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Atoms, Molecules and Stoichiometry – Notes

8 Redox Reactions

The term “redox” is used as an abbreviation for the processes of reduction and oxidation which occur simultaneously. A redox reaction is an oxidation-reduction reaction.

(a) Loss/gain of Oxygen

Oxidation is gain of oxygen.
Reduction is loss of oxygen

(b) Loss/gain of Hydrogen.

Oxidation is loss of hydrogen.
Reduction is gain of hydrogen.

(c) Loss/gain of electrons

Oxidation is loss of electrons.
Reduction is gain of electrons.

(d) Increase/decrease in oxidation number(oxidation state)

Oxidation is increase in oxidation number.
Reduction is decrease in oxidation number.

8.1 Redox Processes in Terms of Electron Transfer

• Consider the reaction between zinc and copper(II) sulfate solution.

Zn(s) + Cu2+(aq) –> Zn2+(aq) + Cu(s) Ionic equation

• The overall reaction can be separated into two simpler processes involving electron transfer.
Oxidation: Zn(s) –> Zn2+(aq) + 2e
Reduction: Cu2+(aq) + 2e –> Cu(s)

• The two separate equations can be termed ion-electron equations but they are more commonly known as half-equations. One half-equation represents the oxidation process while the other represents the reduction process. Addition of the two half-equations gives the overall redox equation.

• In the shown reaction,

Zn acts as a reducing agent. It loses two electrons and is itself oxidised to Zn2+. Cu2+ acts as an oxidising agent. It gains two electrons and is itself reduced to Cu.

8.2 Oxidation Numbers

• An oxidation number is a number that is assigned to an element in a substance to show its state of oxidation

8.3 Rules for Assigning Oxidation Number (O.N)

1. The oxidation number of an atom in the elemental state = 0
E.g. O.N. of Ca, I2 S8 or P4 = 0

2. The oxidation number. of monoatomic ion is simply the charge on the ion.
E.g. O.N. of Cl- = -1; O.N. of Ca2+ = +2

3. The sum of the oxidation number of all the atoms or ions in a neutral compound = 0.
E.g.
CaBr2 +2 + 2(-1) = 0
Al2O3 2(+3) + 3(-2) = 0
CO2 +4+2(-2)=0

4. In polyatomic ions, the sum of the oxidation numbers = the charge on the ion.

E.g.

SO4 2-

: The sum of the oxidation numbers (O.N of S = +6, O.N. of O = -2)
= +6 + 4(-2) = -2 = charge on the ion

CrCl6 3-

: The sum of the oxidation numbers (O.N of Cr = +3, O.N. of Cl = -1)
= +3 + 6(-1) = -3 = charge on the ion

5. In any compound, the more electronegative atom has the negative oxidation number while the less electronegative atom has the positive oxidation number.

• Electronegativity is the ability of an atom in a molecule to attract shared electrons in a bond.

• Electronegativity values : F > O > N > C > H

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A Level GP/General Paper Tuition Singapore

ESSAY CHECKLIST

Question Engagement & Relevance Please tick

The essay plan / essay demonstrates a clear understanding of question keywords and requirements and is relevant to the question.

The scope / breadth defined is sufficient and suitable for the development of the essay.

Introduction

The essay starts in a way which makes the reader want to read more.

Contains a thesis statement that states your position/stand and provides justification.

Provides background information and/or context to show significance of the issue

Content

The thesis statement is supported by at least 3 relevant arguments.
These arguments are well illustrated or supported by a broad range of relevant facts and/or figures.
Appropriate balance is achieved through relevant counter arguments/alternative perspectives that are presented.
Alternative perspectives are effectively addressed.

Coherence
Paragraphs are used appropriately.
Paragraphs have clearly stated topic sentences related to the thesis statement/stand. The other sentences in the paragraph are clearly connected to the topic sentence.
A range of linking devices is used and paragraphing is coherent and controlled, with smooth transitions between paragraphs.
Appropriate transitional words/phrases are used to indicate the direction of the discussion.
The development of the argument can be easily followed from paragraph to paragraph.

Conclusion
The conclusion briefly reiterates the thesis statement and ends effectively (e.g. with a quotation, afterthought, or recommendation).

Overall
Accurate use of Standard English to add to the positive impact of the essay:
a) Tenses
b) Spelling
c) Tone
d) Topical Vocabulary
e) Expressions
The essay is written tidily and legibly.

Final Grade:
Content ( / 30)
Language ( / 20 ) / 50
What is one of the best things about this essay?

What is one important area for improvement in this essay?

