UNIT 3

Review

How can energy changes be represented in chemical reactions?

Thermochemical equations with energy term beside the equatione.g. N2(g) + 2O2(g) → 2NO2(g) ΔHr = +66.4 kJ

• Thermochemical equations with energy term as product or reactantse.g. N2(g) + 2O2(g) + 66.4 kJ → 2NO2(g)

Enthalpy diagrams

UNIT 3 Section 5.2

What to use when

Chapter 5: Energy Changes

When the problem is about:

A solid dropped into water (like the copper penny expt):

Use: Qreleased = Q gained where Q= mc∆T

If it is as above but the container c and m is given use:

Q released by reaction = Q gained by water + Q gained by container

If the question is asking for molar enthalpy of reaction, and gives c and m, then calculate Q first (which is ∆H) and then use ∆H = n∆H

If it is about enthalpy in aqueous solutions, use C= n/V and if as above it is a solid and molar enthalpy is needed, then you need: n=m/M

UNIT 3 Section 5.3

Hess’s Law

- calculating the enthalpy change of reactions using existing data- When using calorimetry is impractical

The enthalpy change of any reaction can be determined if:

• the enthalpy changes of a set of reactions “add up to” the overall reaction of interest

• standard enthalpy change, ΔH°, values are used

UNIT 3 Section 5.3

For example

Chapter 5: Energy Changes

To find the enthalpy change for formation of SO3 from O2 and S8, you can use

UNIT 3 Section 5.3

Techniques for Manipulating Equations

1.You can reverse an equation

• the products become the reactants, and reactants become the products

• the sign of the ΔH value must be changed

2.You can multiply each coefficient

• all coefficients in an equation are multiplied by the same integer or fraction

• the value of ΔH must also be multiplied by the same number

UNIT 3 Section 5.3

Standard Molar Enthalpies of Formation

Chapter 5: Energy Changes

Often used for Hess’ Law is

standard molar enthalpy of formation, ΔH˚fthe change in enthalpy when 1 mol of a compound is synthesized from its elements in their most stable form at SATP conditions

• enthalpies of formation for elements in their most stable state under SATP conditions are set at zero

• since formation equations are for 1 mol of compound, many equations include fractions (for a balanced eq’n)

UNIT 3 Section 5.3

Formation Reactions and Thermal Stability

Chapter 5: Energy Changes

The thermal stability of a substance is the ability of the substance to resist decomposition when heated.

• decomposition is the reverse of formation

• the opposite sign of an enthalpy change of formation for a compound is the enthalpy change for its decomposition

• the greater the enthalpy change for the decomposition of a substance, the greater the thermal stability of the substance

UNIT 3 Section 5.3

Using Enthalpies of Formation and Hess’s Law

Chapter 5: Energy Changes

So the enthalpy of a reaction = the sum of all the products – the sum of all the reactants. For example:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Determine ∆H˚r for the following reaction

using the enthalpies of formation that are provided.

UNIT 3 Section 5.3

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)

∆H˚f of C2H5OH(l): –277.6 kJ/mol∆H˚f of CO2(g): –393.5 kJ/mol∆H˚f of H2O(l): –285.8 kJ/mol

TRY THIS

∆H˚r = [(2 mol)(∆H˚f CO2(g)) + (3 mol)(∆H˚f H2O(l))] – [(1 mol)(∆H˚f C2H5OH(l)) + (3 mol)(∆H˚fO2(g)]

Section 5.3UNIT 3 Chapter 5: Energy Changes

∆H˚r = [(2 mol)(–393.5 kJ/mol) + (3 mol)(–285.8 kJ/mol)] – [(1 mol)(–277.6 kJ/mol) + (3 mol)(0 kJ/mol)]

∆H˚r = (–1644.4 kJ) – (–277.6 kJ) ∆H˚r = –1366.8 kJ

UNIT 3 Section 5.4

5.4 Energy Efficiency and Energy Resources

Chapter 5: Energy Changes

Energy efficiency can be calculated using the equation:

UNIT 3 Section 5.4

Using Energy Efficiently

TO PREVIOUS SLIDE

Chapter 5: Energy Changes

Energy use distribution in Canadian homes

A challenge in the development of energy efficient technology is to find ways to best convert energy input into useful forms.

For example, efficiency of appliances:• conversion of input of electrical energy versus output of

energy usually all that is considered• but should also consider efficiency

of the source of the electricity

UNIT 3 Section 5.4

Conventional Energy Sources in Ontario

TO PREVIOUS SLIDE

Chapter 5: Energy Changes

The distribution of energy sources in Ontario

The three main sources of electrical energy in Ontario:

• nuclear power plants

• power plants that burn fossil fuels (natural gas and coal)

• hydroelectric generating stations

UNIT 3 Section 5.4

Alternative Renewable Energy Sources in Ontario

TO PREVIOUS SLIDE

Chapter 5: Energy Changes

Renewable energy sources in Ontario:

• account for about 25% of energy production

• are projected to increase to as high as 40% by 2025

• include hydroelectric power (major source), wind energy (currently ~ 1% and projected to 15% in 2025), and solar energy (currently low but may be as high as 5% in 2025). Much lower contributors are biomass, wave power, and geothermal energy.

UNIT 3 Section 5.4

What Is a “Clean” Fuel?

TO PREVIOUS SLIDE

Chapter 5: Energy Changes

Different fuels have differing impacts on the environment. One way this impact is measured is through emissions.

For example, CO2(g) emissions per kJ of energy produced.

Fuel kg CO2/ kJ energy

Anthracite coal 108.83

Oil 78.48

Natural gas 56.03

Nuclear 0.00

Renewables 0.00