Chemical Reactions and Energy; Basic Thermodynamics Section 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

Energy that is transferred from an object at high temperature to an object at lower temperature. Measured in Joules (J) - 1 kilojoule (kJ) = 1000J MOLE = A unit of measure in a chemical reaction that contains the same number of particles

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)

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

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

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 Δ H reaction = Δ H products - Δ H reactants

Is…

2H 2 (g) + O 2 (g) → 2H 2 O(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? 2H 2 O(l)  2H 2 (g) + O 2 (g) ∆H = +572 kJ If only 1 mole of water produced? 572kJ = 2 x 286 kJ 286kJ

and-exothermic-reactions.html#lesson and-exothermic-reactions.html#lesson

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

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.

∆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

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.

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. thermodynamics-entropy-and-systems.html#lesson thermodynamics-entropy-and-systems.html#lesson

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.

1)Decrease 2)Decrease 3)Increase 4)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: SO 2(g) + H 2 O (l) H 2 SO 4(aq)