Chemistry S2 - Thermochemistry

Thermochemistry

• study of energy changes of matter

• drive phase changes & heating/cooling process

• chemicals hold potential energy in bonds, release/absorb energy during chemical reactions

Law of Conservation of Energy

states energy can neither be created nor destroyed but can be converted from 1 form to another (1st Law of Thermodynamics)

Heat (q)

energy in process of flowing from warmer to cooler object

What happens when a warmer object loses heat?

temeprature decreases

What happens when a cooler object gains heat?

temperature increases

Conduction

2 substances in direct contact

Convection

transfer using density-driven flows (fluid movement)

Radiation

given off in form of infrared radiation ex. the sun

What is temperature the measure of?

the average kinetic energy of particles in a sample of matter measured in degrees C or K

Calorie (Cal)

amount of heat required to raise temperature of 1 gram of pure water by 1 degree C (always positive)

Kilocalorie

food calorie with a capital C

What is the SI unit of heat & energy?

joule (J), 1 cal = 4.184 J

Specific Heat (c)

amount of energy required to raise temperature of 1 gram of pure water by 1 degree C, water = 4.18 J/g. degrees C or 1.00 call/g. degrees C

What do you use a calorimeter for?

to measure the temperature change

Formula for Specific Heat

q = m*cp*ΔT (T_f -T_i)

Phase Changes

• occur when substance changes from 1 state to another

• energy must be added to/taken away from substance

• physical changes

Reading a Heating Curve

• temperature rises quick when solid, liquid/gas heated

• higher specific heat, shallower line on heating curve & stronger IMFs

• higher enthalpy of change (vap/fuse), longer phase change time

• temp doesn’t change (all added energy used to break apart IMFs holding phase tg)

• all energy change is potential, not kinetic (temp)

• stronger IMFs, longer phase change takes

Can heat also be absorbed/released during a phase change?

Yes

Heat of Vaporization (ΔH_vap)

heat required to vaporize 1 g of liquid (Condensation/Vaporization; g-l, l-g)

Heat of Fusion (ΔH_fus)

heat required to melt 1 g of solid (Melting/Freezing; s-l, l-s)

Heat of Formation (ΔH°_f)

energy needed to form 1 mole of compound from its elements, ΔH°_f of an element = 0

Heat of Reaction (ΔH_rxn)

difference between enthalpy of substances that exist at end of reaction & enthalpy of substances present at start

What is the formula for Heat of Reaction?

ΔH_rxn = ΣnH°_{f prod} - ΣnH°_{f react}

Heat of Combustion (ΔH_c)

amount of energy released as heat by complete combustion of 1 mole of substance

What is the formula for Heat of Combustion?

ΔH_c = ΣnH°_{f prod} - ΣnH°_{f react}

Hess’s Law

• enthalpy not dependent on reaction pathway

• if you can find combination of chemical equations add up to give desired overall equation, can also sum up the ΔH’s for individual reactions to get overall ΔH_rxn

Steps to Remember for Hess’s Law

• 1st decide how to rearrange equations so reactants & products are on appropriate sides of arrows

• if equations must be rearranged, reverse sign of ΔH

• if equations had to be multiplied to get correct coefficient, x ΔH by this coefficient since ΔH’s are in kJ/mole (division applies ~)