Oxidation states are numbers where we can see what has been oxidised, and what has been reduced.
In a compound, each element has a oxidation number. If an element is single, it's number is zero. A positive number shows it has lost electrons and been oxidised, the more positive it is, meaning the more it has been oxidised, whereas a negative shows it has gained electrons and been reduced, the more negative it is being the more it has beeen reduced. Must have +/- sign.
There are some elements that always have the same oxidation states which will need to be remembered. Hence, I'm gonna type em out, becoz I'm silly.
Hydrogen, +1 (Except in metal hydrides where it;s -1)
Group 1's, +1
Group 2's, +2
Aluminium, +3
Oxygen, -2
Fluorine, -1
Chlorine, -1 (unles in a compound with F or O, where it is positive.)
Since all compounds (NOT IONS) are neutral, the sum of oxidation numbers will = zero.
In ions, the sum of the oxidation numbers will equal the charge. In a compound, the one with the most negative number is the most electronegative.
PCl5 = Cl is minus one, and in this, -5, therefore, P must be +5
NH3 = H is always +1, and in this, therefore, three, so N must be -3
HNO3 = O is -2, and therefore, -6, and H is +1, so therefore, to equal zero, N must be +5
H2S = Hydrogen is always +1, therefore, +2, so the sulphur must be -2
SO42- = Four O's = -8, therefore, S must = +6
CuO =O is -2, and therefore Cu is 2+
Cu2O = O is -2, so Cu must be +1
Saturday, 10 April 2010
Redox
Redox means that reduction and oxidation takes place.
Reduction is gain of electrons, Oxidation is loss of electrons.
Oxidation was used for reaction where oxygen was added. E.g.: Cu + 1/2O2 => CuO
The copper has been oxidised to gain oxygen to become coper oxide. (Oxygen is an oxydising agent.)
The reverse is reduction. This would be, in the example, the reaction that turns Copper Oxide back into Copper. CuO + H2 => Cu + H2O. Copper oxide has been reduced,as it has lost oxygen, and therefore, hydrogen, as it made it happen, is the reducing agent.
So : When oxygen is added, it's oxidation, and when oxygen is taken away it's reduction, meaning, when Hydrogen is added, it's reduction, and when Hydrogen is taken away, it's oxidation.
Redox always involves electron transfer, and we represent this with half equations.
Cu + 1/2O2 => CuO
Cu => Cu2+ + 2e-
1/2O2 + 2e- => O2-
Cu + 1/2O2 => (Cu2+ + O-)
CuO + Mg => MgO + Cu
Cu2+ + 2e- => Cu (it looses the oxygen,and is therefore, reduced = gain electrons)
Mg => Mg2+ + 2e- (It gains the oxygen, and is therefore, oxidised= loss electrons.)
Combine them : Cu2+ + Mg => Cu + Mg2+
Oxidisng agents always accept electrons.
Reducing agents always give away electrons.
Questions from book : 1) Ca + Br2 => (Ca2+ + 2Br-)
a) Bromine has gained electrons.
b) Calcium has lost electrons.
c) Calcium has been oxidised.
d) Bromine has been reduced.
e) Ca => Ca2+ + 2e-
Br2 + 2e- => 2Br-
Ca +Br2 => (Ca2+ + 2Br-)
f) The oxidising agent is Bromine, it takes electrons
g) The reducing agent is Calcium, as it gives away electrons.
Rawwwr. Thoughts?
Reduction is gain of electrons, Oxidation is loss of electrons.
Oxidation was used for reaction where oxygen was added. E.g.: Cu + 1/2O2 => CuO
The copper has been oxidised to gain oxygen to become coper oxide. (Oxygen is an oxydising agent.)
The reverse is reduction. This would be, in the example, the reaction that turns Copper Oxide back into Copper. CuO + H2 => Cu + H2O. Copper oxide has been reduced,as it has lost oxygen, and therefore, hydrogen, as it made it happen, is the reducing agent.
So : When oxygen is added, it's oxidation, and when oxygen is taken away it's reduction, meaning, when Hydrogen is added, it's reduction, and when Hydrogen is taken away, it's oxidation.
Redox always involves electron transfer, and we represent this with half equations.
