Chem SAC 3

heterogeneous equilibrium

- concentration of pure solid or liquid is assigned one - the equilibrium constant for heterogeneous is therefore much simpler

collision theory

According to collision theory for a reaction to occur, the reactant particles must Collide with each other Collide with sufficient energy to break the bonds within reactants Collide with the correct orientation to break the bonds within the reactants and so allow the formation of new products If a collision does not meet all these requirements, then no reaction occurs.

The position of equilibrium may be changed by

Adding or removing a reactant or product Changing the pressure by changing the volume (for equilibria involving gases) Dilution (for equilibria in solution) Changing the temperature

irreversible reactions

All of the reactants have been converted into products or either one reactant has been in excess When the products cannot be converted back into reactants, a reaction is considered to be irreversible

increasing the energy of collisions

An increase in temperature not only increases the frequency of collisions, it also increases the kinetic energy of the particles and hence the energy of their collisions

heterogeneous catalysts

Are in different physical state from the reactants and products of the reaction

homogeneous catalysts

Are in the same physical state as the reactants and products of the reaction

explaining equilibrium

As a reaction proceeds the concentration of reactants used will decrease, so the frequency of collisions between molecules decreases and the rate of the production of the products decrease At the same time the product is being formed and they collide and decompose to reform the reactants. Eventually the forward and reverse reactions proceed at the same rate. When this situation is reached, the products are formed at exactly the same rate as it is being broken down. The concentrations of all molecules remains constant In a closed system at constant temperature and temperature, no further change will take place. The reaction has reached a point of balance - an equilibrium. Chemical equilibrium can be described as being in a dynamic state because the forward and reverse reactions have not ceased. Instead, they occur simultaneously at the same rate.

effect of temp on Equilibrium constant

As temperature increases For exothermic reactions, the value of Kc decreases and so the amount of products present at equilibrium decreases For endothermic reactions, the value of Kc increases and so the amount of products presents at equilibrium increases

maxwell boltzmann distribution

At any particular temperature, the particles in a substance have a range of kinetic energies. Although most of the particles have similar kinetic energies, there are always some particles with a high energy or a low energy. This range of energies is shown on a Maxwell-Boltzmann distribution curve (aka kinetic energy distribution diagram) Only a small proportion of the reactant particles have kinetic energy that is equal to or greater than the activation energy and so are able to react. The area under the curve is equal to the total number of particles in the sample - it stays constant when the temperature is changed.

define carbon monoxide

Carbon monoxide is a colourless, odourless and tasteless gas that is formed as a product in the incomplete combustion of fuels. The high toxicity of carbon monoxide is a result of its reaction with haemoglobin

catalysts

Catalysts are able to cause a reaction to occur more quickly because they provide a new reaction pathway, which causes the activation energy barrier of the overall reaction to be dramatically reduced. The colliding particles are therefore more likely to have energies that exceed this lower barrier, causing the bonds in the reactants to be broken more frequently. As a result, a greater proportion of collisions are successful, and reaction rate is increased.

catalysts in industry

Chemists prefer to use heterogeneous catalysts for the industrial process because they are More easily separated from the products of a reaction Much easier to reuse Able to be used at high temperatures

dilution

Dilution by adding water reduces the number of particles per volume. This results in a shift in the position of equilibrium towards the side that produces the greater number of dissolved particles The addition of water momentarily lowers the concentration of each species.

equilibrium law

Equilibrium constant Kc is the concentrations of products divided by the concentrations of reactants at equilibrium Index of each component concentration is the same as the coefficient for the substances in the balanced chemical equation

treatment for carbon monoxide poisoning

Give the person pure oxygen This is an attempt to shift the equilibrium between oxyhaemoglobin and haemoglobin. Addition of oxygen in this equilibrium should shift the reaction in the net forward direction. However the large value of the carboxyhemoglobin equilibrium constant and the relatively slow rate of release of carbon monoxide means that this treatment is not always successful

reaction quotient compared to Kc

Greater than Kc the system shifts to the left to achieve equilibrium and more reactants are formed Smaller than Kc the system shifts to the right to achieve equilibrium and more products are formed Equal to Kc the system is at equilibrium

define haemoglobin

Haemoglobin is a large protein molecule that is the pigment in red blood cells. It is responsible for the transport of oxygen from your lungs to the cells in your body. The haemoglobin complex combines with oxygen to form an equilibrium system with oxyhaemoglobin

