Ch 16Ch 16Reaction EnergyReaction Energy

Standard: Standard: – 7.d. Students know how to solve 7.d. Students know how to solve

problems involving heat flow and problems involving heat flow and temperature changes, using known temperature changes, using known values of specific heat. values of specific heat.

Objective:Objective:– We will define heat, give its units, We will define heat, give its units,

and perform specific-heat and perform specific-heat calculations.calculations.

ThermochemistryThermochemistryThermochemistry: the study of the

transfer of energy as heat that occurs during chemical reactions and changes in state.

Heat: q, is energy transferred from one object to another because of a temperature difference between them.

Heat always flows from a warmer object to a cooler object and will continue to flow until they are in equilibrium.

Endothermic ProcessEndothermic Process: : one that one that absorbs heat from the surroundings absorbs heat from the surroundings (+q(+q))..

Exothermic ProcessExothermic Process: : one that one that releases heat to it’s surroundings releases heat to it’s surroundings (-q(-q))..

CalorimeterCalorimeter: : the insulated device the insulated device used to measure the absorption or used to measure the absorption or release of heat in chemical or physical release of heat in chemical or physical processes.processes.

Specific HeatSpecific Heat Heat flow is measured in joules (J).Heat flow is measured in joules (J). One joule of heat raises the One joule of heat raises the

temperature of 1g of pure water temperature of 1g of pure water 0.23900.2390°C. °C.

Specific HeatSpecific Heat: the amount of heat : the amount of heat needed to increase the temp of 1 g needed to increase the temp of 1 g of the substance of the substance 11°C or 1 Kelvin.°C or 1 Kelvin.

Water’s specific heat is 4.18 J/(g●K)Water’s specific heat is 4.18 J/(g●K)

Calculating Specific Heat (C)Calculating Specific Heat (C) Divide the heat input, Divide the heat input, qq (Joules) by the (Joules) by the

temperature change, temperature change, ΔΔTT ( (°C)°C) times the times the mass of the substance, mass of the substance, mm (g). (g).

C = C = qq = = heatheat mm x x ΔΔTT mass x change in mass x change in

temptemp

ExampleExample The temperature of a 95.4 g piece The temperature of a 95.4 g piece

of copper increases from 25.0of copper increases from 25.0°C to °C to 48.0°C when the copper absorbs 48.0°C when the copper absorbs 849 J of heat. What is the specific 849 J of heat. What is the specific heat, C, of copper?heat, C, of copper?

qq = 849 J = 849 J mm = 95.4g = 95.4g

ΔΔT = (48.0°C -25.0°C)=23.0°CT = (48.0°C -25.0°C)=23.0°C

C = C = qq = = 849 J 849 J mm x x ΔΔTT 95.4 g x 23.0°C 95.4 g x 23.0°C

C = 0.387 J/(g x °C)C = 0.387 J/(g x °C)

Example 2Example 2 How much heat, How much heat, qq, is required to , is required to

raise the temperature of 400.0 g of raise the temperature of 400.0 g of silver 45silver 45°C? The specific heat of °C? The specific heat of silver is 0.24 J/(g x °C).silver is 0.24 J/(g x °C).

ΔΔT = 45°CT = 45°Cmm = 400.0 g = 400.0 g

C= 0.24 J/(g x °C) C= 0.24 J/(g x °C)

qq = C x = C x mm x x ΔΔTT

qq = 0.24 J/(g x °C) x 400.0 g x 45°C = 0.24 J/(g x °C) x 400.0 g x 45°C

qq = 4320 J = 4320 J

Enthalpy of Reaction

Enthalpy Change is the amount of energy absorbed by a system as heat during a process at constant pressure. – The enthalpy change is always the

difference between the enthalpies of the products and the reactants and is called Enthalpy of Reaction.

∆H = Hproducts - Hreactants

∆H is negative for an exothermic reaction because the system loses heat.

∆H is positive for an endothermic reaction because the system gains heat.

Example 3 Enthalpy of Reaction for the formation of water

vapor. – What we already know:

2H2(g) + O2(g) 2H2O(g)

– This equation does not tell us that energy is released as heat during the reaction.

