Back Chapter 16 Reaction Energy West Valley High School General Chemistry Mr. Mata

Back ©Bires, 2005 Slide 2 Standard 7b Students will know that chemical processes can either release (exothermic) or absorb (endothermic) thermal energy.

Back ©Bires, 2005 Slide 3 Essential Question: How is heat used to measure energy in chemical reactions?

Back ©Bires, 2005 Slide 4 Heat vs Temperature Heat measure of energy change in a system. Temperature 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.

Back ©Bires, 2005 Slide 5 Specific Heat Capacity measure of how a substance reacts to heat energy changes. The symbol we use is c p. The “p” stands for pressure (constant) while heat is added or lost. Specific Heat property of matter, diff substances have diff specific heats. Specific Heat Capacity heat energy required to raise 1 gram of pure substance 1 degree Celsius.

Back ©Bires, 2005 Slide 6 Specific Heat Capacity Metals have very low c p, why metals feel cold to touch. Water has a very high c p, J/g· 0 C Substances with lower c p will rise in temperature faster and require less energy to do so than do substances with high c p. Substance J/g/ o C or J/g/K cal/g/ o C or cal/g/K Water (0 o C to 100 o C) Zinc Ice (-10 o C to 0 o C) Steam (100 o C) Brass Wood (typical) Soil (typical) Air (50 o C) Aluminum Tin Glass (typical) Iron/Steel Copper Silver Mercury Gold Lead calorie=4.184 Joules

Back ©Bires, 2005 Slide 7 Calculating Heat Energy C p =Joules / (gram * degree) Change in heat (Joules, J) = Change in temperature (degree, 0 C) x Mass (mass, g) x Specific Heat Capacity

Back ©Bires, 2005 Slide 8 Enthalpy, ΔH Enthalpy heat energy transferred for a specific change to take place. We specify enthalpy with ΔH. “ Δ ” means “change”. Exothermic reaction negative enthalpy ( - ΔH ) Endothermic reaction Positive enthalpy (+ ΔH).

Back ©Bires, 2005 Slide 9 Devices to Measure ΔH comb The Calorimeter (shown) Heat energy transferred from a reaction inside the calorimeter to the water in the calorimeter. The temperature change of the water is observed. A Bomb Calorimeter uses a chamber of pure oxygen to measure heats of combustion to the 1/1000 of a joule.

Back ©Bires, 2005 Slide 10 Reaction Enthalpy If ΔH is negative, reaction is exothermic. C 6 H 12 O 6 + 6O 2  6CO 2 + 6H kJ ΔH rxn = kJ/mol If ΔH is positive, reaction is endothermic. 2H 2 O kJ  2H 2 + O 2 ΔH rxn = kJ/mol energy