Chapter 6: Thermochemistry

Energy Law of conservation of energy - energy can not be created or destroyed, but it can be transformed Similar to how matter can be transformed Energy - the capacity to do work Work is the result of a force acting over a distance Types of Energy Kinetic Potential

Kinetic Energy Kinetic energy - Energy associated with the motion of an object Mechanical energy – the energy of moving macroscale objects Moving cars, planes, baseballs etc. Thermal energy (heat) – the energy of moving microscale objects Moving ions, atoms, molecules etc.

Potential Energy Potenial energy – energy associated with position or composition, stored energy Has the potential to be transformed into kinetic energy Ball held at the top of building, potential to fall and land on something with kinetic energy Chemical energy – relative positions of electrons and nuclei in atoms/molecules represents potential energy

Energy: system vs surroundings To track the energy and transferring of energy you can define a system interacting with its surrounding System - A component or set of components of interest You define the system Surroundings – everything that a system can exchange energy with, everything touching/connected to it That which “surrounds” the system

Units of energy Joule (J) - basic unit of energy in the metric system 1J =1kg •m2/s2 1kJ = 1000 J calorie (cal) – 1cal = the energy required to raise the temp of 1g of H2O by 1ºC 4.184 J = 1 cal 1000 cal = 1kcal = 1Cal Calorie (Cal) – unit associated with “food” calories Calorie = kcal = food calorie

First law of thermodynamics Thermodynamics – the study of energy and its interconversions First law of thermodynamics – the total energy of the universe is constant Law of conservation of energy is built into the definition Perpetual motion machine (a device that puts out constant energy) cannot exist

Internal energy Internal energy (E) – the sum of the kinetic and potential energies of all particles that compose a system E is a state function which is only dependant on the state of the system, not how it got there

Energy of a system Where does lost or gained energy go or come from? The surroundings! Negative change in energy = loss (released) of energy from the system to the surroundings Positive change in energy = gaining (absorbed) of energy by the system from the surroundings

Heat and work Heat and work can be exchanged between the surroundings and the system q = heat, the flow of energy caused by a temperature difference w = work

Heat flow Why does thermal energy (heat) flow due to a temperature difference? Thermal equilibrium – heat is distributed to molecules in contact with them until the thermal energy of the system and surroundings is the same temperature

Quantifying heat We can measure the heat gained or lost by a system Substances can transfer heat, but have different capacities to do so Heat capacity (C) – the quantity of heat required to change a substances temperature by 1 °C Based on the mass, the heat capacity and the change in temperature, we can quantify the heat transferred to or from the system

Problems A thief breaks into a safe at a bank to steal a block of gold in the shape of a coin from Super Mario Bros. The safe keeps the gold at 14.0 °C. Once the thief grabs the gold coin in his hand, the coin eventually reaches body temperature at 37.0°C. How much heat was transferred to the gold coin which has a mass of 11.25g?

Thermal energy transfer When two substances of different temperatures reach thermal equilibrium with each other, the heat from the hotter substance is transferred to the colder substance The hotter substance cools down, as the colder substance warms up, with their final temperatures being the same

Thermal energy transfer problems With the masses and initial temperatures of the metal and water, what will the final temperature be when they both reach thermal equilibrium?

Distribute Kmetal and Kwater Collect Tfinal terms on the leftside, with other terms going to right side Factor out Tfinal Isolate Tfinal

Practice A 32.5g block of aluminum initially at 45.8 °C is submerged into 105.6 g of water initially at 15.4 °C. What is the final temperature once the two substances reach thermal equilibrium?

Pressure-volume work Pressure-volume work – when the force caused is from a volume change against an external pressure If the system does work to increase the volume against a external pressure, the ΔV is positive If ΔV = positive, then w = negative, the system does work

Using the equation Units of pressure = atm Units of volume = L Pressure x volume = L x atm You can convert units of L x atm to units of J 1 L x atm = 101.3 J

ΔE for chemical reactions If a system is at constant volume, then there is no work done If a chemical reaction does no work…. qv = the heat at constant volume

Calorimetry Calorimetry- measurement of the external energy exchanged between a reaction(system) and the surrounding by monitoring the temperature change Bomb calorimeter is a device used in calorimetry The chemical reaction(system) transfers heat to the bomb (surrounding)

Enthalpy (H) Enthalpy – the sum of the internal energy of a system and the product of its pressure and volume For a process at a constant pressure….. ΔH is the heat absorbed or evolved by the system If a reaction produces a significant amount of gas, this can increase the volume, which will do work

Endothermic vs Exothermic If the ΔH of a chemical reaction is positive, the system absorbs heat from the surroundings, this is an endothermic reaction If the ΔH of a chemical reaction is negative, the system releases heat to the surroundings, this is an exothermic reaction

How does a chemical reaction give off heat??? Chemical “potential” energy is stored in the bonds Chemical energy arises from the electrostatic forces between protons and electron that compose atoms and molecules Breaking bonds absorbs energy, while making bonds releases energy Breaking stronger bonds absorbs more energy than breaking weaker bonds

ΔH and thermochemical equations ΔHrxn – the enthalpy of reaction or heat of reaction, the heat transferred in a reaction Dependant on amounts of the reactants that react C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) ΔHrxn = -2044 kJ When the reaction occurs with the corresponding moles from the equation, 2044kJ of heat is released from the system (exothermic reaction) 1 mol C3H8: -2044kJ or 5 mol O2: -2044kJ The relationship acts as a conversion factor to get from moles of a compound to the enthalpy of the reaction

Practice An LP gas tank in a homebarbeque contains 13.2kg of propane, C3H8. Calculate the heat (in kJ) associated with the complete combustion of all the propane in the tank. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) ΔHrxn = -2044 kJ

Measuring ΔHrxn Coffee-cup calorimetry occurs at constant pressure Can calculate ΔHrxn using a coffee-cup calorimeter By recording the change in temperature after areaction occurs, depending on the mass and solution heat capacity, you can calculate ΔHrxn Coffee-cup calorimetry occurs at constant pressure

Calorimetry recap Bomb calorimetry occurs at constant volume and measures ΔE for a reaction Coffee-cup calorimetry occurs at constant pressure and measures ΔH for a reaction.

Chemical equations and ΔHrxn Rules for chemical equations and ΔHrxn 1. If a chemical equation is multiplied bysome factor, then ΔHrxn is also multiplied by the same factor 2. If a chemical equation is reversed, then ΔHrxn changes sign.

3. If a chemical equation can be expressed by the sum of a series of steps, then ΔHrxn for the overall equation is the sum of the heats of reaction for each step Anything on both the product and reactant side cancels out This is known as Hess’s law

Practice The rules can allow us to calculate the ΔHrxn of a new reaction using known reactions that can build up to the new reaction