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A-Level Physics Tuition Singapore/H2 Physics Tuition/JC Physics Tutor

Hi A-level/H2/JC Physics Tuition students

Mastering Qualitative Questions

Chapter 17 Alternating current

1. State an advantage of using alternating current for the transmission of electrical energy. [1]

2. Explain why high voltage is used in the transmission of electrical energy. [2]

3. Explain what is meant by root-mean-square voltage when applied to a sinusoidal alternating voltage.

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A-Level Biology Tuition Singapore/H2 Biology Tuition/JC Biology Tutor

TOPIC 1: CELLULAR FUNCTIONS – Part 14

LEARNING OUTCOME

(n)Explain how uncontrolled cell division can result in cancer, and identify causative factors (e.g. genetic, chemical carcinogens, radiation, loss of immunity) which can increase the chances of cancerous growth. (Knowledge that dysregulation of checkpoints of cell division can result in uncontrolled cell division and cancer is required, but detail of the mechanism is not required.)

KEY WORDS
• Gain of function mutation
• Loss of function mutation
• Accumulation of mutations in a single cell
• Uncontrolled cell division, will eventually contribute to cancer

ESSAY ANSWER

Causative factors

Genetic factors, e.g. inherited oncogenes or defective tumour suppressor genes
Chemical carcinogens, e.g. ethidium bromide
Tar in cigarette smoke
Ultraviolet radiation
Ionising radiation, e.g. X-ray or gamma ray
Loss of immunity due to increasing age

Explanation
(Named factor from above) causes mutation in genes controlling normal cell division, i.e. proto-oncogenes and tumour suppressor genes.

When proto-oncogenes mutate to form oncogenes, this is known as gain of
function mutation. When both copies of a tumour suppressor are mutated, there is a loss of function mutation.

With accumulation of these mutations and other mutations to genes which
encode for enzymes like telomerase in a single aberrant cell, this abnormal cell divides uncontrollably and more rapidly than normal by mitosis. This leads to formation of tumours, i.e. groups of cells which are abnormal and unspecialised. When cells of a tumour invade surrounding tissue, and generally metastasize to other sites, the tumour is considered to be malignant and is defined as a cancer

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A-Level Economics Tuition Singapore/H2/H1 Economics Tuition

Question Type

Discuss how [a policy] in Country A affect the [goal] of another country B.

Answering Technique
i) Introduction

(1) Define the policy in general
(2) Explain the policy’s specific type and what economic problem(s) did A face that induces its lawmakers to implement this particular policy
(3) Define the goal of country B in general
(4) Explain the economic relationships between these two countries

ii) Thesis

(1) Explain how A’s policy can negatively/positively affect B’s goal. Use concepts
such as:

iii) Anti-thesis 1

(1) Explain how A’s policy may have limited impact on B’s goal. Use concepts such as:
(a) Economic Restructuring/Rebalancing – Diversifying trade partners,
diversifying the industry of specialization, diversifying into domestic
sources of growth rather than external sources or diversifying into external sources of growth rather than internal sources
(b) B’s government counter measures – property cooling measures, counter
inflation measures etc

iv) Synthesis

Sample Questions

1. With reference to the data, discuss and compare the effects of China’s revaluation of the Yuan on the other emerging economies and the advanced economies.

2. Suggest how deflation in Japan may affect China’s aggregate demand

3. Discuss how the reduction of interest rates by Federal Reserve affects the Singapore’s economy.

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O-Level Singapore/O-Level/Physics and Chemistry Tuition/Physics Tutor

Key definitions – Kinematics

Kinematics

Scalars and vectors

Scalar quantities are physical quantities that have magnitude only.

Vector quantities are physical quantities that have both magnitude and direction.

Example Scalar Quantities

Distance
Speed
Mass
Energy
Time

Example Vector Quantities

Displacement
Velocity
Acceleration
Force

Distance
• The total length covered by a moving object regardless of the direction of motion
• A scalar quantity (i.e. has magnitude only)
• SI unit: metre (m)

Displacement
• The distance measured in a straight line in a specified direction
• A vector quantity (i.e. has both magnitude and direction)
• SI unit: metre (m)

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Macromolecules – IMPORTANT DEFINITIONS

1. A macromolecule is a large molecule in which there is a very large number of atoms joined together by covalent bonds.

2. A polymer is a large molecule made up of many repeat units joined together by covalent bonds.

3. A monomer is the starting material for making a polymer.

4. Polymerisation is the process of joining up of many small molecules (monomers) to form a long-chain molecule (polymer).

5. Addition polymerisation is a reaction in which monomers join together to form a large molecule (polymer) as the only product.

6. Condensation polymerisation is a reaction in which monomers join together to form polymers with the elimination of small molecules.

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O-Level Singapore/O-Level/Pure Physics Tuition/Physics Tutor

Kinematics

Scalars and vectors

Scalar quantities are physical quantities that have magnitude only.

Vector quantities are physical quantities that have both magnitude and direction.

