ch 171 - chapter 8 (reaction rates & equilibrium) (finals)

Activation energy

the minimum amount of energy required to start a chemical reaction.

Catalyst

substance that speeds up the rate of a chemical reaction.

Activation energy

the minimum amount of energy required to start a chemical reaction.

Catalyst

substance that speeds up the rate of a chemical reaction.

Effective collision

A collision between reactant particles that results in a chemical reaction taking place.

Endergonic process

a process that gains or accepts energy as it takes place.

Entropy

a measure of the disorder of a system.

Equilibrium concentration

Concentration of reactants/products at equilibrium. The concentration of a substance (usually expressed as molarity) in a system that has reached the equilibrium state.

Equilibrium constant (Keq)

the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation (Keq). Temperature dependent!

Does Keq change with temperature

Yes, Keq changes with temperature. Increasing the temperature of a system will increase Keq for an endothermic reaction and decrease for an exothermic reaction.

Does Keq change with the addition of a product?

No, adding more of a product to a reaction at equilibrium will not change the value of the equilibrium constant. The reaction will shift though, in the direction opposite of where the reactant was added.

Equilibrium expression

the expression giving the ratio between the products and reactants. It is equal to the concentration of each product raised to its coefficient in a balanced chemical equation and multiplied together, divided by the concentration of the product of reactants to the power of their coefficients (Keq).

Exergonic process

a process that gives up energy as it takes place.

Heterogenous (surface) catalyst

a catalyst that exists in a different physical state than the reaction it catalyzes.

homogenous catalyst

a catalyst that is in the same phase as all the reactants and products in a reaction system.

Inhibitor

substance that slows down or stops a chemical reaction by reducing the activity of catalysts.

Internal energy

the sum of the kinetic and potential energies of all particles in the system.

Le Chatlier's Principle

states that a system at equilibrium will oppose any change in the equilibrium conditions.

Right. Towards the products to consume some of the added reactant.

If you increase the reactant concentration, in which direction will equilibrium shift?

Left. Towards the reactants to consume some of the added product.

If you increase the product concentration, in which direction will equilibrium shift?

Left. Toward the reactant to produce more of the missing reactant.

If you decrease the reactant concentration, in which direction will equilibrium shift?

Right. Toward the products to produce more of the missing product.

If you decrease the product concentration, in which direction will equilibrium shift?

Left. Toward the reactants because heat is a product in an exothermic reaction, and the system will try to absorb the extra heat.

If you increase the temperature for an exothermic reaction, what direction does equilibrium shift?

Right. Towards the products because heat is a reactant in an endothermic reaction, and the system will try to absorb more heat.

If you increase the temperate in an endothermic reaction, what direction does equilibrium shift?

Towards the products. Since temperature is decreasing, to counter this, the system will shift towards the formation of heat, and in exothermic reactions, heat is a product.

If you decrease the temperate in an exothermic reaction, what direction does equilibrium shift?

Left. Toward the reactants because the system will try to produce more heat.

If you decrease the temperate in an endothermic reaction, what direction does equilibrium shift?

Position of Equilibrium for negative Gibbs Free energy

If negative, spontaneous, produces more PRODUCTS.

Position of Equilibrium for positive Gibbs Free energy

If positive, not spontaneous, more REACTANTS.

Position of Equilibrium for Gibbs Free energy = 0

If around 0 - EQUILIBRIUM (entropy has max value).

Reaction mechanism

the step-by-step sequence of reactions by which the overall chemical change occurs.

Reaction rate

The change in concentration of reactants per unit time as a reaction proceeds.

Spontaneous process

A process that occurs without an overall input of energy; a process that is energetically favorable.

Stable substance

a substance that does not undergo spontaneous changes under the surrounding conditions.

State of equilibrium

exists when forward and reverse reactions occur at equal rates, and when the concentrations of products and reactants are CONSTANT.

Process spontaneity depends on what?

Energy and entropy changes that accompany the process.

What changes to energy and entropy favor spontaneity?

Energy decreases and entropy increases favor spontaneity.

The speed of a reaction is called what?

Reaction rate

How can reaction rate be determined?

By measuring how fast recents are consumed or products are formed.

Explanations of how reactions take place are called what?

They are called reaction mechanisms.

Most mechanism are based on what three assumptions?

Molecules must collide with one another. The collision must involve a certain minimum of energy. Some colliding molecules must be oriented in a specific way during collision in order to react.

What factors affect the rates of all reactions?

The nature of reactants. Reactant concentrations. Reactant temperature. The presence of catalysts.

What defines the position of equilibrium?

relative concentrations of products and reactants in a system.

What is the position of equilibrium indicated by?

The value of the equilibrium constant.

What factors influence the position of equilibrium?

Changes in the amount of reactants/products and changes in temperature influence the position of equilibrium.

Do catalysts change the position of equilibrium?

No! Catalysts have no affect on the position of equilibrium (they aren't consumed during a reaction).

What affects entropy?

Physical state and number of particles. More gas particles means more disorder. Gas is more disordered than a liquid or solid.

In which physical state do reactions occur the quickest?

Liquid

In which physical state do reactions occur the slowest?

Solid

What must concentration units be a HIGH SOLUTE CONCENTRATION for the calculation to work?

Molality, mol/kg

How does the presence of solutes affect water's vapor pressure? Will it increase or decrease?

The presence of solutes decreases vapor pressure because their presence OCCUPIES SPACE at the water's SURFACE, REDUCING the number of WATER MOLECULES able to EVAPORATE and ENETER the VAPOR phase.

Why do reaction rates decrease after longer reaction times?

As a reaction proceeds, the concentration) of reactants will decrease, and the frequency of collisions will decrease as a result. Reactions become saturated after a certain point, so the reaction rates will become stagnant and eventually decrease as the reaction time continues to increase.