U3 Aos 2 Notes (1).docx

RATE OF CHEMICAL REACTIONS Chemical reactions with reference to collision theory, including qualitative interpretation of MaxwellBoltzmann distribution curves. 

Chemical reactions occur as a result of collisions between the reacting particles.



Particles collide and are rearranged to produce new particles.



The collision theory of reactions explains why some collisions result in reactions and others do not.



COLLISION THEORY: for a reaction to occur, the reactant particles must

- collide with each other -collide with enough energy to break the bonds within the reactants -collide with the right orientation to break the bonds within the reactants and so allow the formation of new products. 

A reaction can only occur if it meets all requirements stated in the collision theory.



The minimum energy a collision must have for a reaction to occur is known as the activation energy. This is denoted by Ea .



The energy of a collision must be equal or greater than the activation energy for a reaction to occur.



The transition state occurs when the activation energy is reached/ at the maximum potential energy of the reactant particles.



At the transition state, bonds break and form.



At this stage the arrangement of atoms is unstable.



As the reaction progresses the atom rearrange to form the products and thus the arrangement becomes stable.



Maxwell-Boltzmann distribution curves show the range of energies of the particles in a substance.



this curve is also known as a kinetic energy distribution diagram.



the vertical axis represents the number of particles with kinetic energy.



the horizontal axis represents the kinetic energy.



The area under the graph is always the same because the number of particles is remains the same.



only a small portion of the reactant particles have kinetic energy equal of more than the activation energy and so are able to react.



increasing the temperature increases the collision and thus the energy of particles.



Increasing the temperature increases the portion of particles with energies equivalent of greater than the activation energy of the reaction.

The comparison of exothermic and endothermic reactions including their enthalpy changes and representations in energy profile diagrams. 

Chemical energy is composed of kinetic and potential energy.



Enthalpy is the heat content of a substance under constant pressure.



thermochemistry is the study of energy and heat changes associated with chemical reactions.



∆Hc is the heat of combustion

- ∆H= H(products) – H(reactants) - its units are kJ mol-1 

exothermic reactions involve the release of energy in the form of heat.

-the ∆H is negative -enthalpy of the products is more than the enthalpy of the reactants. - the chemical energy of the products are lower than the chemical energy of the reactants. - the activation energy is lower than the enthalpy of the reaction. 

endothermic reaction is the absorption of energy in the form of heat

-the ∆H is positive -enthalpy of the products is less than the enthalpy of the reactants. -the chemical energy of the products are more than the chemical energy of the reactants. - the activation energy is more than the enthalpy of the reaction. 

energy profile diagrams are energy by reaction progress graph



The heat of combustion of a substance is for exactly one mole of the substance undergoing combustion.



Thermochemical equations include a chemical reaction and ∆H value.



The ∆H value in a thermochemical equation depends on the number of moles of the substance being combusted as indicated by the reaction equation.

Factors affecting the rate of a chemical reaction including temperature, surface area, concentration of solutions, gas pressures and presence of a catalyst. 

Factors the can change the rate of a chemical reaction include:



- surface area of solid reactants



-concentration of reactants in a solution



- gas pressure



- temperature



- the presence of catalysts



A larger surface area increases the rate of a reaction since there is more contct between reactant particles.



smaller particles have a larger surfaces area than larger particles of the same mass.



Smaller particles react much faster.



higher concentrations lead to faster reaction rates.



increasing the pressure of a gas reactant increases the rate of reaction.



increasing the pressure at a constant temperature brings the particles closer together and increases the number of successful collisions.



increasing the pressure increases the concentration as the same mass of particles take up a smaller space.



Volume is inversely proportional to pressure.



For gas particles pressure is directly proportional to concentration.



Volume is inversely proportional to concentration. V=n/c



an increase in temperature increases the number of collisions and thus the rate of a reaction.



catalysts are substances which are added to chemical reactions to speed up the reaction process.

 

Adding an inert gas will decrease the frequency of collisions. Adding an inert gas does not affect the frequency of successful collisions.

The role of catalysts in changing the rate of chemical reactions with reference to alternative reaction pathways and their representation in energy profile diagrams. 

Catalysts are not consumed during the reactions they speed up.



catalysts are neither reactants nor products.



catalysts increase the rate of a reaction as they provide an alternative reaction pathway.



this pathway causes the activation energy of the reaction to reduce dramatically.



the activation energy changes but the ∆H of the reactants and products does not.



Homogenous catalysts are in the same physical state as the reactants and products of the reaction



heterogeneous catalysts are in a different physical state as the reactants and products of the reaction.

