CHE 2B CH 9 Thermochemistry

What is Thermochemistry?

What is Thermochemistry?

study of energy, heat, & work associated with chemical reactions and physical changes

What is internal energy?

microscopic energy contained in a substance—> energy of a substance

How are thermal energy/kinetic energy related?

thermal energy= results in temperature—> associated with random molecular forces= kinetic energy

Higher temp= higher molecular speed= higher kinetic energy. Increase in temp= increase in kinetic energy.

what is chemical energy?

associated with chem. bonds and intermolecular forces (potential energy)

breaking and formation of chemical bonds

What happens when chemical bonds break/bond?

potential energy increases, thermal energy decreases

potential energy decreases, thermal energy increases.

Greatest potential energy= water vapor, because bonds= basically nonexistent.

What is energy?

capacity to do work/produce heat

Law of Conservation of Energy

energy can never be created or destroyed, only converted from one form to another

What is potential energy?

energy based on the position or composition of a substance—> based on attractive/repulsive forces

What is kinetic energy?

energy based on the motion of the object—> depends on mass/velocity

What is heat?

transfer of energy between 2 objects due to temperature differences--> internal energy transferred between a system/surroundings

What is temperature?

reflects random motions of particles in particular substances

What is work?

force acting over a distance—> ex. required to move smth from a lower position to a higher one

What are the two ways to transfer energy?

work & heat

What is a pathway?

way that energy transfer is divided between work & heat (according to the spef. conditions the rxn is occurring in)

What conditions are independent/dependent of a pathway?

Total energy change= independent

Work/Heat= dependent

State function/property

property of a system depending only on the present state of that substance—> independent of present’s past/future state

Examples of state functions/property

energy

Nonexamples of state functions/property

work/heat

What is an endothermic rxn?

rxn absorbs heat, heat flows into system

What is an exothermic rxn?

energy flows out of the system, heat= product of rxn.

What is a system?

part of the universe chosen for study—> chem. rxns usually occur here

What are surroundings?

Surroundings= part of the universe outside of the system

How are systems & surroundings connected?

Connected via heat, work, and matter

What is matter?

flow of molecules across a boundary

What is an open system?

can exchange matter with surroundings (ex. beaker of coffee, water vapor escaping= matter exchange, heat exchange)

What is a closed system?

doesn't allow the flow of matter with surroundings. ( ex. sealed flask) but exchange of heat or work is allowed

What is the difference between closed and isolated systems?

An isolated system cannot exchange any heat, work, or matter with its surroundings, while closed systems can still exchange heat or work.

Define a calorie

the quantity of heat required to change the temp. of one gram of water by one degree C--> Joule= SI unit for heat

How is q positive and q negative determined?

if heat is gained by a system/lost by surroundings, then q= pos. If heat is lost by a system and gained by surroundings, q= neg.

What is the concept of heat capacity?

C= the quantity of heat required to change the temperature of a substance by 1 degree Cel.

What is an extensive property?

proportional to system size

What is an intensive property?

don’t depend on system size

Examples of extensive properties

volume, mass, heat capacity

Examples of intensive properties

temperature, pressure, density

What is a consecutive rxn?

two or more reactions carried out in sequence and an intermediate is any substance produced in one step and consumed in another step of a multi-step process.

What are two things that happen when chemical energy decreases?

released energy—> thermal energy temp. increases

thermal energy= eventually released to surroundings as heat (assuming vol= constant)

What is the effect that exothermic reactions have on the system/surroundings?

chemical energy= converted to thermal energy

system temp increases

heat= released to surroundings

Qrxn<0

What is the effect that endothermic reactions have on the system/surroundings?

