5.3 Narrated Notes

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5.3 - Enthalpy : 

5.3 - Enthalpy

Enthalpy : 

Most commonly, the only kind of work produced by a chemical or physical changes open to the atmosphere is the mechanical work associated with a change in the volume of the system 5.3 Enthalpy

Enthalpy : 

Explain how we can measure the amount of work the reaction between zinc and hydrochloric acid does on the atmosphere. I have provided the picture, you describe what is happening 5.3 Enthalpy

Slide 4: 

5.3

Explanation : 

The hydrogen gas produced by the reaction increases the pressure of the system The pressure of the hydrogen gas pushes the piston up against the pressure of the atmosphere The system is doing work on the surroundings 5.3 Explanation

Pressure-Volume Work : 

The work involved in the expansion or compression of gasses is called pressure-volume work or P-V work 5.3 Pressure-Volume Work

Pressure-Volume Work : 

Formula w = - PΔV P is the pressure V is the change in volume of the system 5.3 Pressure-Volume Work

Pressure-Volume Work : 

When the volume of the system increases, what is the resulting sign of the work? What does this mean? If the volume increase, the value for work is negative This makes sense because the piston moves up if the volume increases and the system is doing work on the surroundings 5.3 Pressure-Volume Work

Pressure-Volume Work : 

When the volume of the system decreases, what is the resulting sign of the work? What does that mean? If the volume decrease, the value for work is positive This makes sense because the piston moves down if the volume decreases and the surroundings have done work on the system 5.3 Pressure-Volume Work

Enthalpy : 

A thermodynamic function called enthalpy accounts for the heat flow in processes occurring at constant pressure when no forms of work are performed other than P-V work 5.3 Enthalpy

Enthalpy : 

Formula H = E + PV H is the enthalpy E is the internal energy of the system P is the pressure of the system V is the volume of the system 5.3 Enthalpy

Enthalpy : 

Enthalpy is a state function because internal energy, pressure and volume are all state functions 5.3 Enthalpy

Change in Enthalpy : 

ΔH = ΔE + P ΔV Show mathematically that ΔH = qp qp means that change occurred at a constant pressure 5.3 Change in Enthalpy

Enthalpy : 

This means that the change in enthalpy (ΔH) equals the heat gained or lost at a constant pressure 5.3 Enthalpy

Enthalpy : 

When ΔH is positive, the system has gained heat from the surroundings If ΔH is positive, the process is endothermic 5.3 Enthalpy

Enthalpy : 

When ΔH is negative, the system has released heat to the surroundings If ΔH is negative, the process is exothermic 5.3 Enthalpy

Practice Exercise : 

Suppose we confine 1 gram of butane and sufficient oxygen to completely combust it in a cylinder like that in the figure below. The cylinder is perfectly insulated so that no heat can escape to the surroundings. A spark initiates the combustion of the butane, which forms carbon dioxide and water vapor. If we use this apparatus to measure the enthalpy change in the reaction, would the piston rise, fall, or stay the same? 5.3 Practice Exercise

Answer : 

The piston must move to maintain the constant pressure in the cylinder 2 C4H10 (g) + 13 O2 (g) 8 CO2 (g) + 10 H2O (g) Since there are more moles of gas on the product side, the piston moves up This reaction is very exothermic so the piston must also move up to accommodate the expansion of the gas as the temperature of the gas rises 5.3 Answer