Scalar
Distance
Speed
Mass
Energy
Time

Vector
Displacement
Velocity
Acceleration
Force

Distance
• The total length covered by a moving object regardless of the direction of motion
• A scalar quantity (i.e. has magnitude only)
• SI unit: metre (m)

Displacement
• The distance measured in a straight line in a specified direction
• A vector quantity (i.e. has both magnitude and direction)
• SI unit: metre (m)

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O-Level Additional Mathematics Tuition Singapore

Conditions for quadratic equation to have

(1) Two real and equal roots (b2 — 4ac = 0)
(2) Two real and distinct roots (b2 — 4ac > 0)
(3) Two real roots (b2 — 4ac 0)
(4) No real roots (b2 — 4ac < 0)

Quadratic Functions and Inequalities.

1. Find the range of values of m for which the equation (m + 2)x2 — 2mx + 1 = 0 has two complex roots.

2. Given that y = 16 — px — x2 = 25 — (q + x)2, for all real values of x, where p and q are both positive
(i) calculate the values ofp and q,
(ii) state the maximum value of y and the value of x at which it occurs
(iii) find the range of values of x for which y is positive.

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O Level E Maths Tuition Singapore/Tuition O Level E Maths/Tutor

Primes, Factors and Multiples

1. A prime number is a number which has only two different factors, 1 and the number itself.
For example, 2, 3, 5, 7, 11, 13 are prime numbers.
Note that “1” is not a prime number.

2. A number which has more than two different factors is a composite number.
For example, 4, 6, 8, 9, 10, 12 are composite numbers.
Note that “1” is not a composite number.

3. Factors of a number are whole numbers that multiply to give that number.
For example, 18 = 1 x 18
= 2 x 9
= 3 x 6
Hence, the factors of 18 are 1, 2, 3, 6 and 18.

4. The common factors of two or more numbers are the factors that the numbers have in common.

For example,
Factors of 18 = 1, 2, 3, 6, 18
Factors of 12 = 1, 2, 3, 4, 6, 12
Factors of 24 = 1, 2, 3, 4, 6, 8, 12, 24

Hence, the common factors of 18, 12 and 24 are 1, 2, 3 and 6. The highest common factor (HCF) of 18, 12 and 24 is 6.

5. When a number is multiplied by a non – zero whole number, a multiple of the number is obtained.
For example,
Multiples of 4 : 4, 8, 12, 16, 20, 24, 28, 32 36, 40„
Multiples of 5 : 5, 10, 15, 20, 25, 30, 35, 40,

6. Common multiples of two numbers are numbers that are multiples of the two numbers.
For example,
The first two common multiples of 4 and 5 are 20 and 40.
It follows that the lowest common multiple (LCM) of 4 and 5 is 20.

7. The decomposition of a composite number into prime factors is known as prime factorisation.
For example,
60 = 2 x 2 x 3 x 5

8. A number, when multiplied by itself, gives the square of the number. For example, 72= 7 x 7 = 49

9. 1, 4, 9, 16, 25, 36 are squares of whole numbers. These numbers are called perfect squares.

10. The square root of a number, when multiplied by itself gives the number. For example, ‘144 = 12

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A-Level Mathematics Tuition Singapore/JC Maths/H2 Math Tuition and Tutor

Hi A-Level/H2 Math Students

Vectors 1

Unit Vector
A unit vector is a vector of magnitude 1.

The unit vector in the direction of a^ is given by a^ = a/lal

Parallel Vectors
Two non zero vectors a and b are parallel if and only if a = kb for some k is element of real number.

Collinear Points
Three points A, B and C are collinear if and only if AB = k x BC for some k is element of real number

Length of Projection
Length of projection of a vector a on a vector b
= | a . ^b |

Angle Between 2 Vectors
cos x = (a.b)/(lal x lbl)
x = the angle between the 2 vectors

Area of Parallelogram and Triangle

Area of parallelogram = l a x b l
Area of triangle = 1/2 l a x bl

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Atoms, Molecules and Stoichiometry – Notes

7 Volumetric (or titrimetric) Analysis

• Volumetric analysis (or titrimetric analysis) is a method of quantitative analysis which depends essentially on the accurate measurements of volumes of two solutions which react together completely.

• A standard solution is defined as one which is of known concentration. This standard solution is used to determine the concentration of another solution through a titration process.

• The equivalence point is then said to have been reached and it marks the completion of the titration.

• In practice, the completion of a titration is detected by a distinct colour change brought about by the use of a suitable indicator. The point at which this distinct colour change occurs is called the end-point of the titration.

• The use of a suitable indicator will ensure that the end-point is very close to the equivalence point.

