Energy Transformations Thermochemistry Energy Transformations

Calorimetry Calorimetry: The measurement of the heat into or out of a system for chemical and physical processes. Based on the fact that the heat absorbed = the heat released The device used to measure the absorption or release of heat is called a Calorimeter

Endothermic Reaction Endothermic reactions – chemical reaction that absorbs energy to break existing bonds Heat goes into the reaction (system) from the surroundings The surroundings will feel colder Temperature of endothermic reactions goes down The sign for the heat change (enthalpy) will be positive Ex: Boiling Water or Melting Ice (absorbing energy)

Endothermic Reaction

Exothermic Reactions Exothermic reactions – chemical reaction in which energy is released Heat goes out of the reaction (system) into the surroundings The surroundings will feel hotter Temperature of exothermic reactions goes up The sign for the heat change(enthalpy) will be negative EX: Condensation (gas to liquid) releasing energy!

Exothermic Reaction

Endo- and Exothermic ENDOTHERMIC EXOTHERMIC Heat Heat qsys < 0 Surroundings Heat qsys > 0 System Surroundings System Heat qsys < 0 DEMONSTRATION example of an ENDOTHERMIC reaction - hydration of ammonium nitrate (NH4)+(NO3)- (s) + H2O -> NH4 (aq) + NO3 (aq) (cools surroundings enough to freeze water) WHY SPONTANEOUS ? ENDOTHERMIC EXOTHERMIC

Endothermic & Exothermic Reactions Are the following reactions endothermic or exothermic? CO + 3H2  CH4 + H2O H= -206kJ I add magnesium metal to some hydrochloric acid. The temperature goes from 23C to 27 C I mix together some vinegar & baking soda. The temperature goes from 28C to 23C

Chemistry Happens in Moles An equation that includes energy is called a thermochemical equation CH4 + O2 → CO2 + H2O ΔH = -802.2 kJ 1 mole of CH4 releases 802.2 kJ of energy STEPS: Convert to moles and then multiply by the energy given over the number of moles of the compound! If 10.3 g of CH4 are burned completely, how much heat will be produced?

The Work

Thermochemical equations S + O2  SO2 H = – 296.9 kJ If we change the equation, then the H also changes … SO2  S + O2 H = + 296.9 kJ If the reaction is reversed the sign is reversed Also, if numbers in the equation change, so will the amount of energy produced/absorbed: 2S + 2O2  2SO2 H = – 593.8 kJ

Heat and Changes of State Heat of combustion (∆H)= the heat of reaction for the complete burning of one mole of a substance Molar heat of fusion (∆Hfus)= the heat absorbed by one mole of a substance in melting from a solid to a liquid at a constant temperature Molar heat of solidification (∆Hsolid)= heat lost when one mole of a liquid freezes to a solid at a constant temperature (equal to the negative heat of fusion) Molar heat of vaporization (∆Hvap)= the heat absorbed by one mole of a substance in vaporizing from liquid to a gas Molar heat of condensation (∆Hcond)= heat released by one mole of a vapor as it condenses

Example (Heat of combustion) The standard heat of combustion (∆H°rxn) for glucose (C6H12O6) is -2808 kJ/mol. If you eat and burn 71 g of glucose in one day, how much energy are you getting from the glucose? C6H12O6 + 6 O2 → 6 CO2 + 6 H2O ΔH = -2808 kJ Step one: convert g of glucose to moles Step two: Use (∆H°rxn) to find amount of kJ gained