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# Ch 17: Reaction Rates - PowerPoint PPT Presentation

Ch 17: Reaction Rates. We define a rate as a change in a quantity divided by the change in time: rate = ∆quantity ∆time Examples of types of rates: Speed of a car Points scored in a game Hot dogs eaten in 5 minutes Pages printed by a printer in 1 minute.

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• We define a rate as a change in a quantity divided by the change in time:

rate = ∆quantity

∆time

Examples of types of rates:

• Speed of a car

• Points scored in a game

• Hot dogs eaten in 5 minutes

• Pages printed by a printer in 1 minute

• Chemicals reactions can also be measured in how they change over time.

• The reaction rate of a chemical reaction is the change in concentration (M, molarity) of a reactant or product per unit time (s, second).

• Given the chemical equation:

CO(g) + NO2(g)  CO2(g) + NO(g)

The average reaction rate of the formation of NO(g) over a period of time t1 and t2 would be

[NO]t2 - [NO]t1

avg reaction rate = t2 - t1

if t1 = 0.0s and t2 = 2.0s and initially there is no NO

and at 2.0s 0.010M NO forms,then:

• Reactions occur when molecules collide together

• The collision theory says that:

• atoms, ions, and molecules must collide in order to react.

• Reacting substances must collide with the correct orientation

• Reacting substances must collide with sufficient energy to form the activated complex

• The activated complex (or transition state) is a temporary, unstable arrangement of atoms that may form products or may break apart to reform the reactants.

• In order for a reaction to occur, the reacting atoms, ions, and molecules must have sufficient energy when they collide in the right orientation.

• The minimum amount of energy required for the activated complex to form and for the reaction to take places is called the activation energy (Ea).

• Although important in determining if a reaction will occur, this tells us little about the actual speed/rate of reaction.

• The compounds themselves

• Concentration

• Reactions speed up when the concentrations of reactants are increased. (Increasing the concentration increases the number of particles available to collide)

• Surface Area

• Reactions can occur faster when more area is exposed to take part in a reaction. (Increasing surface area increased the number of particles available to collide)

• Temperature

• Reactant particles require activation energy in order to react. Raising the temperature increases the average number of particles with sufficient activation energy, increasing the rate of reaction.

• A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed in the reaction.

• Essentially, a catalyst helps lower the activation energy of the reaction. This means that more collisions will then have sufficient energy to react.

• An inhibitor on the other hand is a substance that slows down reaction rates or prevents reactions from occurring at all.

• Given a chemical equation:

N2 + 3H2 2NH3

We say that the reaction goes to completion as all the reactants react and turn into products.

• But, sometimes, the reverse reaction can also occur:

2NH3  N2 + 3H2

• A reversible reaction is one that can occur in both the forward and reverse directions.

• Chemical equations that are reversible use a double arrow instead of the standard single arrow to show that it is reversible:

N2 + 3H2 2NH3

• In the beginning of the reaction, the forward and reverse reactions occur at different rates.

• Eventually, the rate of product formation in the forward reaction will be balanced by the decomposition of product in the reverse reaction. This is called chemical equilibrium.

• Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates

Rate forward reaction= Rate reverse reaction

• Equilibrium is a dynamic process. The reaction does not “stop” even though it seems like nothing is changing.

• Equilibrium of chemical reactions can “shift” in one direction or another.

• Le Châtelier’s Principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

• Increase reactants – forward reaction favored, making more product. The reaction “shifts to the right”

• Increase products – reverse reaction favored, making more reactants. The reaction “shifts to the left”

• Decrease reactants – reverse reaction favored, making more reactants. The reaction “shifts to the left”

• Decrease producs – forward reaction favored, making more products. The reaction “shifts to the right”

• Increase temperature – Endothermic reaction is favored. Reaction shifts towards the endothermic process

• Add catalyst – no effect