Energetics

EnergeticsEnergetics

Energy changes in chemistry.Energy changes in chemistry.

ThermochemistryThermochemistry

What is thermochemistry?What is thermochemistry? Thermochemistry is the study of energy Thermochemistry is the study of energy

changes associated with chemical changes associated with chemical reactions.reactions.

EnthalpyEnthalpy

What is enthalpy?What is enthalpy? Enthalpy (H) is the total energy of Enthalpy (H) is the total energy of

system. It is in the molecules, the system. It is in the molecules, the bonds, and stored chemically.bonds, and stored chemically.

Enthalpy change (Enthalpy change (ΔΔ H) is the measure H) is the measure of the change in the potential energy of of the change in the potential energy of the bonds.the bonds.

Endothermic vs Endothermic vs exothermicexothermic

What is the difference between the What is the difference between the two?two? Endothermic reactions have more Endothermic reactions have more

energy stored in the new bonds energy stored in the new bonds (products) than were in the old bonds (products) than were in the old bonds (reactants) which takes in energy.(reactants) which takes in energy.

Exothermic reactions have more energy Exothermic reactions have more energy in the old bonds (reactants) than in the in the old bonds (reactants) than in the new bonds (products).new bonds (products).

Enthalpy changeEnthalpy change

What is the value of enthalpy change What is the value of enthalpy change for exothermic reactions?for exothermic reactions? Exothermic reaction enthalpy change is Exothermic reaction enthalpy change is

negative due to negative due to ΔΔ H = enthalpy H = enthalpy products – enthalpy reactants.products – enthalpy reactants.

Endothermic reaction would be the Endothermic reaction would be the opposite because of the same equation.opposite because of the same equation.

Enthalpy changeEnthalpy change

How do we show enthalpy change of a How do we show enthalpy change of a reaction?reaction? It is usually written with the equation.It is usually written with the equation. + represents endothermic, - represents + represents endothermic, - represents

exothermicexothermic It is given in the units of kJ/mol (or just kJ) It is given in the units of kJ/mol (or just kJ)

because it changes with the amount of because it changes with the amount of reactants and limiting reactants.reactants and limiting reactants.

This is at thermochemical standard This is at thermochemical standard conditions which are 25 degrees C, 1 atm and conditions which are 25 degrees C, 1 atm and solution concentrations of 1 mol/dmsolution concentrations of 1 mol/dm33..

Enthalpy changeEnthalpy change

What is the difference between heat What is the difference between heat and temperature?and temperature? Temperature is the measure of the Temperature is the measure of the

kinetic energy of the particles and does kinetic energy of the particles and does not depend on the amount present.not depend on the amount present.

Heat is the measure of the total energy Heat is the measure of the total energy of a substance that increases and of a substance that increases and decreases with amount of substance.decreases with amount of substance.

Enthalpy changeEnthalpy change

How do we find heat energy?How do we find heat energy? Heat energy = mass x specific heat Heat energy = mass x specific heat

capacity x change in temperature (m x c capacity x change in temperature (m x c x x ΔΔT).T).

Specific heat capacity depends on the Specific heat capacity depends on the specific material involved.specific material involved.

CalorimetryCalorimetry

What is Calorimetry?What is Calorimetry? This is a technique to measure the This is a technique to measure the

energy change in a reaction.energy change in a reaction. Total heat = (m c Total heat = (m c ΔΔT)liquid + (m c T)liquid + (m c

ΔΔT)solidT)solid

CalorimetryCalorimetry

When 8.00 g of ammonium nitrate dissolves When 8.00 g of ammonium nitrate dissolves in 100.0 cmin 100.0 cm33 of water, the temperature of water, the temperature dropped from 19.0 C to 14.5 C. What is the dropped from 19.0 C to 14.5 C. What is the enthalpy of ammonium nitrate?enthalpy of ammonium nitrate? 100 g x 4.18 J/g K x 4.5 K100 g x 4.18 J/g K x 4.5 K 1881 J1881 J 8.00g/80.06 g/mol8.00g/80.06 g/mol .100 mol.100 mol 1881 J/ .100 mol1881 J/ .100 mol 18.8 kJ/mol18.8 kJ/mol

Hess’s LawHess’s Law

What does Hess’s Law say?What does Hess’s Law say? It states that if you can add 2 or more It states that if you can add 2 or more

different equations to produce the final different equations to produce the final equation, you can add the individual equation, you can add the individual enthalpy changes to find the total.enthalpy changes to find the total.

This means that if you break down the This means that if you break down the steps involved, the overall change will steps involved, the overall change will be the same.be the same.

Hess’s LawHess’s Law

2H2H22OO2(l)2(l) 2H 2H22OO(l) (l) + O+ O2(g)2(g)

2H2H2(g)2(g) + O + O2(g)2(g) 2H 2H22OO(l)(l) ΔΔH= -572 kJH= -572 kJ

HH2(g)2(g) + O + O2(g)2(g) H H22OO2(l)2(l) ΔΔH= -188 kJH= -188 kJ Where do we start?Where do we start?