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Thermochemistry

Thermochemistry. To determine the amount of heat absorbed by the water, we multiply the mass of water and the temperature change by 4.18J/g/°C 4.18 J/g/°C means that it takes 4.18 Joules of energy to raise 1 gram of water 1 degree Celsius. Called the specific heat constant

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Thermochemistry

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  1. Thermochemistry • To determine the amount of heat absorbed by the water, we multiply the mass of water and the temperature change by 4.18J/g/°C • 4.18 J/g/°C means that it takes 4.18 Joules of energy to raise 1 gram of water 1 degree Celsius. • Called the specific heat constant • “c” is a defining physical quantity of a substance.

  2. Units • c = specific heat constant of substance (J/g/°C) • q = Energy released or absorbed by substance (J) • H = Enthalpy (Energy) of system (J/mol) • H° = Enthalpy of system at standard conditions (J/mol) • DH = Change of Enthalpy of system (J/mol) Mass in grams amount in moles

  3. Heat • This can be represented by the following formula: q = cmDT • Where

  4. Heat • ex) 100 grams of water is heated from 20°C to 50°C. How much heat is absorbed by the water? q = cmDT q = (4.18J/g/°C)(100g)(50°C - 20°C) q = (4.18J/g/°C)(100g)(30°C) q = 12540 J

  5. Enthalpy • When ammonium chloride, NH4Cl, dissolves in water, it takes energy to do this (as ions floating freely in water). • The energy is taken from the surroundings (water) causing the water to cool down • The reaction for this is: heat + NH4Cl NH4+1 + Cl-1

  6. Enthalpy • An example of this would be: • What is the change in enthalpy, DH, to dissolve 10 grams of ammonium chloride in 100 grams of water if the temperature of the water went from 50°C to 20°C?

  7. Enthalpy • In order to determine the change in enthalpy, DH we need to break this into two parts. • The amount of heat lost by the water (surroundings). • The amount of energy per mole absorbed by the ammonium chloride (system).

  8. q = mcDT q = (100g)(4.18J/g/°C)(50°C - 20°C) q = -12540 J (The surroundings lost energy therefore, the value for “q” is negative.)

  9. Change in Enthalpy Now we can determine the amount of energy per mole absorbed by the NH4Cl (since the system undergoes an endothermic reaction, the value for “q” is positive for the system) DH = q / g/(molar mass) = 12540J)/[(10g)/(53.5g/mole)] = 66,000 J/mole

  10. H2O energy NH4Cl

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