– Thermochemical Equation:2H2(g) + O2(g) 2H2O(g) + 483.6kJ

– Writing with ∆H: 2H2(g) + O2(g) 2H2O(g) ∆H = -483.6kJ

Enthalpy of Reaction for the decomposition of water vapor.– What we already know:

2H2O(g) 2H2(g) + O2(g)

– This equation does not tell us that energy as heat is absorbed during the reaction.

– Thermochemical Equation: 2H2O(g) + 483.6kJ 2H2(g) + O2(g)

– Writing with ∆H: 2H2O(g) 2H2(g) + O2(g) ∆H = +483.6kJ

Compounds that release a large amount of energy as heat when they are formed are very stable.

Compounds that release a very small amount of energy as heat or absorb a large amount of energy as heat when they are formed are sometimes unstable and may decompose or react violently.

Chapter 17Chapter 17Reaction KineticsReaction Kinetics

Ch 17.1 Ch 17.1

Reaction KineticsReaction Kinetics

Standard: Standard: – 8.a. Students know the rate of 8.a. Students know the rate of

reaction is the decrease in reaction is the decrease in concentration of reactants or the concentration of reactants or the increase in concentration of increase in concentration of products with time. products with time.

Objective:Objective:– We will interpret chemical We will interpret chemical

reactions and define activated reactions and define activated complex. We will draw energy complex. We will draw energy diagrams. diagrams.

Chemical ReactionsChemical Reactions Activation EnergyActivation Energy: the minimum : the minimum

energy that colliding particles must have energy that colliding particles must have in order to react. in order to react.

Activated ComplexActivated Complex: an unstable : an unstable arrangement of atoms that forms arrangement of atoms that forms momentarily at the peak of the momentarily at the peak of the activation-energy barrier. activation-energy barrier.

This is also called the This is also called the Transition State.Transition State.

Energy DiagramsEnergy Diagrams ∆Eforward = energy of products – energy of

reactants

∆Ereverse = energy of reactants – energy of

products

Ea = energy of activated complex – energy of

reactants

Ea’ = energy of activated complex – energy of

products

∆Eforward is positive for endothermic

and negative for exothermic

∆Ereverse is negative for endothermic

and positive for exothermic

Examples!!Examples!!

Ch 17.2Ch 17.2

Standard: Standard: – 8.b. Students know how reaction rates 8.b. Students know how reaction rates

depend on such factors as depend on such factors as concentration, temperature and concentration, temperature and pressure. pressure.

– 8.c. Students know the role a catalyst 8.c. Students know the role a catalyst plays in increasing the reaction rate. plays in increasing the reaction rate.

Objective:Objective:– We will discuss the factors that We will discuss the factors that

influence reaction rate and define a influence reaction rate and define a catalyst. catalyst.

Rate Influencing FactorsRate Influencing Factors The The raterate of a chemical reaction of a chemical reaction

depends upon temperature, depends upon temperature, concentration, particle size, and concentration, particle size, and the use of a catalyst. the use of a catalyst.

The nature of the The nature of the reactantsreactants and and their bonds is also a factor, but their bonds is also a factor, but not one that can be easily changed not one that can be easily changed so we will not talk about it. so we will not talk about it.

TEMPERATURETEMPERATURE Raising the temperature Raising the temperature speeds upspeeds up

the reaction and lowering the the reaction and lowering the temperature temperature slows downslows down the the reaction. reaction.

The The higherhigher the concentration, the the concentration, the more likely collisions will take place, more likely collisions will take place, which which increasesincreases the reaction rate. the reaction rate.

CONCENTRATIONCONCENTRATION

PARTICLE SIZEPARTICLE SIZE The The smallersmaller the particle size, the the particle size, the

more more surface areasurface area, which , which increasesincreases the reaction rate. the reaction rate.

Adding a catalyst will Adding a catalyst will increaseincrease the the rate of reaction, in some cases, rate of reaction, in some cases, better than increasing the better than increasing the temperature. temperature.

Inhibitor:Inhibitor: a substance that interferes a substance that interferes with the action of a catalyst. with the action of a catalyst. – These will slow down or even These will slow down or even stopstop a a

reaction. reaction.

CATALYSTCATALYST