THERMOCHEMISTRY

EnergyEnthalpySpecific HeatCalorimetryPhase Changes

INTRODUCTIONChemical change generally involves energy.

Thermodynamics: the study of energy and its transformations. (energy, work, heat)

Thermochemistry: the study of the relationship between chemical reactions and energy changes involving heat.

UNITS OF ENERGYSI unit for energy: Joule (J). Kilojoules (kJ) are commonly used.

1 J = (1kgm2)/s2

Another unit: calorie (cal).

1 cal = 4.184 J (exactly)

SYSTEM AND SURROUNDINGSSystem: portion we single out for study. Example: just the reacting chemicals in a beaker

Surroundings: everything else. Example: beaker the chemicals are in, the room, the universe

FIRST LAW OF THERMODYNAMICS

“Energy is conserved.” Any energy that is lost by the system must be gained by the surroundings, and vice versa.

WRITING THERMODYNAMIC QUANTITIES

Thermodynamic quantities have three parts:

Numerical value, unit, and sign (+ or -) + means system gained energy from surroundings

- means system lost energy to surroundings

Don’t forget to include the sign when writing thermodynamic quantities!

RELATING HEAT TO A SYSTEM

Chemists look at heat and work in relation to the system. q = heat +q = heat added to system (endothermic) -q = heat removed from system (exothermic)

EXOTHERMIC REACTIONSIn an exothermic reaction, system

gives off heat, surroundings heat up. Ex: combustion reactions

ENDOTHERMIC REACTIONSIn an endothermic reaction, system absorbs heat, surroundings get colder. Ex: portable cold packs

CALORIMETRY Heat flow from a chemical rxn can be determined by measuring temp changes.

Calorimetry: measurement of heat flow

Calorimeter: apparatus used to measure heat flow

HEAT CAPACITY

Heat Capacity (J/K or J/°C): the amount of heat required to raise the temp of an object by 1K (1°C).

Change in K is same as change in °C.

The greater the heat capacity, the greater the heat required to produce a change in temp.

SPECIFIC HEAT CAPACITY Specific Heat Capacity: amount of heat

needed to raise the temp of 1 gram of a substance by 1K.

Units: J/gK or J/g°C Cp means specific heat capacity at constant

pressure

q = Cp m∆T

q = heat (J or cal); m = mass of substance (g);

∆T = final temp- initial temp (K or °C)

Know the specific heat capacity of water:

4.184 J/g°C or 1 cal/g°C

CALCULATIONS WITH SPECIFIC HEAT

Calculate the quantity of heat absorbed by 50.0g of rocks if their temp increases by 12°C. Assume the heat capacity is 0.82 J/g K.

What temp change would these rocks undergo if they emitted 450. kJ of heat?

CALCULATIONS WITH SPECIFIC HEAT

How much heat is needed to warm 250.g of water from 22°C to 98C?

(Remember: specific heat of water is 4.184 J/g K)

CONSTANT-PRESSURE (COFFEE CUP) CALORIMETRY

We can easily measure heat change in a coffee cup calorimeter since the cup is not sealed and we have constant atmospheric pressure.

ASSUMPTIONS WHEN USING A CONSTANT-PRESSURE CALORIMETER

The calorimeter perfectly prevents the gain or loss of heat from the solution to the surroundings so heat gained by the solution comes only from the chemical rxn under study.

The calorimeter itself does not absorb heat. Reasonable because polystyrene cups have very low thermal conductivity and heat capacity.

DETERMINING HEAT IN A CONSTANT-PRESSURE CALORIMETER

The heat gained by the solution is the same magnitude but opposite sign as the heat given off in the rxn: qsol’n= -qrxn

We measure the temp change of the sol’n to measure the q of the reaction.

qsol’n= Cp(sol’n )x g(sol’n) x ∆T(sol’n)= -qrxn

A CALORIMETER CALCULATION! A coffee cup calorimeter contains 150.0 g of water at 25.1⁰C. A 121.0 g block of copper metal is heated to 100.4 ⁰C by putting it into a beaker of boiling water. After the hot copper is added to the calorimeter and the contents are stirred, the temperature of the water in the cup reaches 30.1⁰C. Given the specific heat of water as 4.184 J/g ⁰C, calculate the specific heat of copper.

Q = CP(GAS)M∆T

q = Cp(sol)m∆T

q = Cp(liquid)m∆T

q = m∆Hvap

q = m∆Hfus