IB DP1 ChemistryEnergetics

Why do chemical reactions get hot or cold?

Topic 5: Energetics (8 hours)

5.1 Exothermic and endothermic reactions5.1.1 Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction ( ∆HO ) .5.1.2 State that combustion and neutralization are exothermic processes.5.1.3 Apply the relationship between temperature change, enthalpy change and the classification of a reaction as endothermic or exothermic.5.1.4 Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction.

5.2 Calculation of enthalpy changes5.2.1 Calculate the heat energy change when the temperature of a pure substance is changed.

5.2.2 Design suitable experimental procedures for measuring the heat energy changes of reactions.5.2.3 Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water.5.2.4 Evaluate the results of experiments to determine enthalpy changes.

5.3 Hess’s law5.3.1 Determine the enthalpy change of a reaction that is the sum of two or three reactions with known enthalpy changes.5.4 Bond enthalpies5.4.1 Define the term average bond enthalpy.5.4.2 Explain, in terms of average bond enthalpies, why some reactions are exothermic and others are endothermic.

Thermochemistry

Study of energy changes during chemical reactions

Heat, light, mechanical energy …..

mix ammonium nitrate and water

burn ethanol

See Think Wonder

Why do chemical reactions get hot (or cold)?

What is the difference between energy and enthalpy?

What is the difference between temperature and heat?

How much heat does it take to increase the temperature of a substance?

What is the difference between enthalpy change and standard enthalpy change?

How is calorimetry used to measure enthalpy changes?

How is standard enthalpy change calculated from a temperature change?

Exothermic and endothermic reactions

Ammonium nitrate and water Iron and oxygen

Cold pack

A. The temperature of the cold pack decreases during the reaction

B. The temperature of the cold pack increases during the reaction

C. The cold pack transfers cold to the person touching it

D. Heat is taken in from the person to the cold pack

E. The cold pack has less energy and lower enthalpy after it is used

Exothermic reactions examples

NaOH(s) + H2O NaOH(aq) + heatExothermic

HCl + NaOH NaCl + H2O + heatNeutralisation

Wood + O2 CO2 + H2O + heat Combustion

Endothermic reaction example

Ba(OH)2(s) + 2 NH4SCN(s) + heat

Ba2+(aq) + 2 SCN-

(aq) + 2 H2O(l) + NH3(aq)

Endothermic

Endothermic or exothermic?

In chemical reactions bonds break and form

different amounts of energy are in the bonds before and after the reaction

Exothermic reaction: less energy is in the bonds after the reaction: heat is produced

Endothermic reaction = energy is needed

Distribution of speeds of particles

number of particles

speed

Temperature- average KE per particle

higher average speed higher temperature

more particles at same average speed same temperature

Distribution of speeds

Enthalpy, H

Energy stored in chemical bonds of reactants (in Joules)

PE and KE of particles + energy to make space for substance

Image: https://en.wikipedia.org/wiki/File:Ammonium_Nitrate.jpg

Exothermic reaction

CH4 + 2 O2 CO2 + 2H2O + heat

Energy rich Energy poor

DH = (Energy poor) – (Energy rich) negative value

Exothermic reactions: DH < O more stable products

Endothermic reactions DH > O more reactive products.

Exothermic reactions

Enthalpy, J

reaction coordinate

reactants

products

∆H

Endothermic reactions

Enthalpy, J

reaction coordinate

reactants

products

∆H

Standard enthalpy change of a reaction

∆H° to compare reactions

in kJ/mol

measured at 298K and 1atm

Calculating standard enthalpy changes

2 Mg + O2 2 MgO DH = -1202 kJ/mol Exothermic

The amount of energy released when 0.6 g of Mg is burnt?

Mg

m 0.6 g

M 24.3 g/mol

n 0.025 mol

1202*0.025 = 30 kJ

Calculating standard enthalpy change

What is the enthalpy change in kJ per mole if 45kJ are given out when 0.8g of methane is burned?

What is the enthalpy change in kJ per mole if 1.6g of methanol is used to heat 200mL water from 20C to 38C?

