Download Chemistry 11 Notes on Heat and Calorimetry and more Summaries Chemistry in PDF only on Docsity! Chemistry 11 Notes on Heat and Calorimetry Chemistry 11—Notes on Heat and Calorimetry Page 1 Chemistry 11 Notes on Heat and Calorimetry Some chemical reactions release heat to the surroundings – These are exothermic Some chemical reactions absorb heat from the surroundings – These are endothermic Heat is a form of energy (which cannot be created or destroyed). A chemical reaction cannot create or destroy energy, it just changes it from one form to another. All substances have a type of chemical potential energy stored in them. This is called enthalpy. The actual definition of enthalpy is the total energy contained in a system. For Chemistry 11, it’s easiest to think of enthalpy as mainly chemical potential energy stored in a substance. Gasoline contains more stored chemical potential energy than water. Therefore we say that gasoline has more enthalpy than water. An exothermic chemical reaction converts the enthalpy stored in a substance into heat, which is released to the surroundings. For example, when gasoline burns, the enthalpy in the gasoline (mainly C8H18) is converted into heat: 2C8H18 + 25O2 Æ 16CO2 + 18H2O + heat The symbol for enthalpy is “H” (historically, enthalpy used to be called “Heat Content”) The change in enthalpy during a reaction is called 'H. (The symbol ∆ (delta) means “change in”) The reaction above can be shown on a graph. See the graph on the next page…. Have higher enthalpy Have lower enthalpy Released to the surroundings Chemistry 11 Notes on Heat and Calorimetry Chemistry 11—Notes on Heat and Calorimetry Page 2 An enthalpy diagram for an exothermic reaction: So, to summarize, in an exothermic reaction: ¾ ∆H is negative ¾ Products are lower than Reactants on the Enthalpy Diagram ¾ Heat is released to the surroundings For an endothermic reaction: Eg.) NaHCO3 + HCl + heat Æ H2O + CO2 + NaCl Heat is absorbed from the surroundings and converted into enthalpy. Here is an enthalpy graph for an endothermic reaction: There are two ways to show that a reaction is exothermic: 1. A “ heat term” is written on the right side of a chemical equation to show that heat is given off or produced. This can be the word “ heat” or an actual amount of heat in kJ: eg.) 2C8H18 + 25O2 Æ 16CO2 + 18H2O + 5076 kJ 2. An equation is written and the 'H is written beside it (no “ +” sign between): eg.) 2C8H18 + 25O2 Æ 16CO2 + 18H2O 'H = -5076 kJ 16CO2 + 18H2O 2C8H18 + 25O2 Enthalpy (H) Reaction Proceeds ∆H is Negative H2O + CO2 + NaCl NaHCO3 + HCl Enthalpy (H) Reaction Proceeds ∆H is Positive Notice that for an exothermic reaction, the ∆H is NEGATIVE! Heat term is on the right side of the equation.
