Energy

Chemical reactions

either absorb heat OR give off heat

System

The part of the universe chosen for study (our reaction).

Surroundings

The rest of the universe outside of the system (anything that isn’t our system).

Joule (J)

Energy Unit

Molar Mass Unit

kg/mol

Thermal Chemistry focuses on

the systems transfer of energy between a system and it’s surroundings

Kinetic Energy

the energy of a moving object

Thermal Energy

Kinetic energy associated with molecular motion

Molecular motion is

how much molecules are vibrating. High temp with a lot of vibration = greater thermal energy.

Chemical Energy

the energy with chemical bonds (inside molecules there is chemical energy).

Heat (q)

the energy transferred between a system & its surroundings as a result of temperature difference.

Unit for Heat

q, cal, or Joule

1 cal = ? J

1 cal = 4.184 Joules

The amount of heat required to cause a substance to change temperature FORMULA

q = mC_sΔT

q =

heat change in J

m =

mass in g

C_s =

specific heat of the substance in J/g c

ΔT =

T_final - T_initial temperature in Celsius

Specific heat (of a substance)

is a measure of how easily a substance changes in temperature. Specific heat is different for every substance.

If ΔT is positive,

then q is positive, temperature increases and heat is absorbed.

If ΔT is negative,

temperature decreases and heat is given off.

Enthalpy of a reaction (ΔH) kJ/mol

FORMULA: ΔH = q rxn/(mol reactant (LR). The amount of heat absorbed or given off per mole of a reactant.

q rxn =

ΔH x mol reactant

Exothermic (-q)

ΔH and q rxn are negative, the reaction GIVES OFF heat to surroundings

Endothermic (+q)

ΔH and q rxn are positive, the reaction ABSORBS heat from the surroundings.

Law of Conservation of Energy

The total energy between a system and its surroundings must remain constant (1st law of thermodynamics). This means that heat lost by a system is gained by its surroundings & vice versa. EX: q_system + q_surroundings = 0. q_system = -q_surroundings. Whatever is happening to the systems, the exact oppositive is happening to the surroundings.

Bomb Calorimetry

Heat of reactions for combustions are usually found using bomb calorimetry. When the combustions reaction occurs, chemical energy is converted to thermal energy & this energy is transferred to the bomb.(q rxn = -q calorim and vice versa).

q calorim

the amount of heat absorbed or released by the calorimeter itself.

Because the mass and specific heat of the bomb is constant, we say (FORMULA)

q calorim = (heat capacity) x (ΔT)

Calorimeter

is a scientific instrument that measures heat

Calorimetry

measuring heat flow in a chemical reaction or physical change.

ΔE

It represents the total energy exchanged via heat and work. The change in internal energy of a system during a process or reaction, calculated as the difference between final and initial states

ΔE =

ΔE = q (heat) + w (work).

If a system does work

w is negative

If work is being done on a system

w is positive

Pressure-Volume work formula

(FORMULA: w = -PΔV) It describes how much energy a system "spends" to push against the atmosphere or how much energy it "gains" when the atmosphere pushes on it. P is pressure, delta v is the change in volume (final - initial).