the haber process

Hydrogen and nitrogen molecules both adsorb onto the iron surface. As they attach themselves to the surface, the covalent bonds within their molecules break. The hydrogen and nitrogen atoms now readily combine to form ammonia molecules and move away from the iron surface. (catalyst is unaltered)

equilibrium constant rules

If one equation is the reverse of another, the equilibrium constants are the inverse (or reciprocal) of each other If the coefficients of an equation are doubled, the value of Kc is squared If the coefficients of an equation are halved, the value of Kc is the square root of the original value of Kc

open and closed system

In an endothermic reaction, the system absorbs energy from the surroundings In an exothermic reactions, energy is released to the surroundings Open systems are the most common. In an open system, matter and energy can be exchanged with the surroundings. In contrast, a closed system only exchanges energy with the surroundings

rate considerations

Increased when Temperature is higher Catalyst is present The partial pressures of the gaseous reactants are higher (higher pressure overall)

the meaning of an equilibrium constant

It indicates the extent of reaction at equilibrium (how far the forward reaction proceeds before equilibrium is established) and the equilibrium yield (the amount of products present at equilibrium)

adding catalyst

Lowers the activation energy of the forward and reverse reactions by the same amount. This causes an increase in the number of effective collisions As a result, there is an increase in the rate of both forward and reverse reactions. This occurs because more particles have energies greater than the activation energy barrier of the reaction A catalyst increases the rate of the forward and reverse reactions equally. Therefore, it will not change the relative concentrations of the reactants and products in the equilibrium law expressions. The presence of a catalyst does not change the position of equilibrium or the value of the equilibrium constant. However a catalyst will increase the rate at which an equilibrium is attained. It is for this reason that catalysts are used in many industrial and biological systems.

experimentally determining rate

Need to measure how much of a reactant is being up or how much of a product is being formed in a given time period In graphs (conc vs time) the steeper the gradient the faster the rate of reaction

carbon monoxide poisoning

OXYHAEMOGLOBIN + CARBON MONOXIDE → CARBOXYHEMOGLOBIN + OXYGEN The equilibrium constant for the reaction between carbon monoxide and haemoglobin is nearly 20 000 times greater than for the reaction between oxygen and haemoglobin. This means the forward reaction is more likely to occur and so even small concentrations of carbon monoxide shift the position of equilibrium well to the right The formation of carboxyhemoglobin reduces the concentration of haemoglobin, causing the reverse reaction of oxyhaemoglobin formation to occur. The reactions of oxygen and carbon monoxide with haemoglobin are described as competing equilibria, because both oxygen and carbon monoxide compete for the same substance. The equilibrium reaction with the larger equilibrium constant has a significant effect on the extent of reaction of the other reaction

application of catalysts

Particles at the surface of some solids of high surface area tend to adsorb (form a bond with) gas molecules that strike the surface. Adsorption distorts bonds in the gas molecules or may even break them completely. This allows a reaction to proceed more easily than it would if the solid were absent . These solid surfaces provide a new way for the reaction to occur (a new reaction pathway) that has a significantly lower activation energy A powdered or sponge like form of a solid catalyst is often used to provide the greatest possible surface area. With a larger surface area, more reactant molecules can be adsorbed and the reaction is even faster.

orientation of colliding molecules

Reacting molecules must also collide with each other in the correct orientation in such a way that particular bonds in the reactants are broken and new bonds are formed in the products. If the collision orientation is incorrect the particles simply bounce of each other, and no reaction occurs

reversible reactions

Reactions that can occur in both the forward and reverse directions. Once some products are formed, collisions between product particles can result in the reactants being re-formed Reversible reactions in a closed system eventually reach a situation where the rate of the forward reaction and the rate of the reverse reaction are equal, the system is described as having reached equilibrium

general La Chatelier's principle

States that if an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change

factors that can change the rate of a chemical reaction

Surface area of solid reactants Concentration of reactants in a solution Gas pressure Temperature The presence of catalysts

rate of reaction

The change in concentration of a reactant or product per unit time. The usual unit for a rate of reaction is M/s.

conflicts in chemical manufacturing

The conditions for a high reaction rate must also be optimised to ensure that a chemical plant is operating at full capacity A temperature increase will increase the reaction rate but decrease the product yield of an exothermic reaction because of the effect of temperature on the equilibrium of the system. In such situation moderate temperature and a catalyst is used to increase the rate to obtain satisfactory yields High pressures that favour rapid reaction will sometimes give ow equilibrium yields of product, depending on the equilibrium reaction

percentage yield

The mass of product that is expected to be formed if all reactants react fully according to the equation is known as theoretical yield and it can be calculated using stoichiometry The actual yield obtained from industrial processes is often less than the theoretical yield because of Formation of an equilibrium A slow rate of reaction Loss of reactants and products during transfers between reaction vessels and in separation and purification stages The percentage yield compares the actual yield to the theoretical yield. It tells you how much product is formed in a particular reaction or process. The greater the value of the percentage yield, the greater is the degree of conversion from reactants to products.

define activation energy

The minimum energy that a collision must possess for a reaction to occur is called the activation energy When the energy of a collision is equal to or greater than the activation energy, a reaction can occur.

Increasing frequency of collisions

The rate of a reaction increases as the frequency of collisions increases. The frequency of collisions between reactants can be increased by: - Increasing the concentration of the reactants. Collisions occur more frequently when particles are closer together - Increasing the surface area of a solid reactant

changing concentration or pressure on rate

The rate of a reaction increases when the frequency of collisions between reactants increases. When the concentration of a solution increases, there are more reactant particles moving randomly in a given volume of solution. The frequency of collisions consequently increases and so more successful collisions occur in a given time. - can be done by adding more gas or decreasing volume of container

activation energy and reaction rate

The size of the activation energy determine how easy it is for a reaction to occur and therefore what proportion of collisions result in a successful reaction. Therefore reaction rate dependant on activation energy.

adding inert gas

The total pressure of an equilibrium mixture of gases may be changed by adding a non-reacting gas such as helium,neon or argon Because the presence of the additional gas does not change any of the concentrations of the reactants and products, there is no effect on the position of equilibrium or the equilibrium constant

changing surface area on rate

When a solid is involved in a reaction, only the particles at the surface area of the solid are involved in the reaction. The number of particles at the surface depends on the surface area of the substance. The greater number of exposed particles, the frequency of collisions between these particles and other reactant particles increases, and so the reaction occurs more rapidly.

define transition state

When activation energy is absorbed, a new arrangement of the atoms known as the transition state occurs. The transition state occurs at the stage of maximum potential energy in the reaction, the activation energy. Bond breaking and forming are both occurring at this stage and the arrangement of atoms is unstable. Atoms in the transitions state rearrange into the products as the reaction progresses

explaining reversibility of reactions

When particles collide the energy associated with collisions can break bonds in the reacting particles, allowing them to rearrange to form new products. The energy required to break the bonds of the reactants is known as the activation energy of the reaction. If the newly formed product particles collide with enough energy to break their bonds (equal to the activation energy of the reverse reaction)

effect of temp on rate of reaction

When the temperature increases, the increase in reaction rate due to the increased energy of the collisions (number of particles with sufficient energy to overcome Activation energy)significantly outweighs the increase in reaction rate due to the increased frequency of collisions

pressure with equal number of particles on either side

When there are equal numbers of reactant and product particles, a change in pressure will not shift the position of equilibrium. It does not matter which way the system shifts; the number of particles in the container will remain constant.

Breathing

When you inhale, oxygen from the air combines with haemoglobin in the small blood vessels in the lining of your lungs. According to Le Chatelier's principle, this increase in concentration of oxygen in this environment will cause a net forward reaction, producing greater amounts of oxyhaemoglobin. In your lungs, most haemoglobin is converted to oxyhaemoglobin as a result of the continual addition of oxygen from each breath you take

the reaction quotient

While the reaction quotient can be calculated for any mixture of reactants and products at any time during a reaction, it is only when the mixture is at equilibrium that it gives a constant value. At equilibrium, the value of the reaction quotient is equal to the equilibrium constant Kc

If volume is halved --> Kc impact

becomes 0.5 x Kc momentarily CF is now lower than Kc

Kc value

between 10^-4 and 10^4 -->the extent of reaction is significant - appreciable concentrations of both reactants and products are present at equilibrium >10^4 --> almost complete reaction occurs <10^4 --> Negligible reaction occurs

When Kc is large

he numerator of the equilibrium expression must be large compared to the denominator, which means there is a large amount of products relative to the amount of reactants

when Kc is small

the numerator of the equilibrium expression must be very small compared to the denominator, which means there must be a large amount of reactants relative to the amount of products

during dynamic equilibrium

the reaction in 'incomplete' and all of the substances are present in the equilibrium mixture At the molecular level, bonds are constantly being broken and new bonds are being formed as the reactants and products continue to be converted from one to another