Thermal energy= converted to chem energy

system temp decreases

heat= absorbed by system

Qrxn> 0

What species will have stronger/weaker bonds?

stronger= stable species

weaker= reactive species

What qrxn will the decomposition of a substance have?

qrxn > 0 because the bonds will break—> potential energy increases, temperature decreases

heat flows into system= absorbed

What qrxn will the combustion of a substance have?

qrxn < 0, chem energy decreases, temperature increases

heat flows out of system= released

What do bomb calorimeters measure?

constant vol calorimeter used to measure heat of combustion reactions

heat released by rxn= absorbed by calorimeter

qcal= -qrxn

qcal= Ccal * delta T

What do coffee cup calorimeters measure?

measures heat of solution reaction at constant pressure

qcal= -qrxn

What is pressure- volume work?

type of work done by external pressure through a volume change

w= -P * delta V

What is the relationship between delta V and w=

-P* delta V?

positive delta V= negative w—> system does work on surroundings (ex. gas expanding against constant external pressure of P)

negative delta V= positive w—> surroundings do work on the system (ex. gas compressed by constant external pressure of P)

What is Hess’ Law?

going from a particular set of reactants to particular set of products—> change in enthalpy= same if rxn is in one step, or a series of steps

If rxn= reversed, sign of enthalpy chnages according to if heat was absorbed or released.

Why is the standard state of a thermodynamic property important?

you can only measure changes in THAT specific property, its a defined reference state

How do we calculate the change in enthalpy for an overall rxn?

sum of delta H (standard change in enthalpy) values

delta H of products (added together) minus delta H of reactants (added together).

Calculate heat of combustion of a compound

1. convert to kcal/gram using molar mass of compound

2. Total heat given off= heat given off per mole butane reacted * total moles butane

what is a standard enthalpy of formation?

enthalpy change in formation of one mole of a substance in standard state from reference state

What is the standard enthalpy of formation sign for stabler compunds?

delta Hf < 0 because enthalpy of compound is lower relative to elements—> decomposition needs energy input as heat

What is the standard enthalpy of formation sign for unstabler compounds?

delta Hf >0 because they give off heat energy upon decomposition

Calculate the heat of dissolution (coffee-cup calorimetry)

1) write dissolution equation

2) heat given off by rxn = heat gained by solution + calorimeter

3) create a data table with the masses, Csp, Tfinal, Tinitial, delta T of the solution and calorimeter

4) Calculate heat gained by the solution

(q= mCdeltaT)

5) Calculate heat gained by the calorimeter (qcal= Ccal/ degrees C * delta T)

6) Calculate heat given off by rxn (qxn= -(qsoln + qcal)

7) calculate heat of rxn per gram of substance dissolved

8) calculate heat of rxn per mole using molar mass of substance dissolved

Calculate molar heat of combustion (bomb calorimetry)

1) def= energy released when one mole of compound is burned

2) heat given off by rxn= heat gained by water + calorimeter

(-qrxn= qwater + qcal)

3) create a data table with masses, Csp, Tfinal and Initial, and delta T of water, calorimeter, and sample

4) calculate heat gained by water (qwater= mCdeltaT)

5) calculate heat gained by calorimeter (qcal= Ccal * delta T)

6) calculate heat given off by rxn ( qrxn= -(qwater + qcal)

7) calculate heat of rxn per gram of compound burned

8 ) calculate heat of rxn per mole using molar mass of compound

9) convert J to kJ ( multiply by 1 kj/ 1000 J)

Calculate the heat of combustion of a compound

1) energy from substance= g—> molar mass—> kcal/mol = kcal

2) repeat if need to compare to another compound

How are internal energy and enthalpy related? What is the difference?

Internal energy is roughly stated as the heat of reaction at constant volume, and enthalpy is roughly the heat of reaction at constant pressure.

we can show the relationship through the first law, ΔU = ΔH – PΔV.

Enthalpy is internal energy plus pressure-volume work.

ΔH is the energy required to create a system (ΔU) plus the work required to displace the surroundings (PΔV)

What are the two things that represent what a standard enthalpy of reaction equation includes?

1) decomposition into reference forms

2) formation of products from reference forms

Basically an application of Hess’ Law