Unit 1: Energy Changes and Rates of Reaction Chapters 5: Thermochemistry Chapter 6: Chemical Kinetics

Different forms of energy

Law of Conservation of Energy energy cannot be created or destroyed, only transformed from one form to another (1 st Law of Thermodynamics)

Thermochemistry study of energy changes in physical, chemical and nuclear changes

Energy and Molecules Kinetic Energy: energy of motion/work (i.e. collision theory). Substances may have Thermal Energy (energy from motion of molecules- amount of substance matters). Temperature is the average kinetic energy of the particle (amount of substance negated). Potential Energy: (due to position and composition) stored in molecules because of the arrangement of nuclei and electrons in its atoms, electrical forces b/w charged particles

Endo vs. Exothermic Which process requires energy? releases energy?  breaking bonds or forming bonds What is the net E change if: a) E (required to break bonds) > E (released when bonds are formed)? net change = E is absorbed (endothermic) reactants + energy  products b) E (required to break bonds) < E (released when bonds are formed)? net change = E is released (exothermic) reactants  products + energy

IMPORTANT TERMINOLOGY Chemical System: substances (reactants and products) we are studying, represented by the chemical equations Open System: Energy and matter can move in or out Isolated System: an ideal system where no energy or matter can move in or out Closed System: only energy can move in or out, but not matter Surroundings: all the matter around the system that absorbs or releases heat Universe Surroundings System Exchanges matter and energy

3 TYPES OF SYSTEMS Identify the systems below:

Why is E released during an exothermic reaction? What is the source of E released during a reaction?

Discuss the combustion of gasoline in terms of PE, KE, energy absorbed, energy released, net energy change, exo or endothermic, work. PE of bonds in reactants > PE of bonds in products KE of reactants < KE of products E absorbed to break bonds < E released when bonds are formed  net energy change is release (exothermic) High KE of product gases does work on car parts to make the car move. gasoline + oxygen  carbon dioxide + water + energy

Homework: p. 282 #1,2,4,5 p. 285 A-C p. 291 #1,2,4,7-9

Follow-up to What is the difference between thermal energy and temperature of a substance? 2. Compare the potential energies of the reactants to that of the products for the two types of chemical processes below: (a) endothermic process (b) exothermic process 3. Label the following reactions as “endothermic,” “exothermic,” or “not enough information.” (a) O 2 → O + O (b) O 2 + O → O 3 (c) H 2 + Br 2 → 2HBr (d) NaCl → Na + + Cl − 4. Classify each of the following as an open system or a closed system. (a) a pot of boiling water (b) a sealed bottle of water (c) a helium balloon (d)a hot-air balloon

How do we measure energy changes during a physical or chemical change? A Simple Calorimeter Key Assumptions: 1. Any thermal energy transfer b/w the calorimeter and the universe is negligible. 2. Any thermal energy absorbed by the calorimeter itself is negligible. 3. All dilute, aqueous solutions have d = 1.00 g/mL and c = 4.18 J/g°C.

Calculations Involving Thermal Energy Transfer Understanding the formula: q is thermal energy transferred, in J or kJ m is mass, in g ΔT = T 2 -T 1 the temperature change, in °C c is the specific heat capacity, in J/(g  °C)(Table 1 p. 292) q=mcΔT Substancec Ice Water Steam Aluminum Substancec (J/(g  °C)) Ice2.01 Water4.18 Steam2.01 Aluminum0.900 q > 0 exo q < 0 endo

p. 295: Tutorial 1 Sample Problem 1: A student places 50.0 mL of liquid water at 21.00°C into a calorimeter. She places a sample of gold at °C into the calorimeter. The final temp of the water is 21.33°C. Calculate the quantity of thermal energy, q, absorbed by the water in the calorimeter. G iven R equired A nalysis S olution P araphrase

Sample Problem 2 Using the value of q from #1, calculate the specific heat capacity of the sample of gold if its mass is 6.77 g. Assume that the final temp of the gold sample was the same as the final temp of the water in the calorimeter.

Sample Problem 3 A 50.0 mL sample of 1.0 mol/L HCl (aq) was mixed with 50.0 mL of 1.0 mol/L NaOH (aq) at 25°C in a calorimeter. After the solutions were mixed by stirring, the temperature was 31.9°C. Determine the quantity of thermal energy transferred by the reaction to the water and state whether the reaction was endothermic or exothermic.

p. 297 #1-3

Mini-Investigation: p. 297

Demos KMnO 4 and glycerol in fume hood Ba(OH) 2(s) + NH 4 SCN (s) less stable more stable system surroundings enthalpy change ΔH sys heat (q surr )

Re-draw for an endothermic rxn!

