Chapter 5 Changes in Matter and Energy

Heat and Energy Changes Thermochemistry: Energy changes that accompany changes in matter. Thermal Energy: a form of kinetic energy that results from the motion of the particles. Heat: the quantity of energy transferred between substances. q= quantity of heat Temperature vs. Heat: Temperature is the measure of the average kinetic energy of an object. Heat ≠ temperature

System: object or collection of objects being studied In lab, the system is the chemicals inside the beaker Surroundings: everything outside of the system that can exchange energy with the system The surroundings are outside the beaker Universe: system plus surroundings Exothermic: heat transferred from the system to the surroundings Endothermic: heat transferred from the surroundings to the system

Types of energy Kinetic energy Potential energy Vibrational Translational Rotational Bond energy  

Endothermic reactions absorb energy from the surroundings. D H>0 Heat Potential energy

Exothermic reactions release energy to the surroundings. DH <0 Heat Potential energy

Enthalpy Changes for Chemical Reactions Exothermic reactions generate specific amounts of heat Because the potential energies of the products are lower than the potential energies of the reactants Endothermic reactions consume specific amounts of heat Potential energies of the reactants are lower than the products DH for the reverse reaction is equal, but has the opposite sign to the forward reaction

Calorimetry: Measuring q Measuring the quantity of heat. Use a calorimeter. Two kinds Constant pressure calorimeter (called a coffee cup calorimeter) or a bomb calorimeter

Bomb Calorimeter thermometer stirrer full of water ignition wire Steel bomb sample

Calorimetry: Calculating q q=mcDT OR q= CDT m=mass in g c=specific heat capacity in J /g.K DT = change in temperature in K C= heat capacity of calorimeter (J/K)

Specific Heat Capacity (C) amount of heat required to raise the temperature of 1 gram of a substance by 1 kelvin SI Units: Specific heat capacity = J /g.K Specific heat of water = 4.184 SI unit: Joule (J) 1 calorie = 4.184 J English unit = BTU J g.K

Heat Transfer A 1.6 g sample of metal that appears to be gold requires 5.8 J to raise the temperature from 23°C to 41°C. Is the metal pure gold? J g.K Specific heat of gold is 0.13 Therefore the metal cannot be pure gold. 23

Example The specific heat of graphite is 0.71 J/gºC. Calculate the energy needed to raise the temperature of 75 kg of graphite from 294 K to 348 K. G: c= 0.71 J/gºC m= 75 kg DT= 348 K -294K= =54K R: q A: q=mcDT S: q= 75000 g (0.71 J/gºC )(54K) =2.9 x 103 KJ Review Significant figures and Scientific notation!

Example- mixing of 2, Hot and COLD A 46.2 g sample of copper is heated to 95.4ºC and then placed in a calorimeter containing 75.0 g of water at 19.6ºC. The final temperature of both the water and the copper is 21.8ºC. What is the specific heat of copper? NOTE: Assume the experiment is done correctly so that the system is CLOSED and insulated…ie a calorimeter is used… Thus…heat lost by the hot object=heat gained by the cold object. Water is used since the c of water is well known. The “c” of another substance can be found by comparing to water. -1 x q lost= 1 x q gained -1 x (m c DT) lost= (m c DT) gained

D H Every energy measurement has three parts. A unit ( Joules ). A number how many. and a sign to tell direction. negative - exothermic positive- endothermic

Molar Enthalpies The enthalpy change associated with the change on one mole of a substance. Denoted by a subscript (X) with D H Example: B2H6 (g) + 3O2(g) B2O3(s + 3H2O (g) Hr= -2035 kJ (enthalpy for one mole ) If enthalpy DH is without a subscript, it does not denote per mole. D Hx

Example D Hx = D H/n or n x DHx = DH What is the molar enthalpy of reaction of methane with chlorine gas if 32.6 g absorbs 23.8 KJ of energy? By definition… D Hx = D H/n or n x DHx = DH

Liquid to gas or back- D Hvap or L vap How much energy is added when 100.0 g of ammonia is vapourized? Change of state…check tables at back of textbook --- Solid to liquid or back - D Hfus or L fus Liquid to gas or back- D Hvap or L vap DH= DHfus n, DH= DHvap n, DH= Lfus m, DH= Lvap m

Representing Enthalpy Changes Method 1: Thermochemical Equations with Energy Terms (Example is endothermic) Method 2: Thermochemical Equations with ∆H Values (Example is exothermic)

Method 3: Molar Enthalpies of Reaction (Example is exothermic)

Method 4: Potential Energy Diagrams (Example left is exothermic and Right is endothermic)