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Thermochemistry

Thermochemistry. The study of heat released or required by chemical reactions Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt) CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) + energy . Energy. Kinetic energy (E K ). Potential energy (E P ).

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Thermochemistry

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  1. Thermochemistry • The study of heat released or required by chemical reactions • Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt) CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy

  2. Energy Kinetic energy (EK) Potential energy (EP) Energy due to motion Energy due to position (stored energy) What is Energy?

  3. Chemical Potential Energy: Energy stored within the structural units of chemical substances Law of Conservation of Energy: the total energy of the universe is constant and can neither be created nor destroyed; it can only be transformed.

  4. SYSTEM OPEN ISOLATED CLOSED Systems & Surroundings In thermodynamics, the world is divided into a system and its surroundings A system is the part of the world we want to study (e.g. a reaction mixture in a flask) The surroundings consist of everything else outside the system

  5. Heat The motion of the particles of matter

  6. OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction flask, rocket engine) CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains fixed) e.g. a sealed reaction flask ISOLATED SYSTEM: can exchange neither energy nor matter with its surroundings (e.g. a thermos flask)

  7. Heat Transfer • Heat flows from an area of high heat to an area low in heat.

  8. Three methods of heat transfer Conduction: • Transfer from one substance to another by direct contact of molecules. Example: When you touch a hot stove. • Convection: • Heat carried from one place to another in a liquid or gas as molecules move in currents caused by density differences. Example: Warm air rising. • Radiation: • Heat carried through empty space in the form of infrared rays. Example: When you face the sun and feel warmth on your face.

  9. Temperature: • A measure of the average kinetic energy of molecules. • The faster the molecules of a substance are moving, on average, the higher the temperature. • Thermometer - An instrument for measuring temperature.

  10. Temperature Scales: • Celsius - • Freezing point of water is 0o C. • Boiling point of water is 100o C. • Kelvin - • An "absolute" scale used for kinetic theory calculations. • Zero on this scale, 0 K, is the lowest possible temperature. • oC+ 273 = K • Fahrenheit - • Freezing point of water is 32 oF. • Boiling point of water is 212 oF. • oC = (oF - 30) / 2.

  11. Absolute Zero • - The lowest possible temperature. • The temperature would be 0 K or about -273 oC.

  12. Calorie: • The unit of heat. • The amount of heat needed to raise the temperature of one gram of water one degree Celsius. • A "Food Calorie" is 1000 calories, or a Kilocalorie

  13. Specific Heat • The ability of a substance to absorb heat. • The specific heat of a substance is the number of calories needed to raise the temperature of one gram of the substance one Celsius degree. • The units of Specific Heat are - calories per gram Celsius degree.

  14. EQUATION • Heat gained or lost = (mass) (change in Temp) (specific heat) q = m x DT x Cp S.I. unit of energy is the joule (J) Heat and work ( energy in transit) also measured in joules 1 kJ (kilojoule) = 103 J

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