxn is negative the reaction is exothermic – Hproducts < Hreactants
• When ∆Hrxn is positive the reaction is endothermic - Hproducts > Hreactants
2. Sign of enthalpy reaction• Exothermic Reactions: heat pack
• Endothermic Reactions: cold pack
Practice ProblemThermodynamic Heats of Formationfor one mole at 298K and 1 atmosphere pressure•Substance (form) Enthalpy of•Formation ΔHf (kJ) •NaCl(s) -411.15 •Na+(aq) -240.12 •Cl-(aq) -167.16
•Na+(aq) + Cl- (aq) NaCl(s)
What is the change in enthalpy (ΔH) for this reaction? •∆Hrxn = Hproducts – Hreactants
•∆Hrxn = [-411.15] – [(-240.12)+(-167.16)] = -3.87 kJ•So is the reaction Exothermic or Endothermic?