Lecture Class 28 - Hess's Law and Enthalpies of Formation

Chem Unit 3: Chapter 7-Chapter Chapter 7 Stoichiometry CO, comes from combustion reactions There are 3 requirements for combustion fuel, heat (five) and an oxidizer products of combustion reactions are lower in energy than the reactants so light and heat energy is released in forms of Stoichiometry relative number of molecules, ex 24 to 10 in H.O Big Picture in Chemical reactions bonds rearrange to transform molecules Chemical reactions are represented by chemical equations, which MUST be balanced. converting   cach quantity to moles allows us to use mass to track quantities of reactants and prodvuls Learning Clbjectives balance a chemical equation and then vse implied stoichiomatry to calculate masses of all species involved. perform the classic "gram to gram conversions associated with any species. in a balariced   equation. translate between masses and moles for reactants and products in a balanced reaction. Chemical vs Physical Changes Chemical change in composition or molecular identity through rearrangement of molcular bonds Chimical Property property that changes with change in composition Physical no change in composition or molecular identity Pitysical property property that changes with no change in composition Limiting Reactants and Theoretical Yeilds Rarely will all reactants be present in exact stoichiometric ratios Rather, one reactant will limit the production of products key Concepts Limiting reactant/reagent reactant that is completely consumed and limits the amount of product Reactant/reagent in excess: amount of leftover reactant Theoretical yield: amount of product that can be made from limiting reagent Actual Yield amount of product actually produced Percent Yield actual yield/theoretical yield •Steps to find limiting reagent 1) get balanced chemical equation. 2) convert to moles (or molecules) 3) Figure out how much each product makes Percent Yield = Actual Yield / Theoretical Yield * 100% Given 52g of Mg and 12g O2 what is the limiting reactant and what is the theoretical yield? 2 Ma (5) 0.9) 2490 (5) 529 Mg imol Mg 124.319 Mg 2.13903 mol My/2g M *2.139 mol Mgp 132.0090 0.675 al 0/2 M90 0.75 mol MgO2 15 limiting reactant 0.75 mol MgO- 30.2325 g MgO theoretical yjeld Chapter 9: Heat, Work First Law Energy can be transferred though both heat (q) and work (w) -sign for exothermic reactions (give off energy) Sign for endothermic reactions (take up energy) Heat capacity energy required to raise temperature of I mol of substance 1°C (c) same for I g of subsance (J/°c) Specific heat capauty •Mater the 'stuff of which everything is composed we weigh things (units kg) We characterize matter Force something capable of changing the motion of an objed measure the capacity to induce acceleration, a(unts) Newton's second law: F=mia (Units 붉은-newton, N) *sol N will accelerate I kg of mass 1 m/s²) Energy We meas how much work (and/or hext) we can get out Work when force moves an object a distance, d, we define work as force distance, WoFd (units Joules, J=Nm= Textbook definitions. Energy is the capacity to do work •Work is force acting over a distance w. F.d Energy can be transferred via work (or heat) Energy Something an object/system possesses Heat and Work ways energy is transferred or converted Potential energy energy due to position in a field of attraction/repulsion ex: postion above ground due to force of gravity (water at top of dam) atoms in molecule due to intermolecular attrachon (bond nirgios) PEM Farav Zposition Kinetic Energy energy due to movement (KE = ½mv") Either form can do work (moverunt against a force) •Heat is the average kinetic energy of molecules Heat and Temperature Heat is the motion of matter and it flows from hotter to a colder objects. Types of motion translational, rotational, vibrational Temperature is a measure of the average kinetic energy of the atoms and/or molecules in a substance *Heat is a consequence of motion at the molecular level, whereas temperature is a measure of the average speed of that motion   Tempurature Scales \[T_{F}=\frac{9}{5}(T_{c})\cdot32\] Energy: Capacity To Do Work \[T_{c}=\frac{5}{9}(T_{e}-32)\] \[T_{K}=T_{K}-273~15\] Kinetic Energy Potential Energy: Due to Due to Motion Position or Composition \[T_{k}=T_{k}-273.15\] First Law of Thermodynamics Thermal Energy Chemical Energy Associated with Associated with Positions Tempecature of Electrons and Nuclei - Energy cannot be created or destroyed, but it is always changing forms via heat and work \[\angle E=q+w\] \[(a_{r})\] is the thermal energy due to motion of molecules Consider a rock falling Emg.z PE- KE Tracking Energy Changes Internal Energy sum of PE and KE in a system •State Function: only depends on State of system not on how it got there