Often students are faced with a blockage in many questions in semester 2 due to the inaccessibility of the set of instructions which ultimately make it easy to comprehend questions. Chemistry is often disguised as a subject that requires much understanding and a lot of memorizing, yet this is not t...
Often students are faced with a blockage in many questions in semester 2 due to the
inaccessibility of the set of instructions which ultimately make it easy to comprehend questions.
Chemistry is often disguised as a subject that requires much understanding and a lot of
memorizing, yet this is not the case for many of the top achievers. Top achievers compress their
understanding into a simple set of instructions that they use. Fortunately, this set of instructions
are found and explained in this document.
CONCEPTS COVERED
Chemical kinetics
Chemical equilibrium
Acids and base equilibrium
Aspects of aqueous equilibria
Thermochemistry
, Chemical kinetics
Chemical kinetics- ‘measurement of how quickly reaction occur.’1
Rate of reaction is the change in concentration per unit time.
[𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛] 𝛥𝑚𝑜𝑙𝑎𝑟𝑖𝑡𝑦
Average rate= 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑡𝑖𝑚𝑒
= 𝛥𝑡 = M.s-1
Rate law shows the relationship between rate of reaction and concentrations of the reactants.
For the chemical reaction: aA+bB⇌cC+dD
rate law=k[A]m[B]n.
k is the rate constant
m, n are the numbers indicating the reaction order.
The reaction order indicates the relationship between the concentration of species and the rate of
the reaction.
Finding The Reaction Order:
CASE 1:
If the overall reaction order is not given. Use the coefficients of the reactants.
• When rate of reaction is proportional to the concentration of a substance then it is
a first order reaction, meaning power of A is 1. (a unimolecular process- one mol-
ecule involved)
• When rate of reaction is proportional to the A2, then it is a second order reaction.
(a bimolecular process-two molecules involved)
• When rate of reaction is independent of the concentration of A, then the reaction
is a zero-order reaction.
E.g., aA+bB ⇌cC
Rate law= k[A]a[B]b
CASE 2:
If overall reaction order is given.
Overall reaction order= sum of the individual reaction orders.
E.g., aA ⇌Bb + cC
Overall reaction order=2
Rate law=k[A]2
CASE 3: More than 1 rate of reaction is given (this may be in the form of a table).
1
Chemistry: the central science, Brown et al. (Brown et al., no date)
, Then follow this three-step system to find the reaction order which is the power of the
concentration of the reactants
Step 1: if there is more than one reactant, look for the two rows where only one reactant
concentration changes.
Step 3: repeat above process to find a.
𝑟𝑎𝑡𝑒3 𝐾[𝐴]𝑎 [B]𝑏 3.2𝑥10−4 𝐾[0.21]𝑎 [0.3]𝑏
= ⟹ = ⟹ 1.95=2.1b ⟹ b=log2.1[1.95] ⟹ a= 0.9
𝑟𝑎𝑡𝑒1 𝑘[𝐴]𝑎 [𝐵]𝑏 1.64𝑥10−4 𝑘[0.1]𝑎 [0.3]𝑏
Overall reaction order=0.12+0.9=1.02
.
Half-Life: First Order Reaction
The time needed for a reaction to reach half of its original concentration. t1/2 is independ-
ent of concentration.
0.693
t1/2= 𝑘
Collision Theory: Temperature and Rates
Collision literally means for two objects to crash or hit against each other. For this to happen
well enough to cause a reaction; enough molecules must collide at the right kinetic energy. The
more the collisions, the faster the rate. As the kinetic energy increases, so does the speed at
which it occurs, and the rate consequently increases.
In simple terms: average kinetic energy= temperature ∝ reaction rate
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