7.3 ENERGY CHANGES IN REACTIONS - Lyndhurst School ...

4/23/14 Chapter 7: Chemical Reactions 1

7.3 ENERGY CHANGES IN REACTIONS n  Vocabulary: Chemical energy, exothermic reaction,

endothermic reaction

n  Objectives: n  Describe the energy changes that take place

during chemical reactions. n  Classify chemical reactions as exothermic or

endothermic. n  Explain how energy is conserved during

chemical reactions.


CHEMICAL BONDS AND ENERGY n  Heat produced by propane grill is form of energy

n  Chemical equation for combustion of propane, includes “heat” on right side of equation

C3H8 + 5O2 à 3CO2 + 4H2O + Heat Propane


CHEMICAL BONDS AND ENERGY n  chemical energy - energy stored in chemical

bonds of a substance

n  chemical reactions involve breaking of chemical bonds in reactants & formation of chemical bonds in products n  requires energy


BREAKING BONDS n  that’s why propane grills have igniter à

produces spark à provides enough energy to break bonds of reacting molecules & get reaction started

n  heat & light given off by a propane stove result from formation of new chemical bonds


EXOTHERMIC AND ENDOTHERMIC REACTIONS n  During a chemical reaction, energy is either

released or absorbed


EXOTHERMIC REACTIONS n  exothermic reaction - a chemical reaction

that releases energy to its surroundings

n  energy released as products is > energy required to break bonds in reactants

n  e.g. combustion C3H8 + 5O2 à 3CO2 + 4H2O + 2220kJ

Propane


ENDOTHERMIC REACTIONS n  endothermic reaction - a chemical reaction that

absorbs energy from its surroundings

n  more energy is required to break bonds in reactants than is released by formation of products

n  e.g. decomposition of mercury (II) oxide 2HgO + 181.7 kJ à 2Hg + O2


CONSERVATION OF ENERGY n  law of conservation of energy - total amount

of energy before and after the reaction is the same


7.4 REACTION RATES n  Vocabulary: reaction rates, catalyst

n  Objectives: n  Interpret chemical equations in terms of reactants,

products, and conservation of mass. n  Balance chemical equations by manipulating

coefficients. n  Convert between moles and mass of a substance

using molar mass. n  Calculate amounts of reactants or products by using

molar mass, mole ratios, and balanced chemical equations.


REACTIONS OVER TIME n  TNT explosion happens almost instantaneously n  Reactions causing tree leaves to change color in

autumn happens gradually

n  reaction rate - rate at which reactants change into products over time n  tells how fast a reaction is going


FACTORS AFFECTING REACTION RATES n  Chemical reactions involve collisions between

particles

n  Reaction rate depends on how often these collisions collide

n  Almost any reaction rate can be changed by varying the conditions under which the reaction takes place


FACTORS AFFECTING REACTION RATES n  Factors that affect reaction rates include:

n  temperature n  surface area n  concentration n  stirring n  catalysts


FACTORS AFFECTING REACTION RATES: TEMPERATURE n  Generally increase in temperature will increase

reaction rate n  increases particle movement & therefore

collisions


FACTORS AFFECTING REACTION RATES: SURFACE AREA n  increase in surface area increases exposure

of reactants to one another

n  greater exposure à more collisions there are that involve reacting particles à more particles will react


FACTORS AFFECTING REACTION RATES: STIRRING n  Stirring reactants generally increases

reaction rate e.g. washing machine


FACTORS AFFECTING REACTION RATES: CONCENTRATION n  concentration refers to # of particles in a

given volume

n  more reacting particles present in a given volume à more opportunities for collisions involving those particles à faster reaction rate


FACTORS AFFECTING REACTION RATES: CATALYSTS n  catalyst - substance that affects reaction rate

without being used up in reaction

n  use to speed up reactions or to enable a reaction to occur at a lower temperature


FACTORS AFFECTING REACTION RATES: CATALYSTS n  e.g. reaction between sulfur dioxide & oxygen

very slow. To speed up reaction add catalyst such as vanadium oxide

2SO2 + O2 → 2SO3

V2O5


FACTORS AFFECTING REACTION RATES: CATALYSTS n  For reaction to take place, reacting particles must

collide with enough energy to break the chemical bonds of particles. Catalyst lowers energy barrier.


