Chemistry Chapter 5

Thermochemistry

Study of energy changes occurring in physical or chemical change

Energy

Ability to do work

Work

Amount of energy transferred by a force over a distance, force exerted against the system

Kinetic energy

Energy of moving things 

Potential energy

Stored energy of an object because of its position/bonds

Amount of energy absorbed or released

potential energy of bonds in reactants - potential energy of bonds in products  

Heat

The transfer of thermal energy from warm to cool

Temperature

Average kinetic energy of molecules

warmer substance = faster molecules

Law of conservation energy

 Energy cannot be created or destroyed, just rearranged 

System

The substance undergoing chemical change 

Surroundings

The system’s environment

Open

Matter and energy can move in or out 

Closed 

Energy can move in or out but not matter

Isolated

Neither matter nor energy

Heat of reaction: Bond breaking

Endothermic

Heat of reaction: Bond formation

exothermic 

Exothermic reactions

Heat and energy is released, surrounding temp increases

Endothermic

Heat and energy are absorbed, decreases temp of surroundings

Draw a potential energy diagram of exo and endothermic reactions 

Heat Capacity

Quantity of thermal energy required to raise 1g of a substance by 1℃

SI Units J/ g ℃

Calorimetry

Process of measuring energy changes during a physical/chemical change

Goal is to calculate energy entering or leaving system

Calorimeter

Device used to measure energy changes during physical/chemical change

Assumptions made in calorimetry calculations

• Thermal energy absorbed by calorimeter/outside environment is negligible

• Dilute aqueus soltuoins have the same density and heacapacity of water

  • Density: 1.00g/ml

  • Heat capacity: 4.18J/g℃

Calorimetry calculation for thermal energy absorbed or released in a chemical system

Negative value of q

System transfers thermal energy to surroundings = exothermic

positive value of q

system absorbs thermal energy from surroundings = endothermic

q system relationship to q surroundings

q system + q surroundings = 0

q system = - q surroundings

Enthalpy (H)

Heat content of a system at a constant pressure

Reaction with a gain or loss of energy 

ENTHALPY CHANGE (ΔH), relative difference between Hreactants and Hproducts 

Relationship between thermal energy and heat content 

at constant pressure, enthralpy change equals the flow of thermal energy in or out of system 

Molar Enthalpy Change (ΔHr)

Energy of change that occurs when 1 mol of substance undergoes physical, chemical, or nuclear change

J/mol

for example: ΔHvaporization

Calculation molar enthalpy change

ΔH = nΔHr

Bond Dissociation Energy

Energy required to break a chemical bond (average bond energies)

number of bonds in order of length 

Single > double > triple

number of bonds in order of strength

Triple > double > single

Using bond energies to predict enthalpy change

(# of bonds broken in reactants) - (#of bonds formed in products)

Hess’s Law 

For any reaction that can be written in steps overall ΔH is the same as ΔH sum for each individual steps 

Standard Enthalpy of Formation ΔHf°

Change in enthalpy when 1 mol of substance is formed from its elements at SATP

Calculating enthalpy change

(sum of standard enthalpies of products) - (sum of standard enthalpies of formation of reactants)