Chemistry Friday, March 30 th Monday, April 9 th, 2018
Do-Now: BrainPOP: Heat 1. Write down today s FLT 2. Distinguish between exothermic and endothermic processes. 3. What is the specific heat of water? 4. How much heat must be added to 1450 g of water to raise the temperature from 5.5 C to 29.0 C? Show all work and steps. 5. Number 1-10 underneath your do-now. 6. Take out your calculator, periodic table, planner and ToC
FLT I will be able to solve for enthalpy changes using heats of reaction by completing Ch. 10 CN Part B Standard HS-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known
Ch. 10: Measuring and Expressing Enthalpy Changes
Introduction
Intro A burning match releases heat to its surroundings in all directions. Is this exothermic or endothermic?
Intro Can we determine the amount of heat given off?
Calorimetry
Calorimetry Calorimetry = process of measuring heat into or out of a system during a rxn/change Based on the fact that the heat released = the heat absorbed
Calorimeters = used to measure the absorption or release of heat Ex/ Foam Cups Calorimetry
Calorimeters are used to measure the absorption or release of heat Ex/ Soda Cans Calorimetry
Enthalpy
Enthalpy Enthalpy (H) = equivalent to the total heat content of a system
Enthalpy Change in enthalpy (ΔH) = amount of heat absorbed or released in a rxn Can be calculated
Enthalpy Endo = + ΔH Exo = - ΔH
Enthalpy In a calorimetry experiment: ΔH = - q = heat is being released ΔH = + q = heat is being absorbed Note: enthalpy and heat are different measurements. However, under certain conditions they are equivalent.
Ex/1 50.0 ml of a water solution at 25.0 o C is heated in a calorimeter to 32.0 o C. Calculate the heat released during this reaction if the density of the water solution is 1.00 g/ml. Check: If given L instead of g, use density to convert to g
Ex/2 When 100.0 ml of a water solution is heated from 22.5 o C to 26.0 o C, heat is released. Calculate the change in enthalpy in this reaction. (Recall the specific heat of water is 4.18 J/ g o C, and the density is 1.00 g/ml)
Thermochemical Eqs
Thermochemical Eqs How can you express the enthalpy change for a reaction in a chemical equation?
Thermochemical Eqs In thermochemical equations, the enthalpy change can be written as a reactant or a product Endo: ΔH is a reactant (absorbed) Exo: ΔH is a product (released)
Thermochemical Eqs In thermochemical equations, the enthalpy change can be written as a reactant or a product Do not put negative signs when heat is IN the equation!
Ex/3 Write as a thermochemical equation: CaO + H 2 O à Ca(OH) 2 where ΔH = -65.2 kj
Ex/ 4 Write as a thermochemical equation: 2NaHCO 3 à Na 2 CO 3 + H 2 O + CO 2 where ΔH = +129 kj
Ex/ 5 When 2 mol of solid magnesium combined with 1 mole of oxygen gas, 2 moles of solid magnesium oxide (MgO) is formed and 1205 kj of heat is released. Write the thermochemical equation for this combustion reaction.
Ex/ 6 When 4 mol of iron combines with 3 moles of oxygen gas, 2 moles of iron (III) oxide (Fe 2 O 3 ) is formed and 1625 kj of heat is released. Write the thermochemical equation for this combustion reaction.
Using Heat of Rxn
Thermochemical Eqs We can use balanced thermochemical equations to calculate enthalpy change We can convert between MOLES and KJ using our coefficients
Ex/ 7 Given 2NaHCO 3 + 129 kj à Na 2 CO 3 + H 2 O + CO 2 calculate the amount of heat (in kj) required to decompose 2.24 mol of NaHCO 3.
Ex/ 8 C + 2S + 89.3 kjà CS 2 Calculate the amount of heat (in kj) absorbed when 5.66 g of carbon disulfide is formed. (Hint: convert grams à moles first)
CW Thermochem WS B USE TEXTBOOK! ToC and/or study Ch. 10
Chemistry Tuesday, April 10 th Wednesday, April 11 th, 2018
Do-Now: Ch. 10 Notes Part C 1. Write down today s FLT 2. What is the relationship between ΔH and q? 3. What is the change in enthalpy when 223 ml of water is heated by 12 C? Show all work and steps. 4. If my reaction is exothermic, ΔH should be, and if my reaction is endothermic, ΔH should be. Take out your calculator, planner and ToC
FLT I will be able to solve for enthalpy changes using Hess s Law and heats of formation by completing Ch. 10 CN Part C Standard HS-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known
Ch. 10: Calculating Heats of Rxn
Recall
Enthalpy Change in enthalpy (ΔH) = amount of heat involved in a rxn How can we calculate this in a lab? To calculate in a calorimetry experiment: ΔH = - q = - m x ΔT x C
Enthalpy Are there other ways to calculate ΔH? Can we calculate ΔH without performing an experiment?
Hess s Law
Hess s Law Sometimes we cannot directly measure heat Hess s Law allows us to determine the overall heat of reaction by summing the parts
Hess s Law Hess s Law = The overall ΔH of a rxn equals the sum of the ΔH of its steps What does this mean??