EXTENT OF CHEMICAL REACTIONS The distinction between reversible and irreversible reactions, and between rate and extent of a reaction. 

Open systems exchange energy and matter with the surroundings.



Closed systems only exchange energy with the surroundings.



Irreversible reactions are reactions with products that cannot be converted back to the reactants.



In reversible reactions the products once formed, can react to reproduce the reactants.



a double arrow (↔) is used to show the equation of a reversible reaction.



reversible reactions can only occur in a closed system.



A reaction reaches equilibrium when the rate of the forward reaction equals the rate of the backwards reaction.



as both the forward and backwards reaction occurs, this state is known as the dynamic equilibrium.



at this stage the concentration of the reactants and products remain constant.



equilibrium can only be achieved in closed systems.



reversible reactions can reach a state of equilibrium where the overall concentrations of reactants and products do not change over time.



if the newly formed product particles of a reaction collide with enough energy to break their bonds then it is possible to reproduce the reactants.



as the concentration of reactants decrease, the concentration of products increase.



as more products are being reduced, there is a greater chance of the reactants being formed.



as the concentration of the reactants decreases, the rate of the forward reaction increases.



as the concentration of the products increases, the rate of the backward reaction increases.



different reactions proceed to different extents.



the extent of a reaction does not give any information about how fast a reaction will proceed.



the extent of reaction indicates how much product is formed once the system is at equilibrium.



the ratio of reactants to products are different for different equilibrium systems.



the rate of reaction is a measure in the change in concentration of the reactants and products with time and is not directly related to the extent of the reaction.

Homogenous equilibria involving aqueous solutions or gases with reference to collision theory and representation by balanced chemical or thermochemical equations (including states) and by concentration-time graphs.



When a reaction reaches equilibrium, the quantities and concentrations of the reactants and products in the reaction remain unchanged.

   

When a reaction reaches equilibrium, the rate of their forward and revere reactions are equal. a homogenous equilibrium is when the physical state of all the substances in the reaction are the same. Equilibrium can be achieved in closed systems but not open sys tems. When an equilibrium reaction has all its species in the same physical state, it is known as a homogenous equilibrium.



If the species of an equilibrium reaction are in different physical states, the reaction is known as a heterogeneous equilibrium.

Calculations involving equilibrium expressions and equilibrium constants (Kc only) for a closed homogeneous equilibrium system including dependence of value of equilibrium constant, and its units, on the equation used to represent the reaction and on the temperature 

the equilibrium law states that: - the equilibrium constant is the concentration of products divided by the concentrations of reactants at equilibrium. - the index of each component concentration is the same as the coefficient for the substances in the balanced chemical equation.



Kc= [products]coefficients / [reactants]coefficients= reaction quotient at equilibrium



the unit is M or mol L-1 and depends on the reaction equation



for chemical reactions at equilibrium different chemical reactions have different equilibrium constants (Kc ) .



the size of the equilibrium constant indicates the proportions of reactants and products in a mixture.



for a particular reaction, the equilibrium constant is constant for all equilibrium mixtures at a fixed temperature.



by comparing the two values Kc and Qc , one can indicate which way a reaction should progress to reach equilibrium.



if the reaction quotient (Qc):



- is greater than Kc, more products are formed



- is less than Kc, more reactants are formed



- is equal to Kc, the system is at equilibrium.



the equilibrium constant of a reaction depends on its equation, so if:



- an equation is the reverse of another, the equilibrium constants are the reciprocal of each other.



- the coefficients of an equation are doubled, the value of Kc is squared.



- the coefficients of an equation are halved, the value of Kc is the square root of its original value.



the equilibrium constant of a reaction solely depend on the temperature.



for an exothermic reaction, while the temperature increases the Kc decreases. (forward reaction)



for an endothermic reaction, while the temperature increases the Kc increases. (reverse reaction)

 

Dilution of an aqueous equilibrium does not affect the equilibrium constant of a mixture. Adding an inert gas does not affect the position of equilibrium or the value of Kc.

Le Chatelier’s principle: Identification of factors that favour the yield of a chemical reaction, representation of equilibrium system changes using concentration-time graphs and applications, including completing equilibria involved in the occurrence and treatment of carbon monoxide poisoning resulting from incomplete combustion of fuels.



The mass of the product that is expected to be formed if all the reactants react fully according to the equation is known as the theoretical yield.

  

The theoretical yield can be calculated using stoichiometry. The actual yield is the amount product actually produced. The actual yield obtained is often less than the theoretical yeidl because of - the formation of an equilibrium - a slow reaction rate - loss of reactants or products during transfers. The percentage yield compares the actual yield to the theoretical yield. The percentage yield shows how much product is formed in a particular reaction or process. The greater the value of the percentage field, the greater the degree of conversion from reactants to products. Vice versa A high percentage yield is desired Percentage yield can be calculated using the formula: actual yield/theoretical yield x 100 The more steps for a process, the more the actual yield differs from the theoretical yield.