Enthalpy Change Internal energy - sum of all KE and PE of all the components of the system We can’t measure internal energy, so… measure enthalpy change – the energy absorbed from or released to the surroundings during a reaction Recall: 2 types of energy Potential Energy includes: Protons and neutrons within nuclei Electrons in bonds Kinetic energy includes: Moving e- within atoms Vibrational, rotational, and translational motion

ENTHALPY CHANGE (ΔH) aka heat of reaction, enthalpy of reaction, change in heat content ΔH > 0, q < 0 Endothermic ΔH 0 Exothermic ΔH° 100kPa, pure liquids/solids, 1 M, 25°C or 298K ΔH system = - q surroundings Calorimetry Calculations: q surr = mcΔT ΔH = -q enthalpy per gram = ΔH/m molar enthalpy = ΔH/n where n = m/M

MOLAR ENTHALPY (ΔH X ) ΔH X (molar enthalpy) energy released/absorbed by system per mole of reacting substance units of kJ/mol good for reference or comparison ΔH (enthalpy change) energy released/absorbed by system as R  P units of kJ depends on amount of substance reacting ΔH x =ΔH n ΔH =nΔH x n = ΔH ΔH x

Mini-Lab p. 285 Calculate the molar enthalpy of solution for the salt used in the expt.

Molar Enthalpy Examples Ex 1: The molar enthalpy of vaporization of Freon-12 is kJ/mol. If g of Freon-12 (M = g/mol) is vaporized, calculate the expected enthalpy change. Ex 2: 50.0 mL of mol/L CuSO 4(aq) is mixed with and equal volume of mol/L NaOH (aq). The initial temperature of both solutions is 21.4°C. After mixing, the highest temperature reached is 24.6°C. Determine the enthalpy change of the reaction.

Ex 3: A chemist wants to determine the molar enthalpy of neutralization for the following reaction. HCl (aq) + NaOH (aq)  NaCl (aq) + H 2 O (l) The chemist uses a coffee cup calorimeter to neutralize 61.1 mL of mol/L HCl (aq) with 42.6 mL of NaOH (aq). The initial temperature of both solutions is 17.8 °C. After neutralization, the highest recorded temperature is 21.6°C. Calculate the molar enthalpy of neutralization, in kJ/mol HCl. Ex 4: What mass of KCl must have dissolved if the temperature of g of water increased by 5.5°C? (ΔH sol’n,KCl = 1.7 x 10 4 J/mol)

Investigation Molar Enthalpy of a Chemical Change p. 333 – Title – Purpose – Observations – Analyze and Evaluate (a-g) – Apply and Extend (h,i)

Homework Practice Problems p. 301 #1-4 Section Review p. 306 #1,2,4

REPRESENTING ENTHALPY CHANGES 4 Methods- 1. Include energy value in the thermochemical equation H 2(g) + ½ O 2  H 2 O (l) kJ 2. Write the chemical equation and enthalpy change H 2(g) + ½ O 2  H 2 O (l) ΔH= kJ 3. State the molar enthalpy for a specific reaction & substance ΔH Form = kJ/mol H 2 O (l) 4. Draw a chemical potential energy diagram Exothermic= product side Endothermic= reactant side Exothermic= product side Endothermic= reactant side Equation must be balanced Watch +/- signs Equation must be balanced Watch +/- signs Molar Enthalpy!!! (kJ/mol) Standard molar enthalpies (ΔH° x ) SATP Molar Enthalpy!!! (kJ/mol) Standard molar enthalpies (ΔH° x ) SATP

4. Draw a chemical potential energy diagram Changes to E p as bonds are broken and formed Exothermic Reactions= Energy released=  E p =products lower than reactants Endothermic Reactants= Energy gained=  E p =products higher than reactants REPRESENTING ENTHALPY CHANGES 4 Methods- (kJ) Remember: Title Label axis Units Reactants on LEFT Products on Right Potential Energy Diagram for the Formation of Water

REPRESENTING ENTHALPY CHANGES 4 Methods- What about the reverse reaction, the decomposition of water? 1. Include energy value in the thermochemical equation 2. Write the chemical equation and enthalpy change. 3. State the molar enthalpy for a specific reaction. 4. Draw a chemical potential energy diagram.

REPRESENTING ENTHALPY CHANGES… Ex. What is the thermochemical equation for the following chemical equation and molar enthalpy of combustion for butane: 2 C 4 H 10 (g) + 13 O 2 (g)  8 CO 2 (g) + 10 H 2 O (l) ΔH comb = kJ/mol Remember that thermochemical equations contain ΔH values!

Homework: p. 304 #1-4 p. 306 #3,5-7