The Thread of Life

Everything you need to know about the TL module of A2 OCR Salters Chemistry B! 

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  • Created by: James
  • Created on: 08-05-13 13:56

The effect of concentration on rate

Rate of reaction = change in concentration of reactant or product / time taken 

For the general reaction A + B \rightarrow ( Products, the rate equation would be:

Rate = k[A]^m [B]^n where:

  • [A] and [B] are the initial concentrations of reactants A and B
  • k is the rate constant for the reaction at a specified temperature
  • m is the order of reaction with respect to reactant A
  • n is the order of reaction with respect to reactant B
  • (n+m) is the overall order of the reaction.

The rate of a chemical reaction increases whenever temperature is increased. Since:

Rate = k[A]^m [B]^n

a rise in temperature must increase the value of the rate constant, k.

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Determining a rate equation

Determined through the use of experiments. If there are multiple reactants, multiple experiments must be carried out.

For the reaction A + B \rightarrow ( Products,

  • An initial set of reactions is carried out, varying only concentration of A, keeping B the same;
  • Process is repeated, varying concentration of B, keeping A the same;
  • Rate is determined for each reaction run (by measuring colour change, pH, volume of gas, etc).

The temperature must be kept constant or k will change!

Using concentration-time graphs: Linear = zero order; constant half life = first order; half life not constant = second order.

Using rate-concentration graphs: Parallel to x-axis = zero order; linear = first order; non-linear = second order.

A reaction mechanism describes the step-by-step manner in which the reaction occurs. An intermediate is a chemical formed and then destroyed during the course of the reaction.

The slowest step is called the rate-determining step.

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Optical isomerism

Stereoisomers are molecules tha have the same molecular formula and atoms are bonded in the same order, but they are arranged differently in space.

There are two types of stereoisomers: E/Z isomers and optical isomers.

To exhibit optical isomerism, a molecule must have a chiral centre (usually a central carbon [the chiral carbon] and is denoted by *).

A chiral centre is an atom that has four different atoms or groups of atoms attached to it. Molecules with a chiral centre are called optical isomers or enantiomers.

Molecules that are chiral have non-superimposable mirror images.

Optically active molecules rotate the plane of plane-polarised light in different directions. (d-isomers rotate clockwise, l-isomers rotate anticlockwise)

Optical isomers behave differently in the presence of other chiral molecules - often linked with smells.

If a reaction produces 50:50 d- and l- isomers, the mixture is called a racemic mixture.

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Amino acids and proteins

Amino acids are bifunctional molecules - they have amino (-NH2) and carboxyl (-COOH) functional groups. When these groups are attached to the same carbon, the amino acid is called an α-amino acid.

All proteins are made from the same set of 20 alpha amino acids. The side chain (R) is different in every one. Each amino acid has a 3 letter abbreviation  e.g. Ala for alanine, for which R = CH3.

Apart from glycine (where R = H), alpha amino acids have four different groups attached to the carbon atom, so they exhibit optical isomerism.

Amino acids act as weak acids and bases. The -COOH group donates H+ ions, which the -NH2 group accepts H+ ions. They exist in 3 different forms, depending on the pH of solution.

NH3+CRHCOOH (acidic), NH3+CRHCOO- (zwitterion), NH2CRHCOO- (alkaline)

The amino group is protonated in acidic coniditons, and the carboxyl group is deprotonated in alkaline coniditons. An ion can have both positive and negative groups at the same time - called a zwitterion.

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Amino acids and proteins

Two amino acids join to form a dipeptide. The -NH2 group from one amino acid reacts with the -COOH group from the second amino acid, forming a secondary amide group or peptide link. (See below)

When several amino acids are joined together like this, a polypeptide is formed. 

Proteins are naturall occurring condensation polymers formed when many amino acids join together.

The order that they join in a protein is called the primary structure. When a polypeptide chain forms an alpha helix or beta sheet this is the secondary structure. These arrangements occur as a result of hydrogen bonding.

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Amino acids and proteins

The folding of a polypeptide chain to give the unique shape is called the tertiary structure. There are four types of interaction that maintain the tertiary structure:

  • IDIDs between non-polar side chains;
  • Hydrogen bonds between polar side chains;
  • Ionic bonds between ionisable side chains;
  • Covalent bonds (e.g. 'sulphur bridges').

When a peptide/protein is refluxed with moderately conc. acid or alkali for several hours, the C-N bond in the peptide link is broken. 

Under acidic hydrolysis conditions, the -NH2 groups are protonated to give -NH3+ whereas under alkaline hydrolysis conditions the -COOH deprotonates to give -COO-.

The amino acids produced can be identified using thin-layer chromatography and the use of ninhydrin as a locating agent.

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Enzymes and atom economy

Enzymes are metabolic catalysts that are proteins. They have a high specificity for a given substrate. All enzymes have an active site, where the tertiary structure of the enzyme exactly matches the structure of its substrate.

  • The substrate weakly binds to the surface of the active site;
  • This weakens bonds in the substrate slightly, allowing a reaction to occur;
  • The products leave the active site.

Any changes to the active site (disruption of hydrogen bonding, changes in pH), denature the enzyme and render it useless.

Competitive inhibition is where a substrate bonds strongly to the active site but does not react, preventing another substrate from occupying the site.

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Enzymes and atom economy

Enzyme-catalysed reactions have different rate equations at high and low substrate concentrations.

Low concentrations: rate = k[E][S] where [E] is the concentration of the enzyme and [S] is the concentration of the substrate. Lots of active sites for the substrate to bind to, doubling the [S] doubles the rate of reaction. First order with respect to the substrate.

High concentrations: rate = k[E] because all the active sites on the enzyme have become saturated. Reaction becomes zero order.

Enzymes are used as catalysts in industry because:

  • they are specific - they select particular substrates from a feedstock of a mixture of reactants;
  • they work effectively at low temperatures - reducing energy costs;
  • they work well in aqueous environments - reducing the need for organic solvents (flammable and dangerous to the environment);
  • they often convert reactant to product in a one-step reaction - increasing the % atom economy.
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