Heat in Heat in ReactionsReactions

ThermochemistrThermochemistryy

The study of The study of changes in heat changes in heat in a chemical in a chemical reactionreaction

Part of Part of thermodynamicsthermodynamics

Reactions involve Reactions involve energy because:energy because:

Chemical bonds are BROKEN (requires energy) Chemical bonds are BROKEN (requires energy) think of it as a rope holding things together. It think of it as a rope holding things together. It won’t break until energy is applied.won’t break until energy is applied.

Chemical bonds are FORMED (releases Chemical bonds are FORMED (releases energy) because it is an opposite process.energy) because it is an opposite process.

BondsBonds

Here we see bonds holding molecules Here we see bonds holding molecules together on the left & breaking and then together on the left & breaking and then reforming into new molecules on the rightreforming into new molecules on the right

HeatHeat

Heat is the energy of Heat is the energy of moving particles moving particles transferred between transferred between objects due to a objects due to a difference in difference in temperature.temperature.

HeatHeatIt flows from the higher It flows from the higher

temperature object to the temperature object to the lower temperature object.lower temperature object.

SI unit is Joule (J) but often SI unit is Joule (J) but often measured in kilojoules (kJ)measured in kilojoules (kJ)

Some reactions release Some reactions release energy:energy:

EXOTHERMICEXOTHERMIC

CC33HH88 + 5O + 5O22 →→ 3CO 3CO22 + 4H + 4H22O + energyO + energy

CC33HH88 + 5O + 5O22 →→ 3CO 3CO22 + 4H + 4H22O + 2043 kJO + 2043 kJ

Notice the heat term is on the right !Notice the heat term is on the right !

Some reactions absorb Some reactions absorb energy:energy:

ENDOTHERMICENDOTHERMIC

C + HC + H22O + energy O + energy →→ CO + H CO + H22

C + HC + H22O + 113 kJ O + 113 kJ →→ CO + H CO + H22

Notice the heat term is on the left !Notice the heat term is on the left !

The heat content of any systemThe heat content of any systemDefined by the following Defined by the following

equation: Enthalpy = Energy of a equation: Enthalpy = Energy of a system+ (Pressure x Volume) system+ (Pressure x Volume)

H = E + PVH = E + PVThis holds mostly for gases and This holds mostly for gases and

is often called PV work.is often called PV work.

ENTHALPYENTHALPY

We cannot determine absolute H, We cannot determine absolute H, rather we can determine change in H rather we can determine change in H ((H).H).

H = HH = Hproductsproducts – H – Hreactantsreactants

ENTHALPY ENTHALPY CHANGECHANGE

For our purposes, the For our purposes, the heat absorbed or heat absorbed or released during a released during a chemical reaction is chemical reaction is equal to the enthalpy equal to the enthalpy change for the reaction.change for the reaction.

For easy comparisons,For easy comparisons,

Define:Define:

standard enthalpy change (Ho)

when reactants and products are in their standard when reactants and products are in their standard states (at 25 states (at 25 ooC and 1 atmosphere) for now we C and 1 atmosphere) for now we just have to know that different conditions give us just have to know that different conditions give us different energy changes so we need to state the different energy changes so we need to state the conditions.conditions.

The sign of The sign of H is H is informative.informative.

When When H > 0, (positive)H > 0, (positive)– energy is absorbed, energy is absorbed, – EndothermicEndothermic– The energy term is on the left.The energy term is on the left.

2NH2NH33(g) + 92(g) + 92..4 kJ 4 kJ N N22(g) + 3H(g) + 3H22(g) (g) Check the next slide to see another Check the next slide to see another

way an endothermic reaction is way an endothermic reaction is shown.shown.

Notice the products are higher than the reactants

The sign of The sign of H is H is informative.informative.

When When H < 0, (negative)H < 0, (negative)– energy is released, energy is released, – ExothermicExothermic– The energy term is on the right.The energy term is on the right.

NN22(g) + 3H(g) + 3H22(g) (g) 2NH 2NH33(g) + 92(g) + 92..4 kJ4 kJ Check the next slide to see another Check the next slide to see another

way an exothermic reaction is shown.way an exothermic reaction is shown.

Notice the products are lower than the reactants

Using Enthalpy ChangesUsing Enthalpy Changes

A bombardier beetle uses hydrogen A bombardier beetle uses hydrogen peroxide to create heat and peroxide to create heat and produce a steam spray.produce a steam spray.

