Unit 1: Energy Changes in Unit 1: Energy Changes in Chemical ReactionsChemical Reactions

Not All Reactions Go Off with Not All Reactions Go Off with a Bang!a Bang!

EnergyEnergy The capacity to do work or to produce heatThe capacity to do work or to produce heat

Forms of Energy:Forms of Energy:

1.1. Kinetic Energy– The energy of motionKinetic Energy– The energy of motion

2.2. Potential EnergyPotential Energy

– – The energy of position (gravitational The energy of position (gravitational potential energy)potential energy)

eg. Diefenbaker Dameg. Diefenbaker Dam

– – The energy stored in chemicals The energy stored in chemicals because of their compositionbecause of their composition

eg. Chocolate bars (Yum!)eg. Chocolate bars (Yum!)

Measuring EnergyMeasuring Energy

☼☼ Common UnitsCommon Units

→ → calorie (cal)calorie (cal)

- - food food ≈ 2000 – 3000 kcal/day≈ 2000 – 3000 kcal/day

1000 cal = 1 kcal (kilocal)1000 cal = 1 kcal (kilocal)

1000 cal = 1 1000 cal = 1 CCalorie alorie

→ → British Thermal Unit (BTU)British Thermal Unit (BTU)

- - fuelsfuels

fuel = 20 900 BTU/poundfuel = 20 900 BTU/pound

coal = 9 600 BTU/poundcoal = 9 600 BTU/pound

plastic bags = 18 700 BTU/poundplastic bags = 18 700 BTU/pound

☼ ☼ Scientific UnitScientific Unit

→ → joule (J)joule (J)

1 J = 1 newton 1 J = 1 newton ●● metre (N metre (N●●m)m)

i.e. The work required to move an object i.e. The work required to move an object one metre using a force of one newton one metre using a force of one newton

(N = kg(N = kg●●m/sm/s22). ).

4.184 J = 1 cal 4.184 J = 1 cal

☼ ☼ Most chemical reactions involve energy or Most chemical reactions involve energy or changes in energy.changes in energy.

☼ ☼ In a Chemical Reaction (rxn)… In a Chemical Reaction (rxn)… - existing bonds are broken (requires E)- existing bonds are broken (requires E)

- atoms are rearranged- atoms are rearranged

- new bonds are formed (releases E)- new bonds are formed (releases E)

Thus, almost all chem. Rxns either absorb or Thus, almost all chem. Rxns either absorb or release energy. This results in an exchange of release energy. This results in an exchange of energy (aka HEAT)energy (aka HEAT)

Heat-- Heat-- The energy that is transferred from The energy that is transferred from one object to another due to a difference one object to another due to a difference in temperature (flow of energy is usually in temperature (flow of energy is usually from hot to cold until equilibrium is from hot to cold until equilibrium is reached)reached)

Thermochemistry-- Thermochemistry-- The study of the The study of the changes in heat in chem. rxns (part of changes in heat in chem. rxns (part of thermodynamics-- energy/work)thermodynamics-- energy/work)

→ → thermes = greek for heatthermes = greek for heat→ → we look at systems during changes of we look at systems during changes of heat (i.e. A beaker or flask)heat (i.e. A beaker or flask) → → we also look at surroundings into which we also look at surroundings into which heat may be lost or gainedheat may be lost or gained

→ → A system is part of the universe on A system is part of the universe on which we focus our attention, the which we focus our attention, the surroundings include everything else in the surroundings include everything else in the universeuniverse

→ → energy is neither created or destroyed energy is neither created or destroyed when heat is transferred (1when heat is transferred (1stst Law of Law of Thermodynamics)Thermodynamics)

→ → closed vs open systemclosed vs open system

→ → we can’t tell how much energy is in we can’t tell how much energy is in something until it is releasedsomething until it is released

TemperatureTemperature = degree of hotness or coldness of = degree of hotness or coldness of an object, which is a measure of average an object, which is a measure of average kinetic kinetic energyenergy of the molecules of the molecules

HeatHeat = the energy transferred from one body to = the energy transferred from one body to another because of the temp. differenceanother because of the temp. difference

HeatHeat is a form of is a form of energyenergy; temp. in ; temp. in NOTNOT!!

There is more heat in a large iceberg than in a cup There is more heat in a large iceberg than in a cup of boiling water! This is b/c heat is trapped of boiling water! This is b/c heat is trapped inside as opposed to being released so the ice inside as opposed to being released so the ice berg doesn’t feel warm. berg doesn’t feel warm.

Exothermic Exothermic Reactions Reactions

→ → rxns that rxns that releaserelease heat into the heat into the surroundingssurroundings

→ → PE is converted to PE is converted to

heat energy; temp. ↑ heat energy; temp. ↑

Endothermic Endothermic Reactions Reactions

→ → rxns that rxns that absorbsabsorbs heat from the heat from the surroundingssurroundings

→ → KE decreases; temp. ↓ KE decreases; temp. ↓

Examples:Examples:

1. Combustion of propane1. Combustion of propane

CC33HH8(g)8(g) + 5O + 5O22 → 3CO → 3CO2(g)2(g) + 4H + 4H22OO(g)(g) + 2043 kJ + 2043 kJ

1 mole of propane produces 2043 kJ of heat1 mole of propane produces 2043 kJ of heat

Energy releasedEnergy released as new bonds are formed as new bonds are formed in the products is in the products is greatergreater than the energy than the energy required to break the old bonds in the required to break the old bonds in the reactants.reactants.

