Thermochemistry

Thermochemistry is the study of heat changes that occur during chemical reactions. Heat (q) - energy that is transferred from one object to another because of a temperature difference between them. Heat always flows from a warmer object to a cooler object. Heat cannot be measured directly. Temperature – the average kinetic energy of the particles in a sample of matter Energy - the capacity for doing work or supplying heat.

Energy in Reactions Endothermic process - system absorbs heat from the surroundings. (Energy must be added to the reaction for the reaction to take place) Exothermic process - system releases heat to the surroundings.

Ex. Endothermic or exothermic process? -evaporating alcohol -leaves burning -boiling water -water cooling -melting ice -freezing water

To calculate the heat energy required for a temperature change, use the following formula: Q = (m)(C)(  T) Q = heat energy absorbed or released (J) + q = endothermic (heat absorbed) - q = exothermic (heat released) m = mass (g) C = specific heat of the object (J/g o C) The amount of heat it takes to raise the temperature of 1 g of a substance 1 o C. Water has a uniquely high specific heat compared to other substances.  T= change in temperature of object ( o C)

Example How much energy is required to heat an iron nail with a mass of 7.0 g from 25 o C until it becomes red hot at 752 o C? C Fe = 0.46J/g o C  T = = 727 o C q = mC  T q = 7.0g(0.46J/g o C) 727 o C q = 2300 J

Phase Change Heat energy can be used to not only change the temperature of matter, but also its phase. The energy goes into separating or organizing the molecules into a new state The amount of heat energy necessary to cause a phase change can be calculated using the formula: Q = mLf (solid/liquid) or Q = mLv (liquid/gas)

Q = heat required for phase change (J) m = mass (kg) Lf = latent heat of fusion (J/kg) Energy required to change one kilogram of a material from a solid to a liquid Water Lf = 334,000 (J/kg) Lv = latent heat of vaporization (J/kg) Energy required to change one kilogram of a material from a liquid to a gas Water Lv = 2,260,000 (J/kg)

Example Find the amount of heat needed for melting 1.3 kg ice at 0 ºC?

Notice you calculate each change separately and then add them all up.

Enthalpy (  H) The amount of heat energy absorbed or lost by a system Exothermic reactions have -  H Endothermic reactions have +  H  H is also called the heat of reaction.

Calculating Heat Changes Bond energy is the amount of energy required to break or make a bond Breaking a bond is endothermic (+  H) Forming a bond is exothermic (-  H)  H = H products – H reactants

Example 1 Calculate the energy of the reaction using heats of formation. H 2 (g) + Br 2 (g)  2 HBr(g)  H HBr = kJ/mol All elements and diatomic molecules have an enthalpy of zero.  H = H products – H reactants  H = [2 x kJ/mol] – [0]  H = kJ/mol Exothermic

Example 2  H CH 4 = kJ/mol  H CO 2 = kJ/mol  H H 2 O = kJ/mol  H = H products – H reactants  H = [ kJ/mol + (2 x kJ/mol)] – [-74.9 kJ/mol + (2 x 0)]  H = kJ/mol Exothermic Calculate the energy of the reaction using heats of formation. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(l)