2 ThermochemistryStudy of changes that accompany chemical reactions and phase changesThe Universe is considered to be made of 2 parts:1. System: part that contains the reaction or process2. Surroundings: everything else
3 ENERGY defined as the ability to do work or transfer heat energy. 2 types of energyPotential Energy (PE): Energy at rest due to the position of an object; chemical potential energy is the energy stored in a substance’s bonds.2. Kinetic energy (KE): Energy of the motion of particles in a substance and is directly proportional to temperature. As temperature increases, KE also increases.
4 Law of ConservationLaw of Conservation of Energy states that energy is neither created nor destroyed, just changed in formC8H18 + O2 H2O + CO2 + EnergyStored PE converts to 25% work and 75% heat(ENERGY)
5 Exothermic Reactions HOT PACK An exothermic reaction is when the system releases energy; heat flows out of a reaction and the surroundings get warmer. They have a NEGATIVE H.H products < H reactants 4Fe + 3 O2 2 Fe2O kJ OR4Fe + 3 O2 2 Fe2O H = kJ
6 Endothermic Reactions COLD PACKAn endothermic reaction is when the system absorbs energy; heat flows into a reaction and the surroundings get cooler. They have a POSITIVE HH products > Hreactants 27kJ + NH4NO3(s) NH4(aq)+1+NO3(aq) ORNH4NO3(s) NH4(aq)+1 + NO3(aq)-1H = + 27 kJ
8 What is the difference between Temperature & Heat? Instrument: thermometerUnits: Celsius, Fahrenheit, KelvinDefinition:A measure of the average kinetic energy of the molecules in a substanceA measure of the motions of the moleculesA measure of how hot or cold something is
9 What is the difference between Temperature & Heat? Instrument: calorimeterUnits: calories, joulesDefinition:The total amount of energy in a substance.A form of energy that is transferred between objects because one is warner than the other.Heat transfer is always from hot to coldDepends on 3 things:1. amount of substance (mass)2. Temperature change3. type of material (specific heat)
10 Units of Heat EnergyA calorie is defined as the amount of heat needed to raise the temperature of 1 g of water by 1 C1 cal= JFood “Calories” are kilocalories.1kcal = 1000 calories.
12 Specific HeatAmount of heat required to raise the temperature of 1 g of a substance by 1 CDifferent substances have different specific heats.Water has a specific heat of J/gC.Iron(Fe) has a specific heat of .449 J/gC.Gold (Au) has a specific heat of .129 J/gC.The higher the specific heat the more energy it takes to change its temperature.
13 Calculating Heat mT c= specific heat q = heat in joules or galories m= massT = change in temperature = Tf – Tic= q_mT
14 ExampleA 155 g sample of an unknown substance was heated from 25.0 C to 40.0 C. The substance absorbed 5696 J of energy. What is the specific heat?
15 ExampleHow much heat is needed to change the temperature of 12.0 g of silver with a specific heat of cal/g°C from 25.0°C to 83.0 °C?
16 Measuring Heat in a Calorimeter A coffee cup calorimeter measures heat at constant pressure; works on the premise that the amount of heat released in a reaction(-q) or physical change is equal to the amount of heat absorbed by the water(+q) q = +q Rearrange the specific heat equation:q = m x c x T
17 ExampleA piece of unknown metal with mass g is heated to an initial temperature of °C and dropped into g of water (with an initial temperature of °C) in a calorimeter. The final temperature of the system is 30.05°C. What is the specific heat of the metal? Specific heat of water = J/g°C
18 ExampleA gram sample of vanadium was heated to °C (its initial temperature). It was then dumped into a calorimeter. The initial temperature of the calorimeter’s water was °C. After the metal was allowed to release all its heat to the calorimeter’s water, °C was the final temperature. What mass of distilled water was in the calorimeter?Specific heat of vanadium = J/gC Specific heat of water = J/g°C
20 Heat of Fusion/Solidification Heat of fusion (Hfus ) is the heat energy required to melt one gram of a solid at its melting pointFor water, Hfus = 334 J/gq = Hfus x massHeat of solidification (Hsolid ) is the heat energy lost when one gram of a liquid freezes to a solid at its freezing pointFor water, Hsolid = -334 J/gq = Hsolid x mass
21 Heat of Vaporization/Condensation Heat of vaporization (Hvap) is the heat to vaporize one gram of a liquid at its normal boiling pointFor water, Hvap= J/gq = Hvap x massHeat of condensation (Hcond ) is the heat energy released when one gram of a liquid forms from its vaporFor water, Hcond = J/gq = Hcond x mass
22 ExampleHow much heat is needed to melt 500.0g of ice at 0 C?
23 ExampleHow much heat is evolved when 1255 g of water condenses to a liquid at 100°C?