7.1Acid – Base Titrations

• Acid-base titrations are carried out in order to establish the stoichiometric amounts of acid and base which are required to neutralise each other. There are four main types of acid-base titrations:
(a) strong acid  strong base titration
(b) strong acid  weak base titration
(c) weak acid  strong base titration
(d) weak acid  weak base titration

7.2 Titration Curves (to be further discussed in Ionic Equilibrium)

Below are titration curves showing the pH changes as a 0.1 mol dm-3 solution of a strong or weak alkali is added to a 0.1 mol dm-3 solution of a strong or weak acid. The precise shape of the curves depends on the actual strengths of the alkalis and acids concerned. Slightly different curves are obtained too, if solutions other than 0.1 mol dm-3 are used.

7.3 Indicators

• The indicators used in acid-base titrations are generally organic dyestuffs which are weak acids or weak bases.

• In an acid-base titration, the indicator used is usually added in a small quantity to help to determine the end point of the titration. Each indicator has a pH range over which it changes colour. The choice of an indicator for an acid-base titration depends on the pH range of the indicator and the type of titration involved. It is important to pick an indicator which changes colour over the range in which there is a marked pH change during the titration.

• Essential characteristics of a good indicator:

1. The colour change of the indicator must be clear and sharp i.e. it must be sensitive. Thus it would be useless if 2 or 3 cm3 of the reagent were necessary to bring about the colour change.

2. Its pH range for colour change must coincide with the region of rapid pH change in the titration curve. In other words, the pH range of the indicator must fall on the vertical portion of the titration curve.

7.4 Back Titration

• Back titrations are usually employed when a direct titration is not possible, e.g. solid substances (CaCO3 in colgate) where the end-point is difficult to detect and volatile substances (ammonia, iodine) where inaccuracy arises due to loss of substance during titration.

• In back titration, a known excess of one reagent A is allowed to react with an unknown amount of a reagent B. At the end of the reaction, the amount of A that remains unreacted is found by titration with a standard reagent C. A simple calculation gives the amount of A that has reacted with B and also the amount of B that has reacted.

Steps in back titration calculations:

1. Write balanced equations for the reactions.

2. Calculate the initial no. of mole of reactant A added.

3. Calculate the no. of mole of reactant A remaining (that reacted with standard reagent C).

4. Calculate the no. of mole of reactant A reacted with reactant B.

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A Level GP/General Paper Tuition Singapore

PRACTICE EXERCISES ON SUBJECT-VERB AGREEMENT:

TASKS:
1.Fill in each blank below with the right answer/form of the word given.
2. Underline the subject of each sentence as well.

FUNCTIONS OF A FAMILY:
Enculturation (1)__________( verb to be) the passing on of culture, traditions and customs which (2)________( has) to be taught to children by parents for them to fit into society. This (3)_________(verb to be) sometimes called socialization.

It (4)_________ (verb to be) also argued that forming a family through marriage (5)__________(allow) for the stabilization of adult personalities. This (6)________(occur) when they (7)____________( take) on multiple roles as husband/wife or father/mother and (8)____________( assume) the responsibilities that (9)_____________( come) with these roles. This allows for their personal development.

PERCEIVED CHARACTERISTICS OF YOUTH:
Contrary to the stereotype that today’s youth (10)________(verb to be) self-centred, our young citizens are starting interest groups, non-profit business & enterprises to support issues such as alleviating poverty to give delinquents a second chance.

In a highly globalized world, travel and new media (11)___________ (has) exposed young people to issues and social justice efforts abroad, and have prompted many to get involved .Experts also (12)____________ (suggest) their activism may (13) _____ (stem) from the interest in volunteerism that the education system has tried to inculcate in students.

RELEVANCE OF THE FAMILY’S ROLE:

Institutions like social security (14)___________ ( has) taken away responsibilities of a family because people (15)_____________ ( rely) on them to take care of the elderly instead of doing it themselves.

Childcare is another institution that (16) ______ (has) gradually (17) __________ (become) more popular as a result of the emergence of the dual-income family. Although childcare services (18)___________( provide) much-needed support for families, they at the same time (19)_____________ (reduce) the role(20)______________( play) by the parents in nurturing their children.

The media (21)__________( verb to be) part of our everyday enculturation. We (22)__________( verb to be) bombarded with popular culture through television, radio, advertisement and the internet to help us (23)_____________( determine) our views of the world. Findings in 2008 (24) _____________ ( reveal) that the average child spends nearly 45 hours a week immersed in the media – almost three times the amount of time they spend with their parents.

In short, these institutions (25)_____________( compete) with the family’s role in shaping our young. And yet, there is no doubt that a weak family foundation (26)_______________( engender) a number of social problems such as alcohol and drug abuse, smoking, vandalism, theft, gang violence which (27)_______ (plague) our youth. (28)__________( verb to be) it time to glean wisdom from our forefathers?

ANSWERS:
1. is
2