Energy changes in chemistry. Energetics Energy changes in chemistry.

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

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

Endothermic vs exothermic 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. Exothermic reactions have more energy in the old bonds (reactants) than in the new bonds (products).

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

Enthalpy change How do we show enthalpy change of a reaction? It is usually written with the equation. + represents endothermic, - represents exothermic It is given in the units of kJ/mol (or just kJ) because it changes with the amount of reactants and limiting reactants. This is at thermochemical standard conditions which are 25 degrees C, 1 atm and solution concentrations of 1 mol/dm3.

Enthalpy change 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. Heat is the measure of the total energy of a substance that increases and decreases with amount of substance.

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

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

Calorimetry When 8.00 g of ammonium nitrate dissolves in 100.0 cm3 of water, the temperature dropped from 19.0 C to 14.5 C. What is the enthalpy of ammonium nitrate? 100 g x 4.18 J/g K x 4.5 K 1881 J 8.00g/80.06 g/mol .100 mol 1881 J/ .100 mol 18.8 kJ/mol

Hess’s Law 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. This means that if you break down the steps involved, the overall change will be the same.

Hess’s Law 2H2O2(l)  2H2O(l) + O2(g) 2H2(g) + O2(g)  2H2O(l) ΔH= -572 kJ H2(g) + O2(g)  H2O2(l) ΔH= -188 kJ Where do we start?