Cu + 1/2O2 => CuO
Cu => Cu2+ + 2e-
1/2O2 + 2e- => O2-
Cu + 1/2O2 => (Cu2+ + O-)
CuO + Mg => MgO + Cu
Cu2+ + 2e- => Cu (it looses the oxygen,and is therefore, reduced = gain electrons)
Mg => Mg2+ + 2e- (It gains the oxygen, and is therefore, oxidised= loss electrons.)
Combine them : Cu2+ + Mg => Cu + Mg2+
Oxidisng agents always accept electrons.
Reducing agents always give away electrons.
Questions from book : 1) Ca + Br2 => (Ca2+ + 2Br-)
a) Bromine has gained electrons.
b) Calcium has lost electrons.
c) Calcium has been oxidised.
d) Bromine has been reduced.
e) Ca => Ca2+ + 2e-
Br2 + 2e- => 2Br-
Ca +Br2 => (Ca2+ + 2Br-)
f) The oxidising agent is Bromine, it takes electrons
g) The reducing agent is Calcium, as it gives away electrons.
Rawwwr. Thoughts?
Tuesday, 6 April 2010
Equilibria in industry. (:
For industrial processes which are reversible, such as the haber process, the yield is important, but this is not always the biggest consideration. Think of if you got the biggest yield at a low temperature, but this slowed the reaction way down. Or where the reaction runs, or how much it costs to increase factors like pressure, etc. The conditions often have to be compromised.
Ammonia
Used to make fertilisers, and synthetic materials. It is done at low temperature, as, as Le Chatillier's principle says, 'If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.', This means that if it is at low temperature, it is exothermic, so low temperatures move the equilibrium to the right, as heat is given out. The reaction is a high pressure, as high pressure forces the molecules to areas with less molecules, which is the side which ammonia is on (in the equation.)
Ethanol is made by hydrating ethene, which is a reversible reaction. Catalyst: phosphoric acid abosrbed on silica. Reactants + Products are all gasesous at the temperature used. It is used at a high pressure, as this will force the equilibrium to the right, as it has fewer molecules. (Kinda like it's looking for more room.) A low temperature forces the equilibrium to the right, to give out heat. excess steam forces it to the right to reducethe steam concentration,however, steam will dilute the catalyst. A low temperature will make it loads slower, costs, and high pressure polymerises.
Methanol: a chemical feedstock. Uses a copper cataylst. Also at low temperature and high presure, because low temperature forces equilibrium to the right, as its exothermic and gives out heat, and high pressure forces the equilibrium to the side with less molecules, (right).
Thoughts?
Ammonia
Used to make fertilisers, and synthetic materials. It is done at low temperature, as, as Le Chatillier's principle says, 'If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.', This means that if it is at low temperature, it is exothermic, so low temperatures move the equilibrium to the right, as heat is given out. The reaction is a high pressure, as high pressure forces the molecules to areas with less molecules, which is the side which ammonia is on (in the equation.)
Ethanol is made by hydrating ethene, which is a reversible reaction. Catalyst: phosphoric acid abosrbed on silica. Reactants + Products are all gasesous at the temperature used. It is used at a high pressure, as this will force the equilibrium to the right, as it has fewer molecules. (Kinda like it's looking for more room.) A low temperature forces the equilibrium to the right, to give out heat. excess steam forces it to the right to reducethe steam concentration,however, steam will dilute the catalyst. A low temperature will make it loads slower, costs, and high pressure polymerises.
Methanol: a chemical feedstock. Uses a copper cataylst. Also at low temperature and high presure, because low temperature forces equilibrium to the right, as its exothermic and gives out heat, and high pressure forces the equilibrium to the side with less molecules, (right).
Thoughts?
Equilibria
Reactants are usually thought of as Reactants => Products, but some reactions are reversible, for instance, the Haber Process. If in a closed container, in somereactions, as soon as the products, are formed, they react together again and form the reactants, so eventually, there will be a mixture of reactants /and/products. = Equilibrium mixture.
If water was in a closed container, it would begin to evaporate, the volume of gas increasing, liquid, descreaing proportionally. And then, some gas molecules would start to re-enter the liquid and condense. Eventually evaporating and condensing gets equal. This is Dynamic Equilibrium. (Dynamic, because things are changing and moving, and Equilibrium because values are staying the same.)
Equilibrium conditions:
(:
If water was in a closed container, it would begin to evaporate, the volume of gas increasing, liquid, descreaing proportionally. And then, some gas molecules would start to re-enter the liquid and condense. Eventually evaporating and condensing gets equal. This is Dynamic Equilibrium. (Dynamic, because things are changing and moving, and Equilibrium because values are staying the same.)
