Ap Chemistry Due Next Class: Reading 5.6-5.8 & proof of reading Conduc./Grav. Lab Report Up Coming Due Dates: MC CH. 5 due Sunday 10/8 @midnight MC AF Ch. 5 Due Tuesday 10/10 @ midnight

Woosh Bottle Demo- Warm up Reaction: Isopropanol combusts What should the ∆H sign be for this reaction?

Energy of Chemical Bonds Energy must be added to break bonds; Energy released when bonds are formed!

Endothermic and Exothermic A process is endothermic when H is positive. A process is exothermic when H is negative.

H = Hproducts − Hreactants Enthalpy of Reaction The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants: H = Hproducts − Hreactants

Enthalpy of Reaction This quantity, H, is called the enthalpy of reaction, or the heat of reaction. Periodic Video: Slow Motion H2 + O2 https://www.youtube.com/watch?v=qOTgeeTB_kA

Vocab Review Extensive property - property that changes when the size of the sample changes. Examples are mass, volume, length, and total charge. Intensive property – property that doesn't change when you take away some of the sample. Examples are temperature, color, hardness, melting point, boiling point, pressure, molecular weight, and density.

Units for Enthalpy Units in Thermo- chemical Calculations

2 H2(g) + O2(g) ⟶ 2 H2O(g) ΔH = – 483.6 kJ If the reaction to form water were written H2(g) + ½ O2(g)  H2O(g), would you expect the same value of ΔH as in above)? Why or why not? Yes, because the reactants and products are the same. No, because only half as much matter is involved. Yes, because mass does not affect enthalpy change. No, because enthalpy is a state function. Answer: b

Enthalpy: Summary Enthalpy is an extensive property – changes with the size of the sample. H for a reaction in the forward direction is equal in size, but opposite in sign, to H for the reverse reaction. Enthalpy is a state function - H for a reaction depends only on the state of the products and the state of the reactants – not on the path taken.

In what ways is the balance in your checkbook a state function? It is determined by many factors and involves data such as the number of checks, date, and payee. It is a numerical value calculated from known amounts of checks. It is calculated from multiple transactions. It only depends on the net total of all transactions, and not on the ways money is transferred into or out of the account. Answer: d

Calorimetry Measure H through calorimetry, the measurement of heat flow. The instrument used to measure heat flow is called a calorimeter.

Heat Capacity and Specific Heat Heat Capacity: Amount of energy required to raise the temperature of one mole of a substance by 1 K (1 C). Specific Heat: Amount of energy required to raise the temperature of one gram of a substance by 1 K (or 1 C).

More familiar version: q = m C ∆ T Specific Heat More familiar version: q = m C ∆ T

Constant Pressure Calorimetry By carrying out a reaction in aqueous solution in a simple calorimeter, the heat change for the system can be found by measuring the heat change for the water in the calorimeter. The specific heat for water is well known (4.184 J/g∙K). We can calculate H for the reaction with this equation: q = m  Cs  T

Constant Pressure Calorimetry Reactions can be carried out in a sealed “bomb” such as this one. The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction. qrxn = – Ccal × ∆T

Constant Pressure Calorimetry Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy, E, not H. For most reactions, the difference is very small.

Practice! Ozone, O3(g), is a form of elemental oxygen that plays an important role in the adsorption of ultraviolet radiation in the stratosphere. It decomposes to O2(g) at room temperature and pressure according to the following reaction: 2 O3(g)  3O2(g) ∆H = -284.6kJ What is the enthalpy change for this reaction per mole of O3(g)? What has the higher enthalpy under these conditions, 2O3(g) or 3O2(g)