     

  

Le 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. The position of equilibrium is the relative amounts of reactants and products at equilibrium.



the position of equilibrium can be changed by:



- adding or removing a reactant or product



- changing the pressure by changing the volume



- changing the temperature



- dilution



These changes can increase the amount of reactants or products.



The position of equilibrium is different to the equilibrium constant, which can only be changed by changing temperature.



When a change occurs to the equilibrium system, it is momentarily not at equilibrium until a net reaction occurs to counteract the effect of that change, changing the position of equilibrium.



By adding a reactant, more products form and the equilibrium position shifts to the right of a concentration-time graph.



By adding a product, more reactants form and the equilibrium position shifts to the left of a concentration-time graph.



By removing a product, more products form and the equilibrium position shifts to the right of a concentrationtime graph.



Increasing the volume causes the reaction to shift in the direction of the fewest particles. Decreasing the volume causes the reaction to shift in the direction of the most particles. Volume is inversely proportional to pressure. Increasing temperature results in an endothermic reaction, to oppose an increase in energy. Increasing the temperature results - in a net reverse reaction for an exothermic reaction - a decrease in Kc Increasing temperature results in - net forward reaction for an endothermic reaction - an increase in Kc

   





Haemoglobin is a large protein molecule that is the pigment in red blood cells.



The haemoglobin complex combines with oxygen to form an equilibrium system with oxyhaemoglobin: haemoglobin + oxygen ↔ oxyhaemoglobin.



The high toxicity of carbon monoxide results in its reaction with haemoglobin:



haemoglobin + carbon monoxide ↔ carboxyhaemoglobin.



The equilibrium constant of this reaction is 20 000 times greater than the reaction of oxygen and haemoglobin, making the forward reaction more likely to occur.



the formation of carboxyhaemoglobin decrease the concentration of haemoglobin, causing the reverse reaction of oxyhaemoglobin to take place.



carbon monoxide poisoning occurs when there is almost not oxyhaemoglobin left in the body.



carbon monoxide symptoms include drowsiness, dizziness, headaches, shortness of breath and loss of intellectual skills.



concentrations of carbon monoxide as low as 220 ppm can lead to loss of consciousness and even death.



treatment for carbon monoxide poisoning would be to provide the patient with pure oxygen.



providing pure oxygen allows the shift of equilibria between oxyhaemoglobin and haemoglobin.



adding the oxygen should shift the reaction in the net forward direction and increase the concentration of oxyhaemoglobin.



this treatment is not always successful due to the slow rate of release of carbon monoxide and the large Kc of carboxyhaemoglobin.

PRODUCTION OF CHEMICAL BY ELECTROLYSIS Electrolysis of molten liquids and aqueous solutions using different electrodes. 

Electrolysis involves the passing of electrical energy from a power supply through a conducting liquid, to produce chemical potential energy.



The electrical energy triggers redox reactions to occur that are usually non-spontaneous.



The conducting liquid is known as an electrolyte.



Reactions occur at the surface of both electrodes.



The power supply provides the electrical energy required for the reactions to occur.



The power supply pushes electrons onto one electrode and withdraw electrons from the other electrode.



The negative electrode is connected to the negative terminal of the supply, where reduction occurs.



The cations attract to the negative electrode, which is also known as the cathode.



The positive electrode is connected to the positive terminal of the supply, where oxidation occurs.



The anions attract to the positive electrode, which is also known as the anode.



In the non-spontaneous reactions that occur in electrolysis, the electrical energy converts into chemical energy in the products of the process.



The reverse reactions of the non-spontaneous reactions are spontaneous.



Unlike galvanic cells, due to non-spontaneous reactions, the electrodes of the electrolysis cells can stay in one container.



As the products form, they need to be kept separate from one another to prevent a spontaneous reaction between the two.



There can be several chemicals present at each electrode that can react.



The potential reactants are the strongest oxidant and the strongest reductant.

The general operating principles of commercial electrolytic cells, including basic structural features and selection of suitable electrolyte (molten or aqueous) and electrode (inert or reactive) materials to obtain desired products (no specific cell is required). 

Reactive electrodes are electrodes that are consumed in the cell reaction during electrolysis,



Inert electrodes do not take part in the cell reaction.



The electrolytic cell can be used to produce substance in commercial quantities.



