Energy and Heat

Definitions Thermochemistry: the study of the energy changes that accompany chemical reactions Energy: A property of matter describing the ability to do work Kinetic Energy: related to movement of an object Potential Energy: based on position of an object (stored energy)

Review1 st Law of Thermodynamics 1 st Law of Thermodynamics: The law of conservation of energy ▫Energy cannot be created or destroyed, but transformed from one form to another The Kinetic Molecular Theory ▫Molecules are constantly moving (kinetic energy) ▫When heat is added, the molecules move faster and their kinetic energy increases

Energy Units: ▫Joule (kg*m 2 /s 2 ) ▫1 calorie = J Heat energy (q): energy transferred between substances during a reaction

Temperature vs. Thermal Energy Temperature: A measure of the average kinetic energy of the molecules in a substance (measured) Thermal Energy: a form of kinetic energy that results from the motion of molecules (cannot be measured)

Systems Systems: Where a physical or chemical change occurs Surroundings: the rest of the universe outside the system Open System: Both matter and energy can freely flow into the surroundings Closed System: Energy can flow to surroundings but matter cannot Isolated System: Neither energy nor matter can flow to the surroundings

Types of Reactions Endothermic: When heat is transferred from the surroundings to the system ▫q=(+) Exothermic: When heat is transferred from the system to the surroundings ▫q=(-)

Heat Capacity (C) The amount of heat energy required to raise the temperature of a substance (of any mass) by 1 o C ▫(Units: J/ o C) Specific heat capacity: Amount of heat energy required to raise the temperature of 1g of a substance by 1 o C ▫(Units: J/(g°C)) Molar heat capacity: Amount of heat energy required to raise 1 mole of a substance by 1 o C ▫Units: J/(mol°C))

Practice You are provided with a glass of milk and a swimming pool of milk ▫Which will have the higher heat capacity? ▫Which will have the higher specific heat capacity? Answer: The swimming pool of milk will have the higher heat capacity, because it is dependent on the amount The glass and swimming pool of milk have that same specific heat capacity because that is based on 1g of the substance

Factors Affecting Heat Capacity 1.Mass (m) – the greater the number of molecules, the more heat required 2.Temperature change (∆T) - the greater the temperature increase, the more heat required 3.Type of substance – each substance differs in ability to absorb heat (c)

Calculating Quantities of Heat q=mcT q = quantity of heat transferred (J or kJ) ▫If q is negative, the reaction was exothermic ▫If q is positive, the reaction was endothermic m = mass of substance (grams) C = specific heat capacity (J/(g°C)) t =t 2 -t 1 ( o C)

Practice When 600g of water in an electric kettle is heated from 20 o C to 85 o C to make a cup of tea, how much heat flows into the water?

Practice When 600g of water in an electric kettle is heated from 20 o C to 85 o C to make a cup of tea, how much heat flows into the water? Given: m=600g C = J/(g°C) t 1 = 20°C t 2 = 85°C q=mct q=600g(4.184 J/(g°C))(85°C-20°C) =1.63x10 5 J or 163kJ Therefore, the amount of heat that flowed into the water was 163kJ Unknown: q=? Note: 1000J = 1kJ

Enthalpy Impossible to measure the sum of kinetic and potential energy Instead we measure enthalpy change Enthalpy Change (H): the energy absorbed or released to the surroundings when a system changes from reactants to products ▫H = q (when pressure kept constant) Units: J or kJ

Endothermic: Heat enters the system ( + ΔH) Exothermic: Heat leaves the system ( - ΔH)

Making and Breaking Bonds It takes energy to break bonds and energy is released when bonds are formed Exothermic Reactions: Less energy is required to break bonds in the reactants than is released by formation of new bonds in the products Endothermic Reactions: More energy is required to break bonds in the reactants than is released by formation of new bonds in the products

Molar Enthalpy Molar Enthalpy: the enthalpy change per mole of a substance undergoing a change They are specific to a reaction H x x = type of change that is occurring

Molar Heat of Fusion The amount of heat necessary to melt 1.0 mole of a substance at its melting point. ΔH fus Unit: J/mol or kJ/mol Specific to each substance No temperature change involved q (J) = n (mol) x ΔH fus (J/mol)

Question 11.0 kJ are required to melt 4.2 mol of sodium (Na). What is the molar enthalpy of fusion? q = 11.0 kJ n = 4.2 mol ΔH fus = ? q = n x ΔH fus ΔH fus = q / n ΔH fus = 11.0 kJ / 4.2 mol ΔH fus = 2.6 kJ/mol

