Hey chums, I am aware that this is a FASHION blog but my musings right now are 100% scientifical related. So if you don't like science, look away now and admire this cute t shirt which I totally have to get ASAP.
White or black though? ANYHOW.
Okay, so rates are a measure of how fast a reaction goes with time. You can work it out real simple with change in concentration/ change in time or you can use these snazzy a level things called rate equations.
So if we have a chemical equation that's totally like: 2A+2B-->2AB
The rate equation is Rate=k(a)^2(b)^2. K is a constant, and a and b are the concentrations of a or b and then you raise that to the power of the molar quantities in the stoichiometric equation (clever word innit). Powers are the order in regards to that reactant, and if you add them all up you get the order of reaction.
Whatever's in the rate equation is used in the SLOW STEP OF THE REACTION. Whatever's not is really quick it's like bam flying and joining molecules before your eyes. If only one species is in the rate equation- its first order and goes by SN1 (mechanism wise, pretty much chemical dance moves) . If there's two, its second order and SN2, and so on.
There's a couple of ways you can experimentally measure rates, colorimetry (which is colour change), mass change and volume change (pretty obvious what they are) and then alsooo you can quench and then titrate with whatever to see how much is reacted.
Thennnn you can plot a graph in relation to the falling conc of that reactant (or the rate ya know, whatever) and work out the order of the reaction. Conc time you get a straight line for a zero order reaction- gradient=rate which is constant and half life decreases as conc falls. For first order, you get a exponential curve and half life is constant. First order rate graphs however are straight lines. If you get a freaky curve and a curve with rate/time graphs- its second order! Half life increases as conc falls. if you do rate^2/time graph you probz get a straight line.
Gradient=rate, so if you wanna find rate at some particular momento, draw a tangent!
Ps half life=time taken for any reactant conc to fall to half its initial value.
You can increase the rate by using a catalyst. This lowers Activation energy because it absorbs reactants onto its surface, weakening reactant bonds and holding them together in a small space so its easier to react- providing an alternative pathway for the reaction to occur. Therefore lower Ae, more particles have enough energy to react, rate is quicker! yay.
You can also increase rate (and the rate constant) by making the temperature higher. This is tricky cause hot stuff uses loads of energy from non renewable sources and also what if the reaction is exothermic, it might drive equillibrium backwards (bummer) It works in the same way by providing more Ke to overcome Ae. To link temperature and and rate constants you use the ARRHENIUS EQUATION. This is lnK=-Ae/RT
R is a contsant, so plot lnK on y axis and 1/T (Temp) on x axis, then gradient = activation energy over R. Times gradient by R=Ae. Clever huh.
iodine and propanone
You have to learn one dumb reaction. Iodine plus propanone (plus h= from dil H2S04). The rate equation is first order with propanone and H+, zero with respect to iodine. Measure this beautiful reaction either via colorimetry (did you see the icky brown colour of iodines hair?) or quench and titrate with sodium thiosulphate- giving amount of unreacted iodine still left.
Entropy is the measure of disorder in a system, dispersal, spreading of energy etc. It's only zero when substance=perfectly ordered crystal at absolute zero (that impossible temperature). More heat=more spreading of entropy.
Entropy increases with= heat, energy, moles,dissolving etc. Anything which jumbles up dat shizzle. You gotta know some equations: (triangle) S system= S products- S reactants. S means entropy I think. S system is about the chemical and physical changes, so like if it goes solid--> gas or on RHS there are more moles pour example.
(triangle) S surroundings = -(triangle)H/Temp in kelvin. This is like whether it releases heat or not (endo/exo thermic). You combine these two equations to see whether a reaction is spontaneous or not.
(triangle) S total= (triangle) S system + (triangle) S surroundings. (triangle) S total >0= spontaneous reactiono.
Sometimes things have a really high Ae and therefore are inert or thermodynamically stable- they need catalyst or high heat/UV light to start. Thats why increasing temp increases (triangle) S surroundings and therefore (triangle) S total.
There are two seperate processes in dissolving.
1. Solid lattice seperates into individual ions to dissolve (endothermic)
2. Seperated ions then interact with polar solvent- eg water (exothermic)
The enthalpy of solution of an ionic compound is: (triangle) H solution= -(triangle) H lattice energy + (triangle) H hydration
If (triangle)H solution is positive: Endothermic change, dissolving generally not favoured unless there is an increase in entropy
If it's positive: Exothermic change, dissolving favoured.
Standard enthalpy of hydration is the enthalpy change when one mole of gaseous ions is hydrated under standard conditions with excess water (no more water causes more enthalpy change).
(triangle)H solution= Salt+excess water--> ions(aq)
(triangle)H lattice=salt+water-->ions(g) and water
Lattice enthalpy- depends on charge (bigger charger = bigger forces) size(smaller ions=bigger forces) and covalency.
Hydration enthalpy- depends on ionic radius (small=bigger charge density) and ionic charge (larger charger=bigger charge density). Larger charge densitys=attract more polar molecules giving more ion dipole interactions. These interactions are exothermic and the system shows a decrease in enthalpy.
Compare experimental lattice energy and born haber values- difference=covalent character.
Occurs when forwards and backwards reactions are reversible, and in a closed system.
Kc= equilibrium constant (products to molar powers, over reactants to molar powers). If Kc is >1, products favoured, and if its very large it means that reaction goes to completion. If Kc<1 about="" and="" are="" favoured="" have="" if="" is="" it="" no="" occured.="" p="" rate.="" reactants="" reaction="" really="" small="" tells="" then="" us="" will="" zilch="">
Heterogeneous=more than one physical state present
Homogeneous=all reactants and products are in the same physical state.