7.5 EQUILIBRIUM n  Vocabulary: equilibrium, reversible reaction.

n  Objectives: n  Interpret chemical equations in terms of reactants,

products, and conservation of mass. n  Balance chemical equations by manipulating

coefficients. n  Convert between moles and mass of a substance

using molar mass. n  Calculate amounts of reactants or products by using

molar mass, mole ratios, and balanced chemical equations.


TYPES OF EQUILIBRIA n  Equilibrium (pl equilibria) - state in which the

forward and reverse paths of a change take place at the same rate


PHYSICAL EQUILIBRIUM n  What happens when you pour some water into a jar

and then close the lid?



and then close the lid?



and then close the lid? n  Some of water undergoes a physical change by

evaporating n  As more water evaporates, some of water vapor

condenses & eventually rate of evaporation = rate of condensation (equilibrium).

H2O(l) H2O(g)

n  When a physical change does not go to completion, a physical equilibrium is established between a forward & reverse change.

liquid gas


CHEMICAL EQUILIBRIUM n  Most reactions are reversible to some extent. n  reversible reaction - a reaction in which the

conversion of reactants into products and conversion of products into reactants can happen simultaneously

2SO2(g) + O2(g) 2SO3(g)

n  When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward & reverse reactions.


FACTORS AFFECTING CHEMICAL EQUILIBRIUM n  Can change depending on conditions of reaction. While

reaction rate decreases or increases in response to change, equilibrium shifts (favors either forward or reverse reaction).

n  When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change.

Synthesis of ammonia N2(g) + 3H2(g) 2NH3(g) + Heat

n  Predict how system will be affected by changes in temperature, pressure, and concentration.


FACTORS AFFECTING CHEMICAL EQUILIBRIUM: TEMPERATURE


n  Heat written as product so fwd reaction is exothermic. n  In reverse, heat is a reactant so decomposition of

ammonia is endothermic. n  What would happen if you increased temperature of

system? n  Equilibrium would shift in direction that removes heat

from the system and therefore it would favor the reverse reaction, which is endothermic. So by increasing the temperature you would decrease the amount of ammonia.


FACTORS AFFECTING CHEMICAL EQUILIBRIUM: PRESSURE


n  If increased pressure of system, equilibrium would shift in direction that decreases pressure of system

n  In order to decrease pressure, system would favor reaction that produces fewer gas molecules.

n  So by increasing pressure, you would shift the equilibrium to the right, producing more ammonia.

4 gas molecules 2 gas molecule


FACTORS AFFECTING CHEMICAL EQUILIBRIUM: CONCENTRATION


n  Suppose you removed ammonia from system. n  Equilibrium would shift in direction that produces

ammonia. n  In order to produce ammonia, the system would favor the

forward reaction.

4 gas molecules 2 gas molecule


CHEMICAL CALCULATIONS: CONVERTING MASS TO MOLES n  How much oxygen would you need to make 144

grams of water? n  Start with balanced chemical equation 2H2 + O2 à 2H2O n  Step 1: Determine how many moles of water you are

trying to make. Answer 144 g n  Step 2: Use molar mass of water to convert mass

into moles. 1 mol H2O

144.0 g H2O x 18.0 g H2O = 8.00 mol H2O


CHEMICAL CALCULATIONS: USING MOLE RATIOS n  Step 2: Write conversion factors, or mole ratios. 1 mol O2 2 mol H2O 2 mol H2O 1 mol O2

Ratio on left allows you to convert moles of water to moles of oxygen.

n  Step 3: Calculate how many moles of oxygen are required to produce eight moles of water.

1 mol O2

8.00 mol H2O x 2 mol H2O = 4.00 mol O2


CHEMICAL CALCULATIONS: CONVERTING MOLES TO MASS n  Step 4: Convert moles of O2 to grams of O2 by using the molar mass of O2 as a conversion factor. 32.0 g O2

4.00 mol O2 x 1 mol O2 = 128 g O2 Answer: In order to produce 144 grams of H2O you must supply 128 grams of O2.


REVIEW: n  What is the law of conservation of mass? n  Why do chemist use the mole as a counting unit? n  Ammonia, NH3 can be made by reacting nitrogen

with hydrogen. N2 + 3H2 à 2NH3

How many moles of NH3 can be made if 7.5 moles of H2 react with enough N2.

7.3 ENERGY CHANGES IN REACTIONS - Lyndhurst School ...

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