Chapter 5.1-5.3Thermochemistry

Thermochemistry

Thermo means heat or energy

Energy: Capacity of doing work

Work: Force x displacement

Types of Energy

Kinetic Energy (KE) Energy due to motion Ek = ½ mv2

Where m = mass & v = velocity

Types of Energy

Potential energy (PE) Energy due to position

Gravity higher PE

lower PE

Units: Calorie (cal), Joule (J) 1 Cal = 4.184 J

System Where we focus our attention

Surroundings Everything else

First Law of Thermodynamics

Energy is conserved

Piston pushed up & work (W)

done by the gas (say 6 J)

10 J added(q)

First Law of Thermodynamics

What happens to the difference (10 – 6)? Retained inside. (causes Internal energy

change, ΔE): 10 = 4 + 6

q (Heat applied) = ? q = ΔE + W (first law Thermodynamics)

or ΔE = q-W (here -W, Work done by the gas) ΔE = q +W (here +W, Work done on the gas) Note: Two different sign conventions exist

Enthalpy (H)

q = ΔE + P ΔV

ΔE = q-W (work done by a gas)

ΔH = ΔE + P ΔV (constant pressure)

H = E + PV

q = ΔE + W

Enthalpy or heat Content (H)

Begin End

2H2 + O2 2H2O

Say 484 kJ 1 kJ

Note: 484 kJ is the Heat content of the reactants (HR) and 1 kJ is the heat content of the products (HP).

Enthalpy or heat Content (H)

ΔH = Hp –HR

= 1 – 484

= -483 kJ (exothermic)

Here, initial heat content> Final heat cont. What happens to the difference?

It is given off.

Enthalpy or heat Content (H)

2H2O 2H2 + O2

1 kJ (HR) 484 kJ (HP)

Is heat absorbed or given off?

Heat is absorbed

ΔH = Hp –HR

= 484 – 1

= 483 kJ (endothermic) Here, initial heat cont < Final heat cont