Chemical Reactions and Energy; Basic Thermodynamics Section 20.1

CHAPTER 20 Chemical Reactions and Energy; Basic ThermodynamicsSection 20.1

Objectives ~• Compare and contrast exothermic and

endothermic chemical reactions• Analyze the energy diagrams for typical chemical

reactions• Illustrate the meaning of entropy and trace its role

in various processes

What is heat?

Energy that is transferred from an object at high temperature to an object at lower temperature.

Measured in Joules (J) - 1 kilojoule (kJ) = 1000JMOLE = A unit of measure in a chemical reaction that contains the same number of particles

Endothermic and Exothermic Reactions

Energy changes that occur in a chemical reaction are a result of the bonds that are broken (reactants) and the bonds that are formed (products)

What happens in an Exothermic reaction?• If more energy is released in forming new bonds

than is required to break bonds, the reaction is exothermic

• If reactants go from higher energy to products at lower energy = exothermic

What happens in an Exothermic reaction?

What happens in an Endothermic reaction?• In an endothermic reaction, the energy required to

break the bonds of the reactants is greater than the energy released by forming the bonds of the products.

• If reactants go from lower energy to products at higher energy = endothermic

What happens in an Endothermic reaction?

ENTHALPY ∆H • Symbolizes the difference in the energy between

the products and reactants• At constant pressure, the enthalpy of a reaction

or system is the same as the heat that is gained or lost• Exothermic = -∆H• Endothermic = +∆H

ΔH reaction = ΔH products - ΔHreactants

Law of Conservation of Energy• Is…

Example

2H2(g) + O2(g) → 2H2O(l)

∆H = -572 kJ• In the synthesis of water, 2 moles of hydrogen gas

react with 1 mole of oxygen gas to produce 2 moles of liquid water. The reaction is exothermic and produces 572 kJ of energy for every 2 moles water produced. • What would the enthalpy of the decomposition be? • What is the enthalpy value for 1 mole of water produced?

What does decomposition look like?2H2O(l) 2H2(g) + O2(g)

∆H = +572 kJIf only 1 mole of water produced?

572kJ = 2 x 286 kJ286kJ

• http://education-portal.com/academy/lesson/endothermic-and-exothermic-reactions.html#lesson

Activation Energy• the amount of energy that particles in a reaction

must have when they collide in order for the reaction to occur. Figures 20.4 and 20.5

• https://www.youtube.com/watch?v=OttRV5ykP7A

Activation Energy

Catalyst• Recall that a catalyst will speed up the rate of a

reaction without being consumed• this occurs because the presence of a catalyst

lowers the activation energy of a reaction.

Entropy• ∆S a measure of disorder• The natural tendency of entropy is to increase.

MORE disorder is more favorable; Entropy is affected by temperature

• + ∆S means increasing disorder and is favorable• -∆S means increasing order and is not favorable

Spontaneity• Once activated, a reaction that uses some of its

released energy to continue is considered spontaneous

• When one considers ∆S and ∆H together, we know whether a reaction will be spontaneous or not

• Spontaneity depends on the balance between energy and entropy factors.

Example• An exothermic reaction with increasing disorder is

always spontaneous, BUT an endothermic reaction with increasing order is never spontaneous. (table 20.1 page 714)

• Are reactions that occur spontaneously at room temperature generally exo or endothermic?• Exothermic - Remember the tendency is to change from a

state of high energy to a state of low energy.

• http://education-portal.com/academy/lesson/second-law-of-thermodynamics-entropy-and-systems.html#lesson

Entropy – are these an increase or decrease of entropy?

1) Water in an ice-cube tray freezing?2) You pick up scattered trash along a highway and

pack it in a bag.3) Your campfire burns, leaving gray ashes.4) A cube of sugar dissolves in a cup of tea.

Answers

1) Decrease2) Decrease3) Increase4) Increase

• Which has greater entropy, a tablespoon of dry salt or a tablespoon of salt dissolved in water? Explain.

• The salt dissolved in water has greater entropy because it has a less orderly structure.

• Assuming that temperature does not change, would there be an increase or decrease in entropy in the following reaction:

• SO2(g) + H2O(l) H2SO4(aq)