Thermochemistry

Sayed Hussaini and Jack sun

•Energy

• Thermochemistry: Part of thermodynamics that involves the relationship between chemical reactions and energy changes involving heat.

• Energy: The capacity to do work or transfer heat.

• Work: Energy used to move an object with mass against a force. W = F x d

• Heat: The energy transferred from a hotter object to a colder one.

• The unit of energy is a Joule. 1J=1kg-m2/s2

•Types Of Energy

• Kinetic Energy: The energy an object has just by being in motion. KE=1/2 mv2

• Potential Energy: The energy an object has because of its position and the forces acting on it. Such as the force of Gravity or the electrostatic force that arises between two charged particles.

• PEg=mgh PEe= kQ1Q2/d

• Internal Energy

• The sum of all the kinetic and potential energies of a system. • Since we know Energy is always conserved (first law of thermodynamic) we can say that ΔE= E final – E initial • ΔE= q + w

•Enthalpy

•The work done by the expansion or compression of a gases at constant pressure is given by: W=-PΔV•Enthalpy: The heat flow in processes occurring at constant pressure.•ΔH=ΔE+PΔV ΔH=(qp+W)-W

•Enthalpies of reaction

•Enthalpy of a reaction is the heat of reaction can be exothermic or endothermic. • For example: •2 H2 + O2 2 H2O ΔH=-483.6KJ•The magnitude of H is directly proportional to the amount of reactant.• If you reverse the above equation, ΔH=483.6KJ.

•Calorimetry

• ΔH can be determined by measuring the temperature change the heat flow produces. • Constant Pressure: ΔH=-Cs x m x ΔT=qp• Bomb Calorimetry (Constant-volume):ΔH= -Ccal x ΔT

•Hess’s Law

• If a reaction consists of multiple steps, the sum of the ΔH for each step will equal the overall ΔH.

Example:

•Enthalpy of Formation

•The enthalpy changes associated with the formation of a substance from its components(ΔHf).• Standard enthalpy of formation: Change in enthalpy for a reaction that forms one mole of the compound and is in standard state ( 1atm;298K).

• The standard enthalpy of formation of the most stable form of an element is zero.

•Foods and fuels• Fuel value: Energy release when one gram of a material is combusted; represented by positive values. • The average fuel value of carbohydrates and proteins is 17kJ/g.• The average fuel value of fats is 38kJ/g.

•Quiz!!

•Calculate the change in internal energy of the system for a process in which the system absorbs 140J of heat and does 85J of work.• ΔE=q + w• =140+(-85)=55J

•2H2o2 2H2O + O2 ΔH=-196KJ=qp• Calculate the value of q when 5.00g of H2O2 decomposes at constant pressure.

5gH2O2 -196KJ = -14.4KJ 2molH2O2

34gH2O2

1mol H2O2

•1. What is the specific heat of aluminum if the temperature of a 28.4 g sample of aluminum is increased by 8.10C when 207 J of heat is added?

• ΔT=8.10C q = Cs x m x ΔT• Q=207J Cs=q/mΔT • Cs=0.90J/g-oC

• Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values:

• 2CO2(g) + H2O(g) → C 2H2(g) + 5/2O2(g)

• C2H2(g) + 2H2(g) → C2H6(g) ΔH =-94.5 kJ

• H2O(g) → H2(g) + 1/2O2 (g) ΔH =71.2 kJ • C2H6(g) + 7/2O2(g) → 2CO2(g) + 3H2O(g) ΔH =-283 kJ

• 2 CO2(g) + H2O(g) → C 2H2(g) + 2½ O2(g)

• C2H2(g) + 2 H2(g) → C2H6(g) ΔH = - 94.5 kJ flip• H2O(g) → H2(g) + ½ O2 (g) ΔH = + 71.2 kJ

flip x 2• C2H6(g) + 3½ O2(g) → 2 CO2(g) + 3 H2O(g) ΔH = - 283 kJ flip

• C2H6(g) → C2H2(g) + 2 H2(g) ΔH = + 94.5 kJ • 2 H2(g) + O2 (g) → 2 H2O(g) ΔH = - 142.4 kJ• 2 CO2(g) + 3 H2O(g) → C2H6(g) + 3½ O2(g) ΔH = + 283 kJ• 2 CO2(g) + H2O(g) → C 2H2(g) + 2½ O2(g) 235.1 kJ