Download
1 / 23
230 likes | 305 Vues
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
E N D
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 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? Work
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 (+)
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