Equilibrium conditions:
- Can only be reached in a closed system, so like, a tub. With a lid on. (Or a beaker if it takes place in a solvent. Apparently. I don't get that. Kersplain?)
- Equilibrium can go from either direction and the final equilibrium position will be the same. (Like if the was either water in a tub, or the same amount of water vapour, it would be the same.)
- It is dynamic. Reached when the two processes are the same.
- Equilibrium is reached when the macroscopic properties do not change. Macroscopic meaning visible.
- You have products. Products are reacting. At the begining, the concentration of products is therefore, low.
- As it continues, the concentration of products increases. As there is an increase in concentration, the rate of the reserve reaction speeds up.
- A point is reached where the same amount of reactants > products are going on as the amount of products > reactants.
- This is called dynamic equilibrium
(:
Catalysts.
Catalysts affect the rate of chemical reactions without being chemically changed themselves. They speed up the reactions, as it is cheaper then increasing temperature or pressure, as it is inexpensive as it is not used up.
They provide a different pathway for the reaction, which has lower activation energy. Activation energy is the miniumum amount of energy needed to start the reaction. When the activatation energy is lowered, more particles have the same or greater energy then that level, so more particles react, and the rate is faster.
Cataylsts are in two categories. Heterogenous, and Homogenous. Heterogenous is where the catalyst and reactant are in different phases/states. So, like, a solid catalyst, and liquid reactants. Or a solid catalyst and gas reactants. Homogenous are when they are in the same phase/state. Easy way to remember this, if you can't is to associate them with sexuality (a bit unconventional, but I guess it does the trick, right?) Heterosexual (diff gender - diff phase/state), and Homosexual (same gender - same phase/state) ?
Petrol engine cars have catalytic converters, which reduce the level of polluting gases made. Platinum and Rhodium are the catalysts. They are in a honeycomb shape, which gives a large surface area, on which the reactions take place. It turns nitrogen oxides and carbon monoixde into nigtrogen and carbon dioxide, and hydrocarbons and nitrogen oxides into nitrogen, cardon dioxide and water. The gases make weak bond with the metals. This holds the gases in the right position to react, and then the products are 'released'. The metals, which haven't reacted can then be re-used when these products shift out the way. :D ^_________________^
Zeolites - open structure, allow molecule to fit in it. It means, because they're in small spaces, and, as they're in small spaces, it changes their structure and reactivity. They are used in the petrochemical industry as catalysts.
In hardening fats, a Nickel catalyst is used, which allows the manufacturer to tailor the spreadability of the margerine.
They provide a different pathway for the reaction, which has lower activation energy. Activation energy is the miniumum amount of energy needed to start the reaction. When the activatation energy is lowered, more particles have the same or greater energy then that level, so more particles react, and the rate is faster.
Cataylsts are in two categories. Heterogenous, and Homogenous. Heterogenous is where the catalyst and reactant are in different phases/states. So, like, a solid catalyst, and liquid reactants. Or a solid catalyst and gas reactants. Homogenous are when they are in the same phase/state. Easy way to remember this, if you can't is to associate them with sexuality (a bit unconventional, but I guess it does the trick, right?) Heterosexual (diff gender - diff phase/state), and Homosexual (same gender - same phase/state) ?
Petrol engine cars have catalytic converters, which reduce the level of polluting gases made. Platinum and Rhodium are the catalysts. They are in a honeycomb shape, which gives a large surface area, on which the reactions take place. It turns nitrogen oxides and carbon monoixde into nigtrogen and carbon dioxide, and hydrocarbons and nitrogen oxides into nitrogen, cardon dioxide and water. The gases make weak bond with the metals. This holds the gases in the right position to react, and then the products are 'released'. The metals, which haven't reacted can then be re-used when these products shift out the way. :D ^_________________^
Zeolites - open structure, allow molecule to fit in it. It means, because they're in small spaces, and, as they're in small spaces, it changes their structure and reactivity. They are used in the petrochemical industry as catalysts.
In hardening fats, a Nickel catalyst is used, which allows the manufacturer to tailor the spreadability of the margerine.
