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

Standard 7b Students will know that chemical processes can either release or absorb thermal energy.

Essential Question: How is heat used to measure energy in chemical reactions?

Heat vs Temperature Heat Temperature 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.

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

Specific Heat Capacity 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 Metals have very low cp, why metals feel cold to touch. 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. 1 calorie=4.184 Joules

Calculating Heat Energy Cp=Joules / (gram * degree) Change in heat (Joules, J) = Change in temperature (degree, 0C) x Mass (mass, g) Specific Heat Capacity

Enthalpy, ΔH Enthalpy We specify enthalpy with ΔH. “Δ” means “change”. 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).

Devices to Measure ΔHcomb 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.

Reaction Enthalpy If ΔH is negative, reaction is exothermic. C6H12O6 + 6O2 6CO2 + 6H20 + 2870kJ ΔHrxn = -2870 kJ/mol If ΔH is positive, reaction is endothermic. 2H2O + 571.6kJ  2H2 + O2 ΔHrxn = +571.6 kJ/mol energy

Chapter 16 SUTW Prompt Describe the differences between exothermic and endothermic in a chemical reaction. Complete a 8 -10 sentence paragraph using the SUTW paragraph format. Hilight using green, yellow, and pink. Due Date: Monday, November 28th, 2016 (start of class).