Molar Heat of Vaporization The amount of heat necessary to boil 1.0 mole of a substance at its boiling point. ΔH vap Unit: J/mol or kJ/mol Specific to each substance No temperature change involved q (J) = n (mol) x ΔH vap (J/mol)

Question The ΔH vap of H 2 O is J/mol. How many Joules are needed to completely boil 35.0 g of water at its boiling point? Given: ΔH vap = J/mol m = 35.0 g n = ?* Remember: n = m/M (molar mass) M = 18.0 g/mol (calculated from atomic masses) q = ? q = n x ΔH vap q = m/M (mol) x ΔH vap (J/mol) q = 35.0 g/18.0 g (mol) x (J/mol) q = 7.91 x 10 4 J

Time-Temperature Graph - Heat energy is continuously being added to H 2 O.

Question: How much energy is required to raise the temperature of 45 g of H 2 O from 30 °C to 150 °C ?

Answer: 1) (liquid) ΔT =T 2 –T 1 = 100 °C – 30 °C = 70 °C q 1 = mc ΔT q 1 = 45 g x 4.18 J/g°C x 70 °C q 1 = 1.32 x 10 4 J

Answer: 2) (phase change: Boiling ) Since ΔT = 0 °C q 2 = nΔH vap q 2 = m/M x ΔH vap q 2 = 45 g/18.0 g/mol x J/mol q 2 = 1.02 x 10 5 J

Answer: 3) (gas) T 2 –T 1 = 150 °C – 100 °C = 50 °C q 3 = mc ΔT q 3 = 45 g x 2.07 J/g°C x 50 °C q 3 = 4.7 x 10 3 J

Final Answer: q total = q 1 + q 2 + q 3 = 1.2 x 10 5 J

CALORIMETRY

Calorimetry The science of measuring the change in heat of chemical reactions or physical changes A calorimeter: an insulated reaction vessel in which a reaction can occur and where the change in temperature of the system can be measured ∆E total = ∆E system + ∆E surroundings = 0 ∆E system = -∆E surroundings

Calorimetry Assumptions No heat is being transferred between the calorimeter and the outside environment (isolated system) Any heat absorbed or released by the calorimeter itself is negligible A dilute aqueous solution is assumed to have a density and specific heat capacity equal to that of water

Coffee-Cup Calorimeter Limitations: ▫Cannot be used for reactions involving gases ▫Cannot be used for high temperature reactions

Bomb Calorimeter Sealed metal container Temperature difference of the water is measured

Problem Hot copper was added to a calorimeter. Given the following calorimeter data, calculate the specific heat of copper. ▫Calorimeter:  Mass of water500.0 g  Initial temp. of water20.0 °C  Final temp. of water26.2 °C ▫Copper sample:  Mass of copper g  Initial temp. of copper98.8 °C

Answer: q gained = - q lost q = mcΔT At temperature equilibrium,  m w c w ΔT w = -(m cu c cu ΔT cu ) (500.0 g) (4.18 J/g°C) (6.2 °C) = -( g) (c cu ) (-72.6 °C) c cu = 3.7 x J/g°C

Representing H

1. Thermochemical Equations Ex: 2Na(s) + 2 H2O(l)  2NaOH(aq) + H2(g) H = kJ Exothermic reactions in one direction become endothermic reactions in the reverse direction (the sign of H o is reversed)

2. Thermochemical Equations with Energy Terms “Energy is a reactant” “Energy is a product”

3. Molar Enthalpies of Reaction The enthalpy change associated with 1 mole of a substance The particular reactant or product must be specified

4. Potential Energy Diagrams Exothermic Endothermic

Example 1: A chemist wants to determine empirically the enthalpy change for the following reaction: Mg(s) + HCl(aq)  MgCl 2 (aq) + H 2 (g) The chemist uses a coffee-cup calorimeter to react 0.50 g of Mg ribbon with 100 mL of 1.00mol/L HCl(aq). The initial temperature of the HCl is 20.4˚C. After neutralization, the highest recorded temperature is 40.7˚C. (a)Calculate the enthalpy change ( H), in kJ/mol of Mg (b)Write the thermochemical equation (c)Draw a potential energy diagram (d)State any assumption you made

Example 2: An aluminum-can calorimeter was used to determine the heat of combustion (kJ/mol) of isopropyl alcohol. It was found that combustion of 1.37 g of the alcohol raised the temperature of the can and water from 19.0˚C to 28.6˚C. The mass of the aluminum-can was found to be 45.73g and the mass of the aluminum-can + water was g. Hint: c Al =0.897J/g˚C a)Calculate the heat of combustion (kJ/mol) of isopropyl alcohol (C 3 H 8 O). b)Write the thermochemical equation with energy terms