Factors That Influence Chemical Reactions (In More Detail )
1. The nature of the reacting substances.
§ Ionic bond changes occur faster than covalent bond changes. Ionic bond changes occur in precipitation and neutralization reactions.
§ In general, if more bonds have to be broken and reformed in order for the reaction to take place, the reaction will be slower.
Example 1: Which of the following room temperature reactions are relatively slow?
a. C6H6 + 7.5 O2 à 6 CO2 + 3H2O
b. Ag+1(aq) + F-1(aq) à AgF (s)
c. NaOH(aq) + HCl(aq) à NaCl(aq) + H2O
d. N2(g) + 3 H2(g) à 2 NH3(g)
Answer: (a) and (d). Many bonds have to be broken in each case, whereas (b) only involves 1 bond forming between Ag+1 and F-1.( precipitation reaction) (c) is also fast because it is basically a reaction between OH-1 and H+1 to form water ( a neutralization reaction.)
2. The concentration of the ions or gases.
§ Increasing the concentration of ions or gases ( increasing pressure also works) increases the rate of a reaction.
Example 2a How could you speed up reaction 1d.?
Increase the concentration of hydrogen and nitrogen.
Example 2b How do you increase the rate at which Mg reacts with HCl(aq)?
Use a stronger acid ( HCl).
3. The surface area of contact.
§ Increasing surface area increases the rate because it increases the number of effective collisions between molecules as it exposes more reacting molecules to each other.
Example 3 Which reacts faster: a 200 g sphere of Mg or a 200 g cube of Mg?
Density of Mg = 1.74 g/cm3.
The volume of 200 g of Mg = 200g/ [1.74(g/cm3) ] = 115 cm3.
The cube's dimensions can be figured out from:
s3 = 115
s = 115(1/3) = 4.86.
The surface area of the cube = 6s2 = 6 (4.86)2 = 142 cm2.
The sphere's radius can be figured out from
r = [115(3/4)/ p](1/3)
= 3.0 cm
The surface area of the sphere = 4pr2 = 4p(3.0)2 = 113 cm2.
So the cube will react faster because for the same volume and weight, it has more surface area than the sphere.
4. The presence of a catalyst or inhibitor.
A catalyst is a chemical that speeds up a chemical reaction, but the catalyst itself is not used up in the reaction. An inhibitor does the opposite; it slows down or prevents a reaction from occuring.
Example 4a: The breakdown of ozone is catalyzed by the presence of Cl.
(1) CH3Cl --> CH2Cl + Cl
(2) O3 + Cl --> ClO + O2
(3) ClO + O --> O2 + Cl
In reaction (1), a CFC( found in fridges and AC's) breaks down in the atmosphere to create Cl.
In reaction (2), molecules of O3 from the protective ozone layer get attacked by Cl.
Finally in reaction (3) ClO reacts with atomic O (found in ozone layer because it helps form ozone) to regenerate
Tragically the Cl is available again to break down more ozone. This is what makes the Cl a catalyst. It takes part in the reaction but it's recycled, therefore speeding up the breakdown of a vital molecule.
Example 4b: The growth of bacteria can be inhibited by the presence of penicillin.
Penicillin irreversibly inactivates a key enzyme in bacterial cell wall synthesis.
Without penicillin, bacteria use an enzyme to build a cell wall. The diagram shows the enzyme linking A to B to create a key part of the cell wall. But in the presence of penicillin, bacteria cannot build a cell wall because the penicillin molecule, which is an inhibitor, occupies the spot normally reserved for molecule B.