How much heat will be released if How much heat will be released if 1.0 grams of H1.0 grams of H22OO22 decomposes decomposes according to the following equation:according to the following equation:

2H2H22OO22 2H 2H22O + OO + O2 2 HHoo = -190 kJ = -190 kJ

Help in solving Help in solving Change grams to molesChange grams to moles Remember your stoichiometryRemember your stoichiometry Look to see if you can use a Look to see if you can use a

proportionproportion

Help in solvingHelp in solving

1.0 g of H1.0 g of H22OO2 2 / 34g/mol =/ 34g/mol = 0.0294 mol 0.0294 mol

HH22OO22 H H22O + ½ OO + ½ O22

0.0294 mol x -190 kJ/mol = -5.6 kJ0.0294 mol x -190 kJ/mol = -5.6 kJ

which means 5.6 kJ energy released which means 5.6 kJ energy released because the negative sign means it is because the negative sign means it is an exothermic reaction. an exothermic reaction.

PracticePractice

How much heat will be How much heat will be transferred when 5.81 grams of transferred when 5.81 grams of graphite reacts with excess Hgraphite reacts with excess H22 according to the following according to the following equation:equation:

6 C + 3 H6 C + 3 H22 C C66HH6 6 ((HHoo = +49.03 = +49.03 kJ)?kJ)?

Hess’s LawHess’s Law

If a series of reactions If a series of reactions are added together, the are added together, the enthalpy change for the enthalpy change for the net reaction will be the net reaction will be the sum of the enthalpy sum of the enthalpy changes for the changes for the individual steps individual steps

HHnetnet = = HH11 + + HH22 + +

Example of Hess’s LawExample of Hess’s Law

NN22 + 2O + 2O22 2NO 2NO22 HHoo = ? = ? We do know…We do know… NN22 + O + O22 2NO2NO HHoo = +181 kJ = +181 kJ 2NO + O2NO + O22 2NO 2NO22 HHoo = -113 kJ = -113 kJ Rearrange equations to come up Rearrange equations to come up

with our desired equationwith our desired equation Add the resulting enthalpy changesAdd the resulting enthalpy changes Remember to factor when needed.Remember to factor when needed.

PracticePractice

Given the following enthalpy changesGiven the following enthalpy changes FeFe22OO33 + 3 CO + 3 CO 2 Fe + 3 CO 2 Fe + 3 CO22

HHoo = -27 kJ AND = -27 kJ AND C + COC + CO22 2 CO 2 CO HHoo = +172 kJ = +172 kJ Calculate the Calculate the H for the following H for the following

reactionreaction 2 Fe2 Fe22OO33 + 3 C + 3 C 4 Fe + 3 CO 4 Fe + 3 CO22

CalorimetryCalorimetry

CalorimetryCalorimetry experiments experiments determine the enthalpy (heat) determine the enthalpy (heat) changes of reactionschanges of reactions

Determined by measuring Determined by measuring temperature changes to the temperature changes to the outside environment, using the outside environment, using the Law of Conservation of Energy .Law of Conservation of Energy .

More CalorimetryMore Calorimetry

4.184 Joules = 1 calorie4.184 Joules = 1 calorie 4.184 Kilojoules = 1 Calorie (Cal)4.184 Kilojoules = 1 Calorie (Cal) The Cal is also known as the Food The Cal is also known as the Food

Calorie and is what you see on Calorie and is what you see on Nutritional Labels of the foods you Nutritional Labels of the foods you buy.buy.

A Bomb CalorimeterA Bomb Calorimeter

A Simple CalorimeterA Simple Calorimeter

Heat CapacityHeat Capacity Heat capacity is the amount of heat Heat capacity is the amount of heat

it takes to raise the temperature of it takes to raise the temperature of an object by one degree Celsius.an object by one degree Celsius.

The heat capacity per one gram is The heat capacity per one gram is called the called the specific heatspecific heat..

