5.2 Narrated Notes

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5.2 – The First Law of Thermodynamics : 

5.2 – The First Law of Thermodynamics

The First Law of Thermodynamics : 

The First Law of Thermodynamics states that energy can be neither created nor destroyed In other words, energy is conserved 5.2 The First Law of Thermodynamics

Internal Energy : 

The internal energy of a system is the sum of all the kinetic and potential energies of all its components 5.2 Internal Energy

Internal Energy : 

ΔE = Efinal – Einitial 5.2 Internal Energy

Thermodynamics : 

Thermodynamic quantities have three parts number unit sign The number and unit give the magnitude of change and the sign gives the direction of the change 5.2 Thermodynamics

Δ E : 

A positive value of ΔE indicates the system has gained energy from its surroundings A negative value of Δ E indicates the system has lost energy to its surroundings 5.2 Δ E

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5.2

Δ E, Heat and Work : 

A system may exchange energy with its surrounding as heat or as work The internal energy of a system changes in magnitude as heat is added to or removed from a system or as work is done on it or by it 5.2 Δ E, Heat and Work

Δ E, Heat and Work : 

When a system undergoes any chemical or physical changes, the magnitude and sign of the accompanying change in internal energy, Δ E, is given by the heat added to or liberated from the system, q, plus the work done on or by the system, w. 5.2 Δ E, Heat and Work

Δ E, Heat and Work : 

Δ E, Heat and Work Formula Δ E = q + w 5.2

Heat (q) : 

When is q positive? When heat is transferred to the system from the surroundings When is q negative? When heat is lost by the system to the surroundings 5.2 Heat (q)

Work (w) : 

When is w positive? When work is done on the system by its surroundings When is w negative? When work is done by the system on the surroundings 5.2 Work (w)

Practice Exercise : 

Calculate the change in the internal energy of the system for a process in which the system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings E = q + w = +140 J + (-85 J) = + 55 Joules 5.2 Practice Exercise

Endothermic Processes : 

When a process occurs in which the system absorbs heat, the process is called endothermic During an endothermic process, heat flows into the system from its surroundings 5.2 Endothermic Processes

Exothermic Processes : 

A process in which the system loses heat is called exothermic During an exothermic process, heat exits or flows out of the system and into the surroundings 5.2 Exothermic Processes

State Functions : 

State Function – a property of a system that is determined by specifying the system’s condition, or state The value of a state function depends only on the present state of the system, not on the path the system took to reach that state 5.2 State Functions

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5.2

Explain why ΔE is a state function while q and w are not : 

The specific amounts of heat and work produced depend on the way in which the change is carried out A system may lose energy by losing heat or work to the surroundings, but the total change in energy is the same 5.2 Explain why ΔE is a state function while q and w are not

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5.2