5. Energetics SL

STANDARD TEMPERATURE AND PRESSURE- 298 K (25º C) and 101.3 kPA

Define the terms exothermic reaction, endothermic reaction, and standard enthalpy change of a reaction

Standard enthalpy change of a reaction (ΔH)- The change in the amount of energy that exists within a system

Endothermic- Reaction in which energy goes INTO a system from the surroundings (ΔH+)

Exothermic- Reaction in which energy EXITS a system into the surroundings (ΔH-)

State that combustion and neutralization are exothermic processes.

Combustion Reaction: Reaction between a hydrocarbon and oxygen to produce water vapor and carbon dioxide (Exothermic Process)

Neuralization Reaction between an acid and a base that produces water and a salt (Exothermic Process)

Apply the relationship between temperature change, enthalpy change, and the classification of a reaction as either endo- or exothermic

	Temperature	ΔH	Product Stability	Product Energy	Bond Strength
Endothermic	Decrease	Increase	Less	More	Less
Exothermic	Increase	Decrease	More	Less	More

Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction.

Energy Diagram for Endothermic Process looks like:

ΔH IS POSITIVE

Energy Diagram for Exothermic Process looks like

Calculate the heat energy change when the temperature of a pure substance is changed

Equation for Calculating Heat Enthalpy: q = mcΔt

q = Enthalpy change

m = mass

c = specific heat capacity

Δt = temperature change

Practice: The temperature of a 2.0 g sample of aluminum (Specific heat = 0.90 J g^-1K^-1) increases from 25 degrees C to 30 degrees C. How many joules of heat were added

q = mc Δt

q = 2.0 g * 0.90 J g^-1K^-1 * 5 K

q = 1.8 * 5

q = 9.0 J

Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water AND

Evaluate the results of experiments to determine enthalpy changes

Practice: The heat released from the combustion of 0.0500 g of phosphorous increases the temperature of 15.00 g of water from 25.0 °C to 31.5 °C. Calculate the ΔH of phosphorus in kJ mol^-1

*TAKE NOTE: When doing experiments which involve temperature change of water, mass refers to the mass of the water which is changing temperature

q = mc Δt

q = (15.00 g) (.0418 kJ g^-1K^-1) (6.5 K)

q = 4.0755 kJ

(This gives is the ΔH for 0.0500 grams of P)

4.0755 = 81.5 kJ mol^-1

0.0500

Design suitable experimental procedures for measuring the heat energy changes of reactions

Include the following:

• Known mass of solution in an insulated container (coffee cup, bomb calorimeter, etc…)
• Temperature measured continuously with thermometer
• Substance burned beneath it, transferring heat energy to the solution
• This creates a change in temperature. Use mcΔT to find the energy

In experiments, the results are not always accurate due to error from heat loss.

Determine the enthalpy change of a reaction that is the sum of two or three reactions with known enthalpy changes

Hess’ Law: In a chemical reaction, it takes the same amount of energy to go from Compound A à C as it does to go through intermediate steps from A à B à C.

EXAMPLE!!! Find the amount of energy needed to undergo the following reaction using the equations and Enthalpy change values listed

Define the term average bond enthalpy

Average Bond Enthalpy- the AVERAGE amount of energy needed to break one mole of bonds in the gaseous state. Because this is an AVERAGE value, it is not always reliable

Explain in terms of average bond enthalpies, why some reactions are exothermic and others are endothermic

When bonds are broken: Energy is absorbed [into the system]

When bonds are made: Energy is released [out of the system]

Trick to remember this: BREAKING IN and MAKING OUT

Endothermic reactions result when Energy going in < Energy coming out. It takes more energy to break the bonds than make the bonds, so energy goes INTO the system (creating a positive value)

Exothermic reactions result when Energy going in > Energy coming out. It takes less energy to break the bonds than make the bonds, so energy LEAVES the system (creating a negative value)

Equation for figuring out average bond enthalpy:

Total bonds = Bonds broken – Bonds made

(This is the one exception to the Products – Reactants Rule. Remember Breaking in and making out!)

EXAMPLE!!! What is the ΔH reaction for the following equation in kJ?

CS₂(g) + 3O₂(g) à CO₂(g) + 2SO₂(g)

Energy to make bonds: CS₂ = 110kJ, CO₂ = 390 kJ, SO₂ = 580 kJ

CS₂(g) + 3O₂(g) à CO₂(g) + 2SO₂(g)

1 CS₂                         1 CO₂      2 SO₂

110 * 1                       390 * 1   290 * 2             

   110                           390          580

       Total bonds = Bonds broken – bonds made

            Total =     110 – (  970)

                 Total = -860 kJ

Follow this link for good practice questions (PART A-D)
http://coffman.dublin.k12.oh.us/teachers/teacherpages/brown/mr._browns_chemistry_pages/Topic_5_files/practiceenergetics.pdf