Specific heat of liquid water is Specific heat of liquid water is 4.184 J / (g C4.184 J / (g Coo))

Iron 0.45 J / (g CIron 0.45 J / (g Coo))

Determining the Amount of Determining the Amount of Heat ReleasedHeat Released

Use the formulaUse the formula

q = m x C x q = m x C x TT q is the quantity of heat (J)q is the quantity of heat (J) m is the mass of the water m is the mass of the water

surrounding the reaction (1mL = surrounding the reaction (1mL = 1 g)1 g)

C is the specific heat capacity C is the specific heat capacity (4.184 J / (g C(4.184 J / (g Coo) for water)) for water)

T is the change in temperature T is the change in temperature (final – initial)(final – initial)

Practice Practice How much heat is released How much heat is released

when 5 grams of NHwhen 5 grams of NH44NONO33 is is dissolved in 50 mL of water?dissolved in 50 mL of water?

What is the specific heat of What is the specific heat of Aluminum if the temperature Aluminum if the temperature of a 28.4 gram sample of a 28.4 gram sample increased by 8.1 increased by 8.1 ooC when 207 C when 207 J of heat is added?J of heat is added?

Applying the equation slide Applying the equation slide 11

The equation q = m x C x The equation q = m x C x T can be T can be combined with the Law of combined with the Law of Conservation of Energy to give us Conservation of Energy to give us help with solving some problems.help with solving some problems.

If a piece of hot metal is placed in a If a piece of hot metal is placed in a cup of cold water, we all know that cup of cold water, we all know that the temperature of the metal will the temperature of the metal will decrease and the temperature of the decrease and the temperature of the water will increase.water will increase.

Applying the equation slide Applying the equation slide 22

So for the heat gain of the water we can So for the heat gain of the water we can use the equation: q = m x C x use the equation: q = m x C x T where q T where q equals the heat gained by the water.equals the heat gained by the water.

And for the heat lost by the metal we can And for the heat lost by the metal we can use the equation: q = m x C x use the equation: q = m x C x T where q T where q equals the heat lost by the metal.equals the heat lost by the metal.

According to the Law: The heat lost by According to the Law: The heat lost by the metal has to equal the heat gained by the metal has to equal the heat gained by the water. SO this leads to …….the water. SO this leads to …….

Applying the equation slide Applying the equation slide 33

Heat Loss = Heat Gain or qHeat Loss = Heat Gain or qlostlost = q = qgaingain

Since q = m x Since q = m x T x C we can change T x C we can change this equation to:this equation to:

mmmetalmetal x x TTmetalmetal x C x Cmetalmetal = m = mwaterwater x x TTwaterwater x C x Cwaterwater

This equation allows us to find a lot of This equation allows us to find a lot of different values: Specific Heat of a different values: Specific Heat of a metal, temperature change of a system & metal, temperature change of a system & more.more.

Working our new equationWorking our new equation

Remember to watch which variables Remember to watch which variables are being added. In other words, are being added. In other words, remember to keep all the data for remember to keep all the data for the water together on the same side the water together on the same side of the equation and do the same for of the equation and do the same for the other material as well.the other material as well.

If you are having problems with the If you are having problems with the algebra, don’t forget to watch the algebra, don’t forget to watch the solved problems done in class.solved problems done in class.

Calories in FoodCalories in Food

Caloric content in food is Caloric content in food is determined by food make determined by food make up and digestibility.up and digestibility.

Food is made up ofFood is made up of– Carbohydrates 17 kJ/g or 4 Carbohydrates 17 kJ/g or 4

Cal/gCal/g– Protein 17 kJ/g or 4 Cal/gProtein 17 kJ/g or 4 Cal/g– Fat 38 kJ/g or 9 Cal/gFat 38 kJ/g or 9 Cal/g

Calories in FoodCalories in Food

Calories in food are temporarily Calories in food are temporarily stored in the liver and used when stored in the liver and used when neededneeded

You need to walk ~ 43 minutes to You need to walk ~ 43 minutes to burn off a 300 Calorie ice cream burn off a 300 Calorie ice cream cone, depending on your size.cone, depending on your size.

If Calories are not burned they If Calories are not burned they are stored long term as fats in the are stored long term as fats in the body.body.

Energy SourcesEnergy Sources

Fossil Fuels:Fossil Fuels:– PetroleumPetroleum– Natural GasNatural Gas– LPG & LNGLPG & LNG– CoalCoal– KeroseneKerosene

Renewable EnergyRenewable Energy– SolarSolar– WindWind– GeothermalGeothermal– NuclearNuclear– Fuel CellFuel Cell– WaveWave

Some are more Some are more viable than others.viable than others.