UNIT 3 Review How can energy changes be represented in chemical reactions? Thermochemical equations with energy term beside the equation e.g. N 2 (g) + 2O 2 (g) → 2NO 2 (g) ΔH r = +66.4 kJ • Thermochemical equations with energy term as product or reactants e.g. N2(g) + 2O2(g) + 66.4 kJ → 2NO2(g) Enthalpy diagrams
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UNIT 3 Review How can energy changes be represented in chemical reactions? Thermochemical equations with energy term beside the equation e.g. N 2 (g) +
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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
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.