Kp=equilibrium constant for gases. Use mole fractions (moles of reactant/total moles) and partial pressures (mole fraction x total pressure).
Kp and Kc remain the same at different concentrations and pressures (although equilibrium yield may change) but changes with temperature.
Increase temp, increase (triangle)S surroundings, which increases (triangle)S total. (triangle)S total=RlnK (R is a constant) so increase (triangle)S total, lnK increases, so basically K increases. So products more favoured.
(How can you improve ester yield you may ask? (Or not, whatever) Add excess alcohol and recycle unused reactants)
Immiscible solvents- When a solute distributes itself between two immiscible thingys. The K for this reaction is called Partition coefficient and its worked out byyyyy solute in A/solute in B. If the partition coefficient is large and >1, indicates solute is more soluble in A than B.
Rates and equilibrium in industry
Catalysts- Speed up rate and lower Ae. Reduces time taken to establish equilibrium. No effect on K.
Temp-Exothermic=K falls with higher temperatures. Endothermic=K increases with higher temperatures. You do the math.
Conc and pressure- No effect on K, however shift position of equilibrium according to le Chateliers principle.
Atom economy=mass of atoms in desired product/total mass of atoms in reactants x100. Can be proved by more efficient synthesis etc.
4. ACID/BASE EQUILIBRIUM
Two main people groups developed some acid base theories
1) Arrhenius developed a theory of electrolytic dissociation. Meaning stuff breaks up into ions in aq. So acids-->H+ and bases-->OH-. However this could not be extended to stuff not dissolved in stuff (like liquid ammonia)
2) Bronsted-Lowry said an acid is a substance that donates protons, and a base is a substance that accepts protons.
Also acid base pairs- two species that change into each other by gaining/losing electrons. Each acid has a conjugate base and each base has a conjugate acid. If one is weak, one is strong.
Weak? Partly dissociated into ions.
Strong? Fully dissociated into ions.
ph? Ph=-log10[H+]. H+=conc of strong acid. or use Ka for weak acid.
Ka=H+xO-/Ha or Ka=H+^2/Ha. ph+pOh=14 at room temp. (Know one know the other).pKa=-logKa.
Dilution=Strong acids. Dilute by 10, ph raised by one unit.
Weak acids- increases degree of dissociation into ions.
Theres some stuff about curves but CBA to draw them soz kids. But find indcators-predict end point and then put choose one that changes around then, end point is halfway up the vertical section of the curve.
Weak acid+salt (or base i dunno)
Salt fully ionised, supplying ions that are the conjugate base to the weak acid.
Add acid? H+ combines with conjugate base from salt-->largely undissociated acid. Removes H+ and pH remains same.
Add base? Combines with small H+ amount from undissociated salt. This drives equilibrium to cause acid to dissociate into ions and replace H+ lost. pH doesnt change.
pH of buffer= H+=Ka x HA/salt
5. ORGANIC CHEM
Chiral carbon=4 different functional groups attached. Gives optical isomers. Optical isomers rotate the plane of plane polarised light. Racemic=equimolar mix=no rotation.
Carbonyl Compounds. (React with 2 4 di nitrophenylhydrazine orange yellow precipitate-the crystallised version melts at a specific point according to what it is)
Ketones-RCoR, Aldehydes-RCHO. They have perm dipoles and can hydrogen bond to polar molecules. Solubility decreases with chain length.
Aldehydes can be oxidised to carboxylic acids by fehlings, tollens, K2Cr2O7.
Both can be reduced by LiAlH4 in dry ether conditions-->alcohol.
Also both can react with KCN and HCN- (Nucleophillic addition)-->racemic mix of nitrile.
CH3 next to C=O means a positive result with iodoform test. Yellow precipitate with antiseptic smell.
Carboxylic Acids= RCOOH
Can hydrogen bond and form dimers, high boiling point. Made via oxidation of primary alcohols/aldehydes or hydrolysis of nitriles (giving NH3 as waste)
Neutralised--> carboxylate salts.
+ PCl5 --> Acyl Chloride (+HCL and POCl3)
+ Alcohol --> Ester.
+ water (catalysed by dil HCl, H2So4, boil under reflux) --> acid and alcohol
+base --> carboxylate salts and alcohol. This is used to make soaps (+ alcohol) (saponification) using glycerides
+alcohol --> different alkyl ester. (Trans esterification- also used to harden unsaturated fats forming unharmful fats)
Also triglyceride+alcohol-(base catalyst)->biodiesel +glycerol.
Polyesters- condensation reactions, make nylon terylene etc,
Acyl Chloride RCOCl
+water-->acid and HCl
+alcohol--> ester (not reversible, COCl polar so easy to attack, goes to completion)
+conc ammonia--> Acid amides(RCONH2) and ammonium chloride
+amines--> N substituted amide(RCONHR) and HCl
6. INSTRUMENTAL ANALYSIS
Microwaves- heats polar stuff.
NMR- radio waves, makes Hydrogen nuclei spin/resonate. If they are in different environments=different peaks. N+1= split according to H on adjacent carbon, due to spin spin coupling (spin states of protons)
Mass Spec- molecular ion peak=mass, fragments tell us structure
GLC- Gas mobile phase, attraction to stationary phase determines retention time which seperates volatile components
HPLC- Solvent mobile phase, seperates them more effectively for analysis. Same principles.
Okay so that's unit four. Wait for more to come :)