Energetics Energy changes in chemistry Thermochemistry n What

Energetics Energy changes in chemistry.

Thermochemistry n What is thermochemistry? n Thermochemistry is the study of energy changes associated with chemical reactions.

Enthalpy n What is enthalpy? Enthalpy (H) is the total energy of system. It is in the molecules, the bonds, and stored chemically. n Enthalpy change (Δ H) is the measure of the change in the potential energy of the bonds. n

Thermochemistry n What is a thermochemical equation? n It is a reaction written with physical states that shows the overall energy change of a reaction or phase change. n This (-). is an exothermic reaction because heat is lost

Endothermic vs exothermic n What is the difference between the two? Endothermic reactions have more energy stored in the new bonds (products) than were in the old bonds (reactants) which takes in energy. n Exothermic reactions have more energy in the old bonds (reactants) than in the new bonds (products). n

Enthalpy change n What is the value of enthalpy change for exothermic reactions? Exothermic reaction enthalpy change is negative due to Δ H = enthalpy products – enthalpy reactants. n Endothermic reaction would be the opposite because of the same equation. n

Enthalpy change n How do we show enthalpy change of a reaction? It is usually written with the equation. n + represents endothermic, - represents exothermic n It is given in the units of k. J/mol (or just k. J) because it changes with the amount of reactants and limiting reactants. n This is at thermochemical standard conditions which are 25 degrees C, 1 atm and solution concentrations of 1 mol/dm 3. n

Enthalpy change n What is the difference between heat and temperature? Temperature is the measure of the kinetic energy of the particles and does not depend on the amount present. n Heat is the measure of the total energy of a substance that increases and decreases with amount of substance. n

Enthalpy change n How do we find heat energy? Heat energy = mass x specific heat capacity x change in temperature (m x c x ΔT). n Specific heat capacity depends on the specific material involved. n

Calorimetry n What is Calorimetry? This is a technique to measure the energy change in a reaction. n Total heat = (m c ΔT)liquid + (m c ΔT)solid n

Calorimetry n When 8. 00 g of ammonium nitrate dissolves in 100. 0 cm 3 of water, the temperature dropped from 19. 0 C to 14. 5 C. What is the enthalpy of ammonium nitrate? n n n 100 g x 4. 18 J/g K x 4. 5 K 1881 J 8. 00 g/80. 06 g/mol. 100 mol 1881 J/. 100 mol 18. 8 k. J/mol

Hess’s Law n What does Hess’s Law say? It states that if you can add 2 or more different equations to produce the final equation, you can add the individual enthalpy changes to find the total. n This means that if you break down the steps involved, the overall change will be the same. n

Hess’s Law n 2 H 2 O 2(l) 2 H 2 O(l) + O 2(g) 2 H 2(g) + O 2(g) 2 H 2 O(l) ΔH= -572 k. J n H 2(g) + O 2(g) H 2 O 2(l) ΔH= -188 k. J n n Where do we start?

Reaction spontaneity n What is a spontaneous reaction? A spontaneous reaction will happen naturally. n Iron naturally rusts when left exposed. n

Reaction spontaneity n How can we tell if a reaction will be spontaneous? n Gibbs free energy equation will tell us. If the sign of G is negative, the reaction is spontaneous. n Be sure units are the same. n Is there anything in this equation you don’t know? n

Reaction spontaneity n What is entropy? Entropy is the randomness in a system. n The more “freedom” particles have, the more entropy the system has. n

Reaction spontaneity n What are situations when Gibbs free energy equation is negative (spontaneous)?