Thermochemistry Feeling hot, hot, hot

heat chemical temp. difference joule Thermochemistry Thermochemistry – concerned with the changes that occur during a reaction. Heat ( ) - that transfers from one object to another because of a between them. SI unit is the ( ). Heat always flows from a object to a object. heat chemical energy q temp. difference joule J warmer cooler

Heat transfer Example: You place an ice cube into a bowl of hot soup. Describe the direction of heat flow. The hotter soup transfers heat to the colder ice. It is possible to transfer HEAT, but never COLD. (There is no such thing as cold transfer!)

(Stored within the chemical structure of molecules) Energy Energy – the for doing or supplying . capacity heat work Kinetic energy Potential energy Due to motion Due to position (Temperature) (Stored within the chemical structure of molecules)

release ALL absorption Law of COE created destroyed transferred Law of Conservation of Energy – Energy is neither nor ; it can . from one form to another. chemical reactions involve a or of heat. created destroyed transferred ALL release absorption

releases heat absorbs heat Reactions exo - exit Exothermic process – to its surroundings Endothermic process – releases heat absorbs heat endo - enter

Reactions Direction of heat flow Sign Reaction Type Heat flows OUT of the system Negative – “Losing heat” Exothermic Heat flows INTO the system Positive + “Gaining heat” Endothermic

Thermochemical Equations 4Fe (s) + 3O2 (g)  2Fe2O3 (s) + 1625 kJ Does this reaction give off heat or absorb heat? Exothermic or endothermic?

Thermochemical Equations What does kJ mean? (measurement of ) ΔH - change in heat content for a reaction at constant pressure. Kilojoule = 1000 J heat Enthalpy

Thermochemical Equations C (s) + 2S (s) + 89.3 kJ  CS2 (l) Is heat released or absorbed in this chemical reaction? Exothermic or endothermic?

Heat Capacity and Specific Heat The heat energy required to raise the temperature of 1 g of a substance by 1 degree Celsius. calorie –the quantity of heat to needed to raise the temperature of one g of pure water to one °C. Q = mcΔT

Where Q = the change in heat m = the mass of water (in grams) c = the specific heat capacity of water ΔT = the change in temperature (Tfinal - Tinitial)

Conversions Convert 444 calories to joules.

Measuring/Expressing Heat Changes Thermochemical equations – treat heat change ( ) just like any other or Chemistry problems involving H are similar to problems – depends on number of of reactants and products involved. H reactant product stoichiometry moles

Energy Change in Exothermic and Endothermic Reaction

Calculate: Examples 1. Convert the following heat quantities recalling 1 cal= 4.184J 350 cal to joules 515 j to calories 1.6 kcal to joules 2. How much heat is required to change the temperature of 150 g of water by 20 C? 3. How much heat will be given off when 1500 g of water cools down by 20 C?

You Do….Pair What is the total number of joules of heat energy absorbed by 15 g of water when it is heated from 30 C to 40 C?