Thermochemistry Energy Transformations

Definitions Thermochemistry – The study of energy changes that occur during chemical reactions and changes in state. Energy changes occur as either heat transfer or work, or a combination of both. Heat – Represented by q The change in heat is represented by ∆H The energy that transfers from one object to another because of a temperature difference between them. Always flows from a warmer object to a cooler object. System – A part of the universe on which you focus your attention Surroundings – In thermochem, the region in the immediate vicinity of the system

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

Endothermic Reaction

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

Exothermic Reaction

Endo- and Exothermic Surroundings Heat q sys > 0 System ENDOTHERMICENDOTHERMIC Heat q sys < 0 q sys < 0 Surroundings System EXOTHERMIC

Endothermic & Exothermic Reactions Are the following reactions endothermic or exothermic? CO + 3H 2  CH 4 + H 2 O  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 CH 4 + O 2 → CO 2 + H 2 O ΔH = kJ 1 mole of CH 4 releases kJ of energy STEPS: Convert to moles and then multiply by the energy given over the n umber 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 = – kJ If we change the equation, then the  H also changes … SO 2  S + O 2  H = 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 + 2O 2  2SO 2  H = – 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 (∆H fus )= 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 (∆H solid )= 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 (∆H vap )= the heat absorbed by one mole of a substance in vaporizing from liquid to a gas Molar heat of condensation (∆H cond )= 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 kJ/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 O 2 → 6 CO H 2 O ΔH = kJ Step one: convert g of glucose to moles Step two: Use (∆H° rxn ) to find amount of kJ gained