Hess s Law Reactions often occur in steps
Hess s Law Hess s Law allows you to determine the ΔH of a rxn by using the known heats of reaction of two or more thermochemical equations.
Hess s Law Example/ C (diamond) à C (graphite) Find ΔH. Is the rxn exo or endothermic? Related steps: C (graphite) + O 2 à CO 2 C (diamond) + O 2 à CO 2 ΔH = -393.5 kj ΔH = -395.4 kj
Step 1 C (diamond) à C (graphite) C (graphite) + O 2 à CO 2 ΔH = -393.5 kj C (diamond) + O 2 à CO 2 ΔH = -395.4 kj If needed, flip eqs (ΔH changes sign) CO 2 à C (graphite) + O 2 C (diamond) + O 2 à CO 2 ΔH = +393.5 kj ΔH = -395.4 kj
Step 2 C (diamond) à C (graphite) CO 2 à C (graphite) + O 2 ΔH = +393.5 kj C (diamond) + O 2 à CO 2 ΔH = -395.4 kj If needed, multiply eqs by any factor (ΔH as well) CO 2 à C (graphite) + O 2 ΔH = +393.5 kj C (diamond) + O 2 à CO 2 ΔH = -395.4 kj
Combine! Step 3 Same side = add together Opposite side = subtract C (diamond) à C (graphite) CO 2 à C (graphite) + O 2 ΔH = +393.5 kj C (diamond) + O 2 à CO 2 ΔH = -395.4 kj
Ex 2/ Calculate the value of ΔH for C + O 2 à CO 2 C + ½ O 2 à CO ΔH = -110.5 kj CO + ½ O 2 à CO 2 ΔH = -283.0 kj
Ex 3/ Calculate ΔH for C 2 H 5 OH à CH 3 OCH 3 C 2 H 5 OH + 3O 2 à 2CO 2 + 3H 2 O ΔH = -1234.7 kj CH 3 OCH 3 + 3O 2 à 2CO 2 + 3H 2 O ΔH = -1328.3 kj
Ex 4/ Calculate ΔH for 2F 2 + 2H 2 O à 4HF + O 2 H 2 + F 2 à 2HF ΔH = -542.2 kj 2H 2 + O 2 à 2H 2 O ΔH = -571.6 kj
Is There Yet Another Way??
ΔH ΔH = Change in Enthalpy I can calculate ΔH by Using ΔH = -q Using Hess s Law ΔH = sum of parts Using Standard Heats of Formation!
Standard Heats of Formation
Standard Heats of Formation Elements and compounds have specific ΔH values when formed under standard conditions See handout
Standard Heats of Formation Knowing these values, Hess s Law allows us to indirectly calculate the heat of a reaction under standard conditions
Standard Heats of Formation Note: Pure substances at standard state have a ΔH 0 f = 0. Ex/ Br 2, O 2, Fe, etc.
Standard Heats of Formation Standard Heats of Formation (ΔH f0 ) = Predetermined values at standard conditions Use the table! Formula for using standard heats of formation:
Examples
Ex/1 Calculate the standard heat of reaction (ΔH 0 ) for the reaction 2NO + O 2 à 2NO 2
Ex/2: You Try Calculate the standard heat of reaction (ΔH 0 ) for the reaction CaCO 3 à CaO + CO 2
Ex/3: You Try Calculate the standard heat of reaction (ΔH 0 ) for the reaction 7O 2 + 4NH 3 à 4NO 2 + 6H 2 O (g)
CW Ch. 10 Review WS Use textbook and notes ToC and/or study Ch. 10
Chemistry Thursday, April 12 th Friday, April 13 th, 2018
Do-Now: Lab Day Do-Now 1. Write down today s FLT 2. What is the equation for heat? 3. What are the two different units we can use for heat? 4. What is the specific heat of water? 5. If you have 50.0 ml of water, how many grams do you have? 6. How do we safely use bunsen burners? Take out your calculator, planner and ToC
FLT I will be able to calculate the amount of heat transferred in a reaction by completing Calorimetry Lab Standard HS-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known
Lab Background
Today s Focus Question How do we determine the amount of calories in the food we eat?
Specific Heat The amount of energy that is transferred to a material is dependent on the nature of the material receiving the energy. Specific Heat The amount of energy required to raise the temperature of one gram of substance one degree Celsius Represent by the symbol C p or C
Calorimetry Calorimetry is the science of measuring the heat of a chemical reaction. q = heat (J or cal) C = specific heat (J/g o C) ΔT = Final Temp o C Initial Temp o C If we can measure the mass and temperature, then we can determine the heat gained/lost in a reaction
Calories in Flamin Cheetos Q can be measured in calories or Joules Food scientists measure the calories contained within food by measuring the amount of energy it takes to raise 1 g of water 1 o C. 1000 calories = 1 Calorie or Kilocalorie
Lab The more calories a food contains, the more heat is given off when burned Foods high in calories will release large amounts of energy