Maxwell Boltzmann Distribution. :D
Yea, in a bit they'll be a tonnnneeeee of unit one bio stuff on it's way for all yous retaking. But fo' nowwww...
A Maxwell-Boltzmann distribution curve expresses the 'energy' against the 'fractions of particles that have that energy', because in a solution all particles are moving at different speed, and, as speed and energy are relative to each other, there are varying energies.
It tells us that no particles have zero energy, and also, that as the curve peaks to the middle, intermidiate levels of energy is the most frequent. The right hand side of the graph is small, which means that not many have high energies.
With the Ea (Activation energy) marked on the graph, the area under the curve to the right of this, is the amount of particle with enough energy to overcome it. (And then like, react.)
Fuels are safe at room temperature, but a spark can provide energy for them to react. Match heads do not combust without friction. Nor lighters, come to think of it.
When a Maxwell-Boltzmann curve is affected by temperature, the curve shape changes. With higher temperatures, the curve gets shorter and fatter, and the amount of particles meeting the Ea increases. At lower temperatures, the curve gets taller and thinner, so like, nearer to the axis on the right? So, at high temperatures, more particles have equal or higher energy to the Ea, whereas at lower temperatures, less do.
A Maxwell-Boltzmann distribution curve expresses the 'energy' against the 'fractions of particles that have that energy', because in a solution all particles are moving at different speed, and, as speed and energy are relative to each other, there are varying energies.
It tells us that no particles have zero energy, and also, that as the curve peaks to the middle, intermidiate levels of energy is the most frequent. The right hand side of the graph is small, which means that not many have high energies.
With the Ea (Activation energy) marked on the graph, the area under the curve to the right of this, is the amount of particle with enough energy to overcome it. (And then like, react.)
Fuels are safe at room temperature, but a spark can provide energy for them to react. Match heads do not combust without friction. Nor lighters, come to think of it.
When a Maxwell-Boltzmann curve is affected by temperature, the curve shape changes. With higher temperatures, the curve gets shorter and fatter, and the amount of particles meeting the Ea increases. At lower temperatures, the curve gets taller and thinner, so like, nearer to the axis on the right? So, at high temperatures, more particles have equal or higher energy to the Ea, whereas at lower temperatures, less do.
Collision Theory, etc.
Kinetics- how factors affect chemical reactions. Lots of variations, lots of reasons.
For a reaction to happen, particles have to collide. This collision must have the energy to break the bonds, and must take place between the parts that are actually going to react. For many collisions, a high concentration of particles in a small space is ideal. They also need to move quick, to collide with enough force.
According to collision thoery;
Did that make any sense? xP
Many factors affect the rate of reaction.
If there's a large activation energy, it takes a lot to get the ball to the top of the hill, to start rolling down, but if its a lower activation energy it takes less to get it to the top of the hill to get it rolling down. The rolling down part being the reaction.
In endothermic reactions, the energy of the products is higher then the energy of the reactants, as endothermic reactions take /in/ heat, and therefore energy, so there is going to be more.
For a reaction to happen, particles have to collide. This collision must have the energy to break the bonds, and must take place between the parts that are actually going to react. For many collisions, a high concentration of particles in a small space is ideal. They also need to move quick, to collide with enough force.
According to collision thoery;
Most collisions between molecules or other particles do not lead to reaction. They either do not have enough energy, or are in the wrong orientation.This makes a fair amount of sense. If the particles did not have enough energy, the eA could not be reached and the reaction could not take place, as bonds could not be broken to form products. (eA being 'Activation Energy'). Also, orientation wise, as stated above, the collsion must take place between the parts of the molecule that are actually going to react, therefore, if must be orientated the right way, or, in english, pointing itself in the right direction. Kind of makes me think of car accidents. If the collision was represented as someone getting hit by a car, and the car hit them on the shoulder 'bond' this wouldn't be likely to break their leg... 'bond'.
Did that make any sense? xP
Many factors affect the rate of reaction.
- Temperature - it increases the kinetic energy of the particles. This increase the speed with which they move, which increases the amount of force (and in turn, energy) the collide with, and also the amount of collisons, as they are moving quicker.
- Increasing the concentration of a solution - it means there are more particles, and that means that collisions are more likely to happen. As a reaction goes on, the reactants are used up, which means (as the concentration drops) the rate slows down as the reaction happens.
