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Energy Relationships in Chemical Reactions

Energy Relationships in Chemical Reactions. Chapter 6. Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances

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Energy Relationships in Chemical Reactions

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  1. Energy Relationships in Chemical Reactions Chapter 6

  2. Energyis the capacity to do work • Thermal energy is the energy associated with the random motion of atoms and molecules • Chemical energy is the energy stored within the bonds of chemical substances • Nuclear energy is the energy stored within the collection of neutrons and protons in the atom • Electrical energy is the energy associated with the flow of electrons • Potential energy is the energy available by virtue of an object’s position 6.1

  3. Thermochemical Definitions Heat (q) : Is the energy transferred between a system and it’s surroundings as result in the differences in their temperatures only! Work (w) : The energy transferred when an object is moved by a force. Thermochemistry is the study of heat change in chemical reactions.

  4. E is a state function, q and w are not.

  5. Temperature = Thermal Energy 900C 400C Energy Changes in Chemical Reactions Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy. greater thermal energy 6.2

  6. 2H2(g) + O2(g) 2H2O (l) + energy H2O (g) H2O (l) + energy energy + H2O (s) H2O (l) Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. Endothermic process is any process in which heat has to be supplied to the system from the surroundings. 6.2

  7. Specific Heat

  8. The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. • in the SI system has units of J/gC. • in the metric system has units of cal/gC. Heat (q) absorbed or released q = m x s xDt Calculate the Temperature Change (DT) q = C x Dt Dt = tfinal - tinitial

  9. Heat Capacity The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius. Heat Capacity: C = m x s Heat Capacity = mass x specific heat

  10. Heat Equation • Rearranging the specific heat expression gives the • heat equation. • Heat = g x °C x J = J • g°C • The amount of heat lost or gained by a substance is calculated from the: • mass of substance (g). • temperature change (T). • specific heat of the substance (J/g°C).

  11. cal/g°C 0.214 0.0920 0.0308 0.108 0.0562 0.125 0.488 0.588 0.207 0.100

  12. How much heat is given off when an 869 g iron bar cools from 940C to 50C? s of Fe = 0.444 J/g •0C Dt = tfinal – tinitial = 50C – 940C = -890C q = msDt = 869 g x 0.444 J/g •0C x –890C = -34,000 J 6.4

  13. Constant-Pressure Calorimetry Reaction at Constant P qrxn = - (qwater + qcal) qwater = msDt qcal = CcalDt No heat enters or leaves! 6.4

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