Revision Chemistry Questions For University Freshers July 2020

University/High School Chemistry Tutorials

CHEMISTRY (CHE 101)​​ 

 

 

 

 

 

1.​​ Question on Chemical Kinetics:

If doubling the concentration of a reactant increases the rate of reaction four times, what is the order of reaction with respect to the reactant?

 

ANSWER

N/B;​​ Order of​​ reaction​​ is the power or exponent to which the reactant is raised in a rate equation. It is determined experimentally, and not stiochiometrically (or from the stiochiometrical equation). For example:

​​ If ​​ 

aA+bBcC+dD

Then​​ 

r=k[A]x[B]y

​​ (where x+y is the order of reaction)

N/B;​​ Rate Law:​​ the law states that the rate of reaction is directly proportional to reactant concentration with respect to the order of reaction; the mathematical expression of the rate law is known as​​ rate equation.​​ i.e

r[A]x ………………….. (1) rate law


r=k[A]x ………………….. (2) rate equation

Transformations

from equa (2) ……..

k= rAxr1A1x=r2A2x

  ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​​​ 

r1r2=[A]1[A]2x

but…A2=2A1doubling ​​​​ r2=4r1four times

r14r1=[A]12A1        14=12x       122=12x         x=2

 ​​​​ 
the reaction is therefore a second order reaction since​​ x=2

 

2. Question on Chemical Equilibrium

  • Calculate the equilibrium concentration of NO2​​ in a​​ solution prepared by dissolving​​ 0.20mol​​ of N2O4​​ in 400ml of chloroform. For the reaction  ​​ ​​​​ N2O4g   2NO2gin chloroform, ​​  ​​ k=1.07×10-5

  • What will be the percentage decomposition of the original​​ N2O4?

ANSWER

  • keq=rate of backward reversereaction concentrationrate of forward reaction concentration ​​ ​​​​  ​​ (where​​ keq=equilibrium constant)​​ 

keq=[NO2]2N2O4

keq=1.07×10-5NO2=?                    N2O4=0.20mol400ml1000=0.5M

NO22=keq×N2O4=1.07×10-5×0.5=5.35×10-6

NO2=5.35×10-6=2.31×10-3 or 0.0023M

 

  • N2O4(g)        2NO2(g)

percentage composition = N2O4NO2×1001  =  0.462%

 

3. Question on Electromagnetic Radiation

  • What is an​​ electromagnetic radiation?

  • What is the wavelength (in nanometers) that corresponds to an electron transition from the​​ n=5 ​​​​ level to the​​ n=3​​ level in the hydrogen atom?​​ 

ANSWER

3a. Electromagnetic Radiation (EMR) is the propagation of energy in wave form, with periodic changes in magnetic and electric fields. The following are characteristics of EMR:

  • Wavelength (λ)

  • Velocity (speed of light [c])

  • Frequency (f or v)

  • Wavenumber (v-)

N/B:c=fλ c=velocity or speed of light,       f=frequency,              λ=wavelenght

3b.

1λ=R1n12-1n22                                        where;  n2>n1

λ=wavelength=?,          R=Rydberg's constant=10967758m-1,       n2=5,       n1=3

1λ=10967758132-152      =     1096775819-125       =        1096775825-99×25

1λ=10967758×16225   =   175484128225  =  779929.4578

λ=1779929.4578  =  1.2822×10-6m  =  1282.2nm

 

 

4. Question on Gas Laws​​ ​​ 

  • State Graham’s law of diffusion of gases. Express the equation in terms of rate, density, molar mass and time.

  • A mixture of two gases, A and B, of equal masses is made to diffuse through a porous pot. If it takes 30secs for A to diffuse through the pot, how long​​ will it take B?​​ [molar masses of A and B are 28 and 44 respectively]

  • Express the Van Der Waal’s equation for real gases.

ANSWER

  • Graham’s law of diffusion of gases states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its density at constant​​ temperature and pressure.​​ 

i.e.

r1d ,          thus,   r=kd ,           therefore,  rd =k,       hence,  rArB=dBdA

 

The expression of the equation in terms of rate, density, molar mass and time follows as

rArB=dBdA=MBMA=tBtA

 

  • MBMA=tBta          tB=tA×MBMA           tB=30×4428            tB=37.61secs

 

  • Expression of the Van Der Waal’s equation is also known as the real gas equation

 

P+an2v2V-nb=nRT

 

 

5. Question on Acids/ Bases/ pH & pOH Analysis/ Nuclear Reactions​​ 

  • Water is said to be amphoteric. Illustrate this with a suitable example and equation

  • Prove that​​ pH=pOH=7

  • What are the differences between nuclear reactions and other chemical reactions?

ANSWER

  • An amphoteric​​ substance is one that has the ability to donate hydrogen ion (H+) or accept hydrogen ion (H+) in solution. E.g. water (H2O), ammonia (NH3) etc. They behave both like an acid and a base in solution.

H2O  H++OH-                                     

H2O+H+H3O                                        

 

  • pH+pOH=14       

kw=ka+kb  ,                 kw=1.0×10-14

H2O  H++OH-

kw=H+OH-but when H+=OH-=x,

kw=x2      x=1.0×10-14 =1.0×10-7

H+=1.0×10-7      :pH=-logH+=7

OH-=1.0×10-7    :pOH=-logOH-=7

pH=pOH=7

 

  • Differences between chemical and​​ nuclear reactions

 

CHEMICAL REACTIONS

NUCLEAR REACTONS

1

Only the electronic structure of the atoms are affected

The nucleus of the atoms are affected​​ 

2

Does not result in formation of a new element

Results in conversion of one element to another

3

Are affected by factors that affect rate of chemical reactions such as temperature and pressure

Are not affected by factors that affect rate chemical reactions

4

Small amount of energy is released or absorbed by the reaction

Large amounts of energy is​​ released or absorbed by the reaction

5

Isotopes of an element show identical behaviour

Isotopes of an element behave differently​​ 

 

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