chem unit 3 knowledge

What is chemical energy of a substance

the sum of potential and kinetic energy

What is the change in H value for exothermic reactions

H < 0

What is the change in H value for endothermic reactions

H > 0

Activation energy

the minimum amount of energy required to break the bonds of reactants

Thermochemical equations

show the energy released or absorbed during a chemical reaction

What is energy measured in

joules (J) or kilojoules (kJ)

What does the H value correspond to

it corresponds to the mole quantities specified by the coefficients in the equation

What happens when the coefficients of an equation change

the change in H changes by that factor

What happens to the H value when the equation is reversed

the sign of the H value is reversed

What increases the rate of reaction

• concentration of solutions

• pressure of gases

• surface area of a solid reactant

• the temperature

• using a catalyst

Open system

A system in which matter and energy can be exchanged with the surroundings.

Closed system

only energy is exchanged with the surroundings

Irreversible reactions

when reactants form products which cannot be converted into reactants (only occurs in one direction)

reversible reactions

the products can react to reform reactants (occurs in both directions)

Examples of reversible systems

• evaporation and condensation of water

• saturated sugar solution

• oxygen transport in blood

• synthesis of ammonia

activation energy of reverse reaction

equilibrium arros

show that the reactions can occur both forwards and reverse

equilibrium

• when the forward and reverse reactions proceed at the same rate

• the concentrations of the reactants and products no longer change

what happens during dynamic equilibrium

• the reaction is incomplete and all substances are present in the equilibrium mixture

• bonds are constantly being broken and formed as the reactants and products continue to be converted from one to another

extent of reactions

describes how much product is formed when the system reaches equilibrium

What is the position of equilibrium

the relative amounts of reactants and products

What can change the position of equilibrium

• adding/removing a reactant or product

• changing the pressure (via changing the volume of a sealed container)

• dilution (via adding water)

• changing the temp

What allows maximum equilibrium yield of a product

controlling the reaction conditions via moving the position of equilibrium to the right

Le Chatelier's Principle

if an equilibrium system is subject to change the system will adjust itself to partially oppose the effect of the change

What happens when something changes the position of equilbrium

the system will shift to counteract the change and establish a new position of equilibrium

What is the pressure of a gas ... to the volume

inversely proportional

How can the pressure of a gas be changed

via changing the volume (at constant temperature)

What can affect the pressure

the number of particles in an equation (mole ratio)

Gas Pressure

measure of the force per unit area

is proportional to the number and frequency of collisions with the side of the container

Which way will the equilibrium shift regarding pressure

will move in the direction of the fewer particles in order to equalize

What happens after the system adjusts to the change in pressure

there is a change in concentration of each species until equilibrium is reached

What happens when you reduce volume

it increases the pressure meaning gas molecules are closer and collisions will become more frequent

What happens when more products form due to the rate of the forward reaction increasing

the rate of the reverse reaction increases and the rate of the forward reaction decreases

What isn't affected by pressure changes

systems in the liquid or solid phase

What happens when there are equal number of particles (to the equilibrium)

there is no shift

What happens when a system is diluted

reduces the number of particles hence decreases the concentration

the concentration of each species will lower - a net reverse reaction will occur

What happens when there is a temp increase

increases the energy of the substances in the system - more collisions

In an exothermic reaction what does an increase in temp result in

a net reverse reaction (fewer products)

decreasing holds opposite effect

In an endothermic reaction what does an increase in temp result in

a net forward reaction (more products) for endothermic reactions

decreasing holds opposite effect

What effects does a catalyst have

• lowers the activation energy of the forward and reverse reactions

• increases the rate of reaction for the forward and reverse equally

• does o=not change the position of equilibrium

• increases the rate at which equilibrium is attained

What is the Kc

equilibrium constant

[products] / [reactants]

What do the Kc values mean

Kc = 1 then [products] = [reactants]

Kc < 1 then [reactants] > [products]

Kc > 1 then [products] > [reactants]

What happens to the Kc when the reaction is reversed

1 is divided by Kc

What happens to the Kc when the products and reactants are doubled

Kc value is squared

What happens to the Kc when the products and reactants are ahlved

Kc is square rooted

What does Kc mean

indicates the extent of reaction at equilibrium (how far the forward reaction proceeds before equilibrium is establish)

What is Kc affected by

temperature only

What is Kc not affected by

• addition of products or reactants

• changes in pressure

• use of catalyst

What happens to the Kc in exothermic reactions

the Kc decreases hence the amount of products present decreases

What happens to the Kc in endothermic reactions

the Kc increases and the amount of products increases

What is the reaction quotient

the concentration ratio at a point during the reaction (not equilibrium)

What happens to the Qc and Kc at equilbrium

Qc = Kc

What can Qc be used for

to predict how the reaction will shift as it approaches equilibrium

If Qc > Kc

the equilibrium shifts R->L to increase [reactants]

If Qc<Kc

the equilibrium shifts L->R to increase [products]

What is the formula for Qc

What is a redox reaction

an equation showing the movement of electrons from one molecule to another

oxidation

loss of electrons

reduction

gain of electrons

What happens when electrons are gained

they appear as reactants

What happens when electrons are lost

they appear as products

Why is an electron lost/gained

Dependent on the species' charge:

• if it is a positive charge it will lose electrons

• if it is a negative charge it will gain electrons

OIL RIG

Oxidation

Is

Loss

Reduction

Is

Gain

Steps for writing redox equation

1. split equation into half equations

2. balance half-equations with electrons

3. add half equations together and minus electrons

4. balance final equation

What can't oxidation and reduction do

cannot occur independtly of one anotehr as they occur simultaneously