Learn what catalysts are and how they affect the activation energy and reaction rate of a chemical reaction.
A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed. This is called catalysis. A catalyst is not consumed by the reaction and it may participate in multiple reactions at a time. The only difference between a catalyzed reaction and an uncatalyzed reaction is that the activation energy is different. There is no effect on the energy of the reactants or the products. The ΔH for the reactions is the same.
Usually when someone refers to a catalyst, they mean a positive catalyst, which is a catalyst which speeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalysts or inhibitors, which slow the rate of a chemical reaction or make it less likely to occur.
A promoter is a substance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.
Catalysts permit an alternate mechanism for the reactants to become products, with a lower activation energy and different transition state. A catalyst may allow a reaction to proceed at a lower temperature or increase the reaction rate or selectivity. Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalyst. Note that the catalyst may be consumed during one of the intermediate steps, but it will be created again before the reaction is completed.
This reaction is a summary of an ecologically friendly process for halogenating alkynes. The reaction works with both hydrobromic and hydrochloric acid, and produces water as its only waste product. It also gives a good yield of the halogenated product.
Instead of undergoing anti-Markovnakov addition of HBr, the alkynes are halogenated. This is due to the way the reactants are mixed. Mixing a hydrohalogenic acid with a solution of t-butylhydroperoxide (TBHP) and hydrogen peroxide will oxidize the halogens, causing them to become positively charged. The charged halogens will then attack the alkynes, and a halogenation reaction will occur.
The most common medical ester is aspirin (ASA; acetyl salicylic acid). Other drugs such as Worm Guard (anti-wormer), Maxicaine (local anesthetic), Malathion (organophosphate), Mebendazole (antihelmenthic), Demerol (narcotic analgesic) and Equinil (sedative) are also esters.
The starting reactants for this experiment are salicylic acid and acetic anhydride (structures are shown above).
Salicylic acid reacts better with acetic anhydride than acetic acid, so acetic acid will provide the acetyl group which will react with the alcoholic -OH group on the salicylic acid. (The reaction is on the top of the post.)
Chemicals needed for the reaction: Salicylic acid, Acetic anhydride, and Concentrated sulfuric acid.
Equipment: 250 mL Erlenmeyer flask, Hot water bath, Ice bath, Buchner funnel and filter paper, Glass stirring rod, and Electronic pan balance and weighing boat.
Alkenes are named as if they were alkanes, but the “-ane” suffix is changed to “-ene”. If the alkene contains only one double bond and that double bond is terminal (the double bond is at one end of the molecule or another) then it is not necessary to place any number in front of the name.
butane: C4H10 (CH3CH2CH2CH3)
butene: C4H8 (CH2=CHCH2CH3)
If the double bond is not terminal (if it is on a carbon somewhere in the center of the chain) then the carbons should be numbered in such a way as to give the first of the two double-bonded carbons the lowest possible number, and that number should precede the “ene” suffix with a dash, as shown below.
correct: pent-2-ene (CH3CH=CHCH2CH3)
incorrect: pent-3-ene (CH3CH2CH=CHCH3)
The second one is incorrect because flipping the formula horizontally results in a lower number for the alkene.
If there is more than one double bond in an alkene, all of the bonds should be numbered in the name of the molecule – even terminal double bonds. The numbers should go from lowest to highest, and be separated from one another by a comma. The IUPAC numerical prefixes are used to indicate the number of double bonds.
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A) 1. Enthalpy ( delta H) is the amount of heat content.
1. Heat content is accounted for by a change in “heat flow” or enthalpy of the reaction system.
1. Endothermic reaction: delta H > 0
(i.e., H products >H reactants).
Heat absorbed goes to increase the enthalpy of the reaction system.
2. Exothermic reaction: delta H < 0
(i.e., H products < H reactants).
Heat is evolved at the expense of the reaction system.
2. Thermochemical Equation: specify delta H in kilojoules/mole.
1. CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l) + 890.3 kJ
delta H = -890 kJ
6.00kJ + H2O(s) –> H2O(l)
delta H = +6.00kJ
! In some textbooks delta H is written as a product or reactant !
The preceding is based upon the Law of Conservation of Energy (James Joule, 1818-1889, Joule also developed the First Law of Thermodynamics): energy is neither created nor destroyed in ordinary chemical or physical changes.
2. Quantitative delta H
delta H = qreaction mixture (at constant temperature only)
q = (m)( delta t)(Cp)
q = heat absorbed by the water in joules (J)
m = mass of substance
delta t = tfinal – tinitial
Cp = specific heat of water = 4.184 J/g oC
When using moles, molar heat capacity is used. The units are kJ/mol K
1 cal = 4.184 J
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