Using temperature to calculate ∆Hᶱ

Heat energy = mass x specific heat capacity x temperature change

Q = mc∆T

How much energy is in a cracker?

1408kJ per 100grams

7 grams per cracker

How much will a cracker increase the temperature of 100mL of water when we burn it?

Calculating uncertainties

How do you calculate uncertainties when:

adding or subtracting variables?

multiplying or dividing variables?

Calculate the standard enthalpy change for the reaction between hydrochloric acid and sodium hydroxide

To calculate the uncertainty in the standard enthalpy for the reaction between zinc and copper sulphate...

absolute uncertainty in temperature change =

percentage uncertainty in temperature change =

percentage uncertainty in mass of liquid =

percentage uncertainty in specific heat capacity =

pecentage uncertainty in enthalpy =

percentage uncertainty in number of moles =

total percentage uncertainty =

total absolute uncertainty in standard enthalpy =

Neutralization- acid-base reaction

Write word and chemical equations for the following reactions:

hydrochloric acid + sodium hydroxide

sulphuric acid + sodium hydroxide

nitric acid + potassium hydroxide

Standard enthalpy of neutralization

H+ (aq) + OH-

(aq) H2O(l)

standard enthalpy change of neutralization for a strong acid and base is -55.90 kJ/mol

Bond enthalpy

The enthalpy change when one mole of bonds is formed in the gaseous state:

X (g) + Y(g) X-Y(g)

Forming bonds is exothermic (negative ∆H)

Breaking bonds is endothermic (positive ∆H)

Calculate a theoretical standard enthalpy of combustion of methane

Enthalpy of combustion

Bond Average bond enthalpy kJ/mol

C-C 347

C=O 746

C-H 413

O=O 498

O-H 464

C-O 358

Image: https://en.wikipedia.org/wiki/Burning

How does the theoretical standard enthalpy of combustion depend on the number of carbon atoms for the alkanes?

Compare your answers with experimental values.

Standard enthalpy of combustion of alkanes

C no. n name alkane ΔH

1 methane CH4 -890

2 ethane C2H6 -1560

3 propane C3H8 -2219

4 butane C4H10 -2877

5 pentane C5H12 -3509

6 hexane C6H14 -4163

7 heptane C7H16 -4817

8 octane C8H18 -5470

name alcohol ΔHcomb

methanol CH3OH -726

ethanol CH3CH2OH -1367

propan-1-ol CH3(CH2)2OH -2021

butan-1-ol CH3(CH2)3OH -2676

pentan-1-ol CH3(CH2)4OH -3329

hexan-1-ol CH3(CH2)5OH -3984

heptan-1-ol CH3(CH2)6OH -4638

Ammonia, NH3

Draw a Lewis diagram of the molecule

State the bond angles and shape

State whether the N-H bonds are polar, and whether the molecule is polar

Calculate the standard enthalpy of formation from data booklet data

Standard enthalpy change of formation ΔHfᶱ

the standard enthalpy change when a compound is formed from its elements

CH4 -74.4kJmol-1

CO2 -393.5kJ/mol-1

H2O -285.8kJ/mol-1

Formation of ammonia

N2 + 3 H2 2 NH3

N N triple bond 944kJmol-1

H-H 436 kJmol-1

N-H 388 kJmol-1

DH = (cost of bond breaking) – (gained for bond forming) =

( 944 + 3*436) – (2*3*388) = -76 kJmol-1Exothermic

Hess’s Law

conservation of energy applied to chemistry

total enthalpy change is the same whatever the route of a chemical reaction

Combustion of carbon to form Carbon dioxide

C + ½ O2 CO DH1 = -110kJmol-1

CO +½ O2 CO2 DH2 = -283kJmol-1

C + O2 CO2 DH3 = DH1+DH2 =

Plan investigation…

Links

Ionic bonding http://www.teachersdomain.org/asset/lsps07_int_ionicbonding/

Covalent bonding http://www.teachersdomain.org/asset/lsps07_int_covalentbond/