EnergyEnergy

► Changes form but Changes form but does not change in does not change in amount or amount or disappear.disappear.

EnergyEnergy

Chemical potential energy is Chemical potential energy is stored in chemical bonds of a stored in chemical bonds of a

substancesubstanceCl

Cl

Cl2 Bond

Heat: Energy is released or Heat: Energy is released or absorbed as heat during chemical absorbed as heat during chemical

processes and reactions.processes and reactions.

Let’s Review What We Let’s Review What We Know… Know…

► Specific Heat formula is: Specific Heat formula is:

q = mCq = mCΔΔTT When do we use this? What for?? When do we use this? What for??

► Practice: Practice: Calculate the heat required to raise Calculate the heat required to raise the temperature of 1,250 g of ice -25 C to 0 the temperature of 1,250 g of ice -25 C to 0 C. The specific heat capacity of ice is 1.84 C. The specific heat capacity of ice is 1.84 J/gC.J/gC.q = (1,250)(1.84)(25)q =

Now we will see what else we Now we will see what else we can do with heat… can do with heat…

►During a phase change, we don’t have During a phase change, we don’t have a change in temperature. a change in temperature.

►Plug in “0” for your Plug in “0” for your ΔΔT in your specific T in your specific heat formula. What happens?heat formula. What happens?

►We need a new formula!!! We need a new formula!!!

Heat (enthalpy) of Heat (enthalpy) of VaporizationVaporization

►Hvap: the heat required to change a Hvap: the heat required to change a liquid to a gas or liquid to a gas or L to G. L to G.

Formula: q = mFormula: q = mΔΔHHvapvap

Heat (enthalpy) of FusionHeat (enthalpy) of Fusion

►Hfus: the heat required to change a Hfus: the heat required to change a liquid to a solid (liquid to a solid (L to a S) or back.L to a S) or back.

Formula: Formula: q = mq = mΔΔHHfusfus

Let’s Practice!Let’s Practice!

►Calculate the amount of energy required Calculate the amount of energy required to melt 30.0 grams of ice.to melt 30.0 grams of ice.

►Calculate the amount of energy required Calculate the amount of energy required to freeze 250 grams of water.to freeze 250 grams of water.

►Calculate the amount of energy required Calculate the amount of energy required to vaporize 750 grams of liquid water.to vaporize 750 grams of liquid water.

Heat in ReactionsHeat in Reactions

► Sometimes we want to know how Sometimes we want to know how much heat is in a whole reaction. much heat is in a whole reaction.

►Two words we will need to know:Two words we will need to know:

Activation Energy: the energy needed for Activation Energy: the energy needed for a rxn to take placea rxn to take place

Catalyst:Catalyst: substance we can add to lower the substance we can add to lower the activation energyactivation energy

Two Types of Rxns: Two Types of Rxns:

► If a rxn LOSES HEAT it is EXOTHERMICIf a rxn LOSES HEAT it is EXOTHERMIC

► If a rxn GAINS HEAT it is If a rxn GAINS HEAT it is ENDOTHERMICENDOTHERMIC

►Endothermic Rxn Video

Exothermic RxnExothermic Rxn

►Reactants higher in Reactants higher in energy than productsenergy than products

►Energy is RELEASED in Energy is RELEASED in the form of heat the form of heat

►EXOTHERMIC reaction EXOTHERMIC reaction ►ΔHrxn = -ΔHΔHrxn = -ΔH

Hproducts – HreactantsHproducts – Hreactants = negative = negative

Endothermic Endothermic RxnRxnGAINING heat – heat GAINING heat – heat is kept in rxn.is kept in rxn.•Products higher in Products higher in energy than reactantenergy than reactant•Energy is ABSORBED Energy is ABSORBED in the form of heatin the form of heat•ENDOTHERMIC ENDOTHERMIC reaction reaction (ENDO=inside)(ENDO=inside)

ΔHrxn = ΔHrxn = ++ΔΔH H

Hproducts – Hproducts – Hreactants = positive Hreactants = positive

5 Factors that affect reaction 5 Factors that affect reaction rates:rates:

1.1. ConcentrationConcentration

2.2. ReactivityReactivity

3.3. Surface AreaSurface Area

4.4. Temperature Temperature

5.5. Catalyst (lower activation energy)Catalyst (lower activation energy)

1. Concentration1. Concentration► Think Molarity. Think Molarity. ► The higher the The higher the

concentration of the concentration of the reactants, the larger reactants, the larger the reaction.the reaction.

How could we make How could we make watered down Kool-Aid watered down Kool-Aid taste better?taste better?

2. Reactivity2. Reactivity

►How “reactive” are How “reactive” are the elements? the elements?

► The more reactive The more reactive they are, the bigger they are, the bigger the reaction! the reaction!

3. Surface Area3. Surface Area

► The more surface The more surface area, the more area, the more places for a reaction places for a reaction to take place. More to take place. More COLLISIONS.COLLISIONS.

► 3GB to 8GB3GB to 8GB

4. Temperature4. Temperature

►Why do we heat Why do we heat things in things in Chemistry Class?Chemistry Class?

► Increase in temp Increase in temp increases number increases number of collisions. of collisions.

►Faster particles = Faster particles = more collisions.more collisions.

5. Catalyst5. Catalyst

►Something we Something we add to make add to make something something happen FASTER.happen FASTER.

►Gives rxn lower Gives rxn lower activation energy activation energy – the rxn doesn’t – the rxn doesn’t need as much need as much energy to start energy to start the rxn.the rxn.

Quick Review… Quick Review…

►Reversible reaction: can take place in the Reversible reaction: can take place in the forward and reverse directionsforward and reverse directions aA + bB cC + dDaA + bB cC + dD

Chapt 18 Key PointsChapt 18 Key Points

EquilibriumEquilibrium

►When the forward and reverse When the forward and reverse directions have equal rates and the directions have equal rates and the concentrations of the concentrations of the reactants/products don’t change reactants/products don’t change anymore.anymore.

►Equilibrium Constant (KEquilibrium Constant (Keqeq))

Le Chatlier’s principleLe Chatlier’s principle

►How equilibrium can shift in response to a How equilibrium can shift in response to a stress or disturbance.stress or disturbance. If you increase any reactant the equilibrium will If you increase any reactant the equilibrium will

shift towards the products (right or in the forward shift towards the products (right or in the forward direction).direction).

If you increase any product the equilibrium will If you increase any product the equilibrium will shift towards the reactants (left or in the reverse shift towards the reactants (left or in the reverse direction).direction).

An increase in pressure will cause the equilibrium An increase in pressure will cause the equilibrium to shift towards the direction with the most # of to shift towards the direction with the most # of moles of a gas.moles of a gas.

►Equilibrium Constant (KEquilibrium Constant (Keqeq))

►aA + bB cC + dDaA + bB cC + dD►KKeqeq = = [C][C]cc[D][D]dd

[A][A]aa[B][B]bb

►Ex)Ex) NN22(g) + 3H(g) + 3H22(g)(g) 2NH2NH33(g)(g)

KKeqeq = [NH = [NH33]]22

[N[N22][H][H22]]33