Transfers of energy as heat in chemical reactions and physical changes 17-1 Thermochemistry Transfers of energy as heat in chemical reactions and physical changes

Remember… The First Law of Thermodynamics: energy cannot be created or destroyed only converted from one form to another

Heat vs.Temperature Heat (Joule, J) measure of energy change in a system. Temperature (Celsius, °C or Kelvin, K) measure of the kinetic energy (movement) of the particles in a system. Gaining or losing heat energy in a substance can change its temperature. Exothermic System loses energy to surroundings Endothermic System gains energy from surroundings

Specific Heat is a property of matter, and different species have different Specific Heat. The heat energy required to raise one gram of a pure substance 1° C or 1 K The symbol we use is cp

Specific Heat Metals have very low cp, Water has a very high cp, which is why metals often feel cold to the touch. Table on page 513 Water has a very high cp, 4.184 J/g·0C Substances with lower cp will rise in temperature faster and require less energy to do so than do substances with high cp.

Specific Heat 1 calorie= 4.184 Joules Substance J/g/oC or J/g/K cal/g/oC or cal/g/K Water (0 oC to 100 oC) 4.186 1.000 Zinc .387 0.093 Ice (-10 oC to 0 oC) 2.093 0.500 Steam (100 oC) 2.009 0.480 Brass .380 0.092 Wood (typical) 1.674 0.400 Soil (typical) 1.046 0.250 Air (50 oC) Aluminum .900 0.215 Tin .227 0.205 Glass (typical) .837 0.200 Iron/Steel .452 0.108 Copper 0.0924 Silver .236 0.0564 Mercury .138 0.0330 Gold .130 0.0310 Lead .128 0.0305 1 calorie= 4.184 Joules

Specific Heat Equation cp = q/(m ΔT) OR q = cp x m x ΔT

Heat of Reaction Quantity of energy released or absorbed during a chemical reaction Thermochemical equation – shows the quantity of heat Example: 2H2 + O2  2H2O + 483.6 kJ Energy released or absorbed?

Heat of Reaction Example: 2H2 + O2  2H2O + 483.6 kJ Energy released (on product side) EXOTHERMIC What about when it’s on the reactant side?

Heat of Reaction Example: 2H2O + 483.6 kJ  2H2 + O2 Energy absorbed (on reactant side) ENDOTHERMIC

ΔH = Hproducts - Hreactants Enthalpy Change (ΔH) The amount of energy absorbed or lost ΔH = Hproducts - Hreactants Thermochemical equations usually written this way 2H2 + O2  2H2O ΔH = -483.6 kJ/mol When ΔH is negative, the system loses energy and it is EXOTHERMIC

ΔH = Hproducts - Hreactants Enthalpy Change (ΔH) The amount of energy absorbed or lost ΔH = Hproducts - Hreactants 2H2O  2H2 + O2 ΔH = +483.6 kJ/mol When ΔH is positive, the system gains energy and it is ENDOTHERMIC

Endothermic or Exothermic? C6H12O6 + 6O2 6CO2 + 6H20 ΔHrxn = -2870 kJ/mol

Heat of Formation (ΔHf) energy released or absorbed to form 1 mole of a compound from its elements p. 902 If a large amount of energy is released when compound is formed… Endothermic or exothermic? Positive or Negative ΔHf?

Heat of Formation (ΔHf) energy released or absorbed to form 1 mole of a compound from its elements p. 902 If a large amount of energy is released when compound is formed… Endothermic or exothermic? Positive or Negative ΔHf? HIGH NEGATIVE ΔHf = VERY STABLE!

Heat of Formation (ΔHf) POSITIVE ΔHf = UNSTABLE! Pure elements ΔHf = O ΔHf of carbon dioxide = -393.5 kJ/mol More stable than C and O alone HgC2N2O2 ΔHf = +226.7 kJ/mol Very unstable, used in explosives

Heat of Combustion (ΔHc) Reactants? Products? Exothermic or Endothermic? Positive or Negative?

Heat of Combustion (ΔHc) Reactants? C and H with O2 Products? CO2 and H2O and heat and light Exothermic or Endothermic? Positive or Negative? P. 896

Stability of these compounds? Al2O3 (s) -1676.0 kJ/mol CaCO3 (s) -1206.92 kJ/mol NO (g) 90.29 kJ/mol O3 142.7 kJ/mol