Thermochemistry

The heat of the matter

Energy

• The capacity to do work or produce heat

Law of Conservation of Energy

• Energy can be converted from one form to another, but cannot be created nor destroyed.

Potential Energy

• Potential energy is due to position or composition

Examples• Water behind a dam

that may push a turbine

• Gasoline

Kinetic Energy

• Energy due to the mass and speed of an object

• KE=1/2 mv2

Heat

• The transfer of energy from kinetic to heat.

• Energy cannot be created nor destroyed so where does it go when the ball hits the ground?

HEAT

aka frictional heating

Heat

• Heat involves the transfer of energy between two objects observed through temperature changes

Work

• Work is the force acting over a distance

• However, the way the energy transfer is divided between heat and work depends on certain conditions or the PATHWAY.

• Regardless of the pathway, the total energy remains constant. Why?

State Function

• A state function refers to the property of the system that depends only on its present state. It doesn’t matter how you got there, only that you are there.

• Energy is a state function.

Chemical Energy

• In discussing reactions we need to identify our present state.

• System• Surroundings• Universe

Chemical Energy

• Exothermic: Energy flows out of the system into the surroundings. Expressed as (-)

• Endothermic: Energy flows from the surroundings into the system. Expressed as (+)

Exothermic and Endothermic

Thermodynamics

The First Law of Thermodynamics:

• The energy of the universe is constant

Internal Energy

• The internal energy of a system is the sum of the kinetic and potential energies of all the “particles” in the system. The internal energy of a system can be changed by a flow of heat or work or both.

ΔE=q+w

• ΔE is the change in energy

• q is the heat and w is the work

Example

• Calculate ΔE for a system undergoing an endothermic process in which 15.6 kJ of heat flows and where 1.4 kJ of work is done on the system.

Result

ΔE=q+w

• q= +15.6kJ (endothermic) w=+1.4kJ

ΔE=15.6kJ + 1.4 kJ = 17.0 kJ

Work and Pressure

• Work may be done by a gas (inflation) and work may be done on a gas (compression).

• Pressure is Force per unit area

P=F/A

• Work is force applied over a distance.Work=force * distance=F * Δh (height)

Work and Pressure

• Since P=F/A or F=P*A then,

Work = F* Δh* = P*A* Δh

• This results in a change in volume,

ΔV= final volume – initial volume =A* Δh

• Substitute ΔV =A* Δh and

Work = P*A* Δh= PΔV

What about the sign of work? (+ or -)

Work and Pressure

• When the gas is expanding, work is done on the surroundings by the system.

w= -PΔV

• When the gas is compressed, work is done on the gas by the surrounding.

w= PΔV

Example

• Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external pressure of 15 atm.

Result

• w= -PΔV as the gas is expanding.

• P=15 atm, ΔV = 64-46 = 18 L

• w= - 15atm *18L = -270 atm

To do tonight

Pressure from Work