- Increasing surface area of solid reactants - this give more of a surface of which to react, so the reaction will take place quicker. At first, I found this a tad confusing. But, thinking of it like baking cakes kinda helped. If you're baing cakes, you'll get more made in a bigger oven then a small one, right? So, if the cakes are reactants, and the oven is the solid reactant, if it's bigger, you can make more cakes. And everyone likes cakes. ^_____^
- Anyway, increasing the pressure - it forces more molecules into a give volume, so, like increasing the concentration, there is a natural increase in collsions, which means more chance of sucessful collsions, and more reactions taking place quicker.
- Catalysts - They lower the activation energy of the reaction by providing an alternate pathway. This means that, with a lower activation energy more particles with be able to colide with /that/ amount of energy to break the bonds, so the reaction will happen faster.
If there's a large activation energy, it takes a lot to get the ball to the top of the hill, to start rolling down, but if its a lower activation energy it takes less to get it to the top of the hill to get it rolling down. The rolling down part being the reaction.
In endothermic reactions, the energy of the products is higher then the energy of the reactants, as endothermic reactions take /in/ heat, and therefore energy, so there is going to be more.
Bond Enthalpies.
But first, just to recap.
Enthalpy of Combustion; The enthalpy change it takes to burn one mole of compound in oxygen under standard conditions, all products/reactants in their standard states.
Enthalpy of Formation; The enthalpy change it takes to form one mole of compund from its constituent elements, under standard conditions, all products/reactants in standard states.
(I can finally remember these without looking. YEA! :D)
So yea, bond enthalpies.
Energy has to be put in to break a bond, endothermic. (takes in heat.) Bond Dissasociation energy is the value of energy required to break a covalent bond, with all species in a gassious state. And in reverse, the same amount of energy would be given out, exothermically. (When it is formed)
Each bond may have slightly different enthalpies in different molecules. Becuase of this, bond enthalpies are normally averages, to make things a bit of accurate. However, using them for calulations, answers won't be entirely accurate, as I'm sure you can tell, but they are quicker to use.
We can use bond enthalpies to work out the enthalpy changes in reactions. We do this by counting up the types of bonds, and how many there are, and filling in their average values. All the bonds have to break, in the reactants, so add these up to get the energy that must be put in. The energy given out is the added up reactants. The difference between these two values is the enthalpy change. Also, if more energy is given out then put in, it is exothermic and needs a '-' sign at the start.
Although, a way to shorten it down is to only calculate with the bonds that actually take part in the reaction, as this will really cut down calculation times.
In a lot of past exam questions I've seen recently there's a load of stuff about comparing the average bond enthapies to thermodynamic type things. This would be that, in a thermodynamic cycle, all the calculated (delta)Hf/c values are used of the actual compounds, whereas with bond enthalpies they are average, so thermos are more accurate.
Enthalpy of Combustion; The enthalpy change it takes to burn one mole of compound in oxygen under standard conditions, all products/reactants in their standard states.
Enthalpy of Formation; The enthalpy change it takes to form one mole of compund from its constituent elements, under standard conditions, all products/reactants in standard states.
(I can finally remember these without looking. YEA! :D)
So yea, bond enthalpies.
Energy has to be put in to break a bond, endothermic. (takes in heat.) Bond Dissasociation energy is the value of energy required to break a covalent bond, with all species in a gassious state. And in reverse, the same amount of energy would be given out, exothermically. (When it is formed)
Each bond may have slightly different enthalpies in different molecules. Becuase of this, bond enthalpies are normally averages, to make things a bit of accurate. However, using them for calulations, answers won't be entirely accurate, as I'm sure you can tell, but they are quicker to use.
We can use bond enthalpies to work out the enthalpy changes in reactions. We do this by counting up the types of bonds, and how many there are, and filling in their average values. All the bonds have to break, in the reactants, so add these up to get the energy that must be put in. The energy given out is the added up reactants. The difference between these two values is the enthalpy change. Also, if more energy is given out then put in, it is exothermic and needs a '-' sign at the start.
Although, a way to shorten it down is to only calculate with the bonds that actually take part in the reaction, as this will really cut down calculation times.
In a lot of past exam questions I've seen recently there's a load of stuff about comparing the average bond enthapies to thermodynamic type things. This would be that, in a thermodynamic cycle, all the calculated (delta)Hf/c values are used of the actual compounds, whereas with bond enthalpies they are average, so thermos are more accurate.
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