Thermochemistry Energy Transformations Calorimetry Calorimetry The measurement of

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 Surroundings System Heat qsys > 0 ENDOTHERMIC Surroundings Heat qsys < 0 System EXOTHERMIC

Endothermic & Exothermic Reactions • Are the following reactions endothermic or exothermic? • CO + 3 H 2 CH 4 + H 2 O H= -206 k. J • 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 CH 4 + O 2 → CO 2 + H 2 O ΔH = -802. 2 k. J • 1 mole of CH 4 releases 802. 2 k. J 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 CH 4 are burned completely, how much heat will be produced?

The Work

Thermochemical equations S + O 2 SO 2 H = – 296. 9 k. J • If we change the equation, then the H also changes … SO 2 S + O 2 H = + 296. 9 k. J • If the reaction is reversed the sign is reversed • Also, if numbers in the equation change, so will the amount of energy produced/absorbed: 2 S + 2 O 2 2 SO 2 H = – 593. 8 k. J

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 (C 6 H 12 O 6) is -2808 k. J/mol. If you eat and burn 71 g of glucose in one day, how much energy are you getting from the glucose? • C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O ΔH = -2808 k. J • Step one: convert g of glucose to moles • Step two: Use (∆H°rxn) to find amount of k. J gained