Enthalpy (5.3)

Objectives – Today I will be able to: Define state function Calculate the enthalpy of a system Informal assessment – monitoring student interactions and questions as they complete the practice problems Formal assessment – analyzing student responses to the practice problems

Lesson Sequence Evaluate: Warm Up Explain: Enthalpy Elaborate: Enthalpy Calculations Evaluate: Closure

Warm Up Calculate the change in internal energy for a process in which a system absorbs 30 J of heat from the surroundings and does 44 J of work on the surroundings.

Answer E = q + w q = 30 J because heat was absorbed w = -44 J because work was done by the system E = 30 J + (-44 J) E = - 14 J

Objectives Today I will be able to: – Define state function – Calculate the enthalpy of a system

Homework Organic Functional Groups Quiz – Thursday, October 2 Bring textbook to exchange Finish practice problems

Agenda Warm Up Enthalpy Notes Practice Problems Exit Ticket

Enthalpy (5.3)

What two components make up the total energy of a system?

Work Mechanical work is the focus for chemical and physical changes Associated with a change in volume Constant pressure is maintained Zn(s) + 2H + (aq)  Zn 2+ (aq) + H 2 (g)

Work cont. Work involved in the expansion or compression of gases is called pressure – volume work w = - PΔV Units: L-atm Conversion factor 1 L-atm = J

Practice Problem A fuel is burned in a cylinder equipped with a piston. The initial volume of the cylinder is L, and the final volume is L. If the piston expands against a constant pressure of 1.35 atm, how much work (in J) is done?

Answer w = -PΔV W = - (1.35 atm)(0.730 L) = L-atm L-atm (101.3J / 1 L-atm) = J W = J

Practice Problem 2

Answer

Enthalpy (H) Internal energy plus the product of the pressure and volume of a system H = E + PV The equation is used to account for the absorption/release of heat and work during a chemical or physical change Relates mainly to heat flow

Enthalpy is a state function State function – A property of a system that is determined by specifying the systems condition or state – Value of a state function depends only on the present state of the system, not on the path the system took to reach the state

Potential energy of hiker 1 and hiker 2 is the same eventhough they took different paths. 6.7 Example

Which of the following variables are examples of state functions? ΔE q w H PV

Enthalpy Change (ΔH) Change in heat exchange between a system and its surroundings at constant external pressure ΔH = ΔE + PΔV

Keep in mind… ΔH = ΔE + PΔV ΔH = (q p + w) – w ΔH = q p For most reactions the difference between ΔH and Δ E is small because there is not a lot of work If PΔV is small it can be ignored from calculations

Closure Complete practice problems: – 5.31, 5.32, 5.37,