All combustion rxns are All combustion rxns are exothermicexothermic!!

2.2. Water Gas ReactionWater Gas Reaction

CC(s)(s) + H + H22OO(g) (g) ++ 113 kJ 113 kJ → CO→ CO(g)(g) + H + H2(g)2(g)

1 mole of solid carbon requires 113 kJ of 1 mole of solid carbon requires 113 kJ of heat to produce 1 mole of carbon heat to produce 1 mole of carbon monoxide.monoxide.

This is an This is an endothermicendothermic rxn. rxn.

Chemical equations with heat values Chemical equations with heat values incorporated into them are known as incorporated into them are known as thermochemical equationsthermochemical equations..

EnthalpyEnthalpy (H) is… (H) is…

… … the heat content or the amount of heat a the heat content or the amount of heat a substance has at a given temp. and pressure substance has at a given temp. and pressure (the total energy stored by a substance is the (the total energy stored by a substance is the sum of KE & PE. The enthalpy is this energy sum of KE & PE. The enthalpy is this energy plus a small added term that takes into account plus a small added term that takes into account the pressure and volume.)the pressure and volume.)

∆∆H = Change in enthalpy H = Change in enthalpy

∆∆H = HH = Hproductsproducts – H – Hreactantsreactants

∆∆H = + (pos.) = heat absorbed = endoH = + (pos.) = heat absorbed = endo

∆ ∆H = H = (neg.) = heat released = exo (neg.) = heat released = exo

Energy DiagramsEnergy Diagrams

For reporting enthalpy changes, chemists use For reporting enthalpy changes, chemists use

1 atm = 101.3 kPa and 25°C (298K) as standard 1 atm = 101.3 kPa and 25°C (298K) as standard enthalpy changes ∆H°.enthalpy changes ∆H°.

CHCH4(g)4(g) + 2O + 2O2(g)2(g) → CO → CO2(g)2(g) + 2H + 2H22OO((ll)) ∆H° = ∆H° = 890.3 kJ 890.3 kJ

½H½H2(g)2(g) + ½I + ½I2(g)2(g) → HI → HI(s)(s) ∆H° = + 26.5kJ∆H° = + 26.5kJ

Equations are stoichiometrically correct!Equations are stoichiometrically correct!

SATP

Your Turn!Your Turn!

1.1. Draw the Energy diagram for… Draw the Energy diagram for…

a) the combustion of propanea) the combustion of propane

b) the formation of COb) the formation of CO(g)(g)

2. Write a balanced equation and state the2. Write a balanced equation and state the

∆ ∆H° value (i.e. don’t include the value in H° value (i.e. don’t include the value in the eqn but state it separately as pos. or the eqn but state it separately as pos. or neg.)neg.)

SummarySummary

ExothermicExothermic ( (∆H°)∆H°)

→ → energy is releasedenergy is released

→ → energy appears as a productenergy appears as a product

→ → surroundings increase in temp. (warmer)surroundings increase in temp. (warmer)

EndothermicEndothermic (+∆H°) (+∆H°)

→ → energy is absorbedenergy is absorbed

→ → energy appears as a reactantenergy appears as a reactant

→ → surroundings decrease in temp. (cooler)surroundings decrease in temp. (cooler)

☼ ☼ Equations are stoichiometrically correct!Equations are stoichiometrically correct!

Calculation Questions:Calculation Questions:

1.1. How much heat is transferred when 9.22 g How much heat is transferred when 9.22 g of glucose (Cof glucose (C66HH1212OO66) in your body reacts ) in your body reacts

with Owith O22 according to the following eqn.? according to the following eqn.?

CC66HH1212OO6(s) 6(s) + 6O+ 6O2(g)2(g) → 6CO → 6CO2(g)2(g) + 6H + 6H22OO((ll))

∆∆H° = H° = 2803 kJ 2803 kJ

9.22g C9.22g C66HH1212OO66 x x 1 mol 1 mol x 2803 kJ x 2803 kJ

180.0 g 1 mol C180.0 g 1 mol C66HH1212OO66

= 143.57589 kJ = = 143.57589 kJ = 144 kJ released144 kJ released

2. How much heat is transferred when 147g 2. How much heat is transferred when 147g of NOof NO2(g) 2(g) is dissolved in an excess of His dissolved in an excess of H22O?O?

3NO3NO2(g) 2(g) + H+ H22OO ( (ll)) → 2HNO → 2HNO3(aq)3(aq) + NO + NO(g)(g)

∆ ∆H° = H° = 138 kJ 138 kJ

147g NO147g NO22 x x 1 mol NO 1 mol NO22 x 138 kJ x 138 kJ

46.0 g 3 mol NO46.0 g 3 mol NO22

= = 147 kJ released147 kJ released

AssignmentAssignment: Pg. 187, #10 & 11 : Pg. 187, #10 & 11

Pg. 191 – 195, #30 – 33 Pg. 191 – 195, #30 – 33

(Addison-Wesley Text)(Addison-Wesley Text)