To obtain substances in commercial quantities, commercial cells can use - aqueous or molten electrolytes - reactive or inert electrodes.



The cathode is always inert, meaning the cathode does not take part in the cell reaction.



Most of the electrodes in commercial electrolytic cells are inert.



The design features of industrial/commercial electrolytic cells must consider the - chemical nature of the reactants and products - physical conditions under which electrolysis takes place - energy requirements to operate the cell.



Using a molten electrolyte can be disadvantageous as the electrolysis process will require more energy than when using an aqueous electrolyte.



In the electrolysis of a molten electrolyte, water is absent so it cannot interfere with the desired reactions.



In the electrolysis of a molten electrolyte, reduction occurs at the anode, the positive electrode.



In the electrolysis of a molten electrolyte, oxidation occurs at the cathode, the negative electrode.



Aqueous electrolytes are used preferably to molten electrolytes.



Aqueous electrolytes are more cost-effective.



A membrane cell is placed in the middle of the two reactants to prevent their contact.



The membrane cell is made up of a polymer that only allows cations to pass through it.



The polymer in the membrane cell prevents the products that form from mixing at the electrodes.



Using an aqueous electrolyte limits the cost as the electrolysis process can occur at a lower temperature.

The use of the electrochemical series to explain or predict the products of an electrolysis, including identification of species that are preferentially discharged, balanced half-equations, a balanced ionic equation for the overall cell reactions, and states. 

To predict the products of the electrolysis and the overall equation of the reaction in the electrolysis cell, take the following steps: 1. Identify which species are present in the solution. 2. Identify what the electrodes are made of. 3. Refer to the electrochemical series and identify the possible reactions. 4. Write the half equations of the reaction in the order they appear in the electrochemical series. 5. Determine the reactions that could occur at the anode. 6. Determine the most likely reaction at the anode, which is the strongest reductant. 7. Determine the reactions that could occur at the cathode. 8. Determine the most likely reaction at the cathode, which is the strongest oxidant. 9. Combine the two half-equations to obtain the overall reaction in the electrolysis cell, adding the states. 10. State the products of the overall reaction.



The comparison of an electrolytic cell with a galvanic cell with reference to the energy transformations involved, and basic structural features and processes. 

The following table summarises the differences between galvanic and electrolytic cells:

The application of stoichiometry and Faraday's Laws to determine amounts of product, current or time for a particular electrolytic process.



Faraday's first law states that the mass of a metal produced at the cathode is directly proportional to the electrical charge passed through the cell.



m∞Q



Q=I x t



Faraday's second law of electrolysis is that for a mole of a metal, 1, 2, 3 or another whole number of moles of electrons must be consumed, deposited, evolved or dissolved.



The charge on I mole of electrons must be 96 500 C.



1F = 96 500 C, which is known as I faraday.



Q= n(e-) x F

RECHARGEABLE BATTERIES The operation of rechargeable batteries (secondary cells) with reference to discharging as a galvanic cell and recharging as an electrolytic cell, including the redox principles (redox reaction and polarity of electrodes) and the factors affecting battery life with reference to components and temperature (no specific battery is required). 

Rechargeable batteries are also known as multiple secondary cells.



The batteries are designed to be used several times.



To recharge a cell, the cell reaction must occur in the reverse, as in the products of the reaction must convert back to the reactants.



The products must stay in contact with the electrodes for the reverse cell reaction to occur.



During discharge, the cell acts as a galvanic cell.



During discharge, chemical energy is converted to electrical energy, as done in a galvanic cell.



The anode is at the negative terminal and the cathode is at the positive terminal.



During recharge, the cell acts as an electrolytic cell.



During recharge, electrical energy is converts to chemical energy, as done in an electrolytic cell.



The anode is at the positive terminal and the cathode is at the negative terminal.



The rechargeable battery connects to a charger that supplies electrical energy.



The positive terminal of the charger connects to the positive electrode.



The negative terminal of the charger connects to the negative electrode.



The battery life of a secondary cell describes its performance.



The battery life is the number of charge-discharge cycles before the battery can no longer be used.



Battery life decreases over time. Factors that can influence this include:

 Loss of active materials (reactants and products of cell reaction)  Progressive conversion of small crystals of active materials into larger crystals at the electrodes can

increase resistance to current flow.  Formation of other chemicals in side reactions that can affect the efficient functioning of the cell.  Impurities in cell materials which can react with active materials.  Decrease in contact of the electrolyte with electrodes.  Corrosion or failure of internal components.  Increase in temperature increases rate of deterioration and side reactions- decreasing the battery life.  Under cold conditions, batteries deliver less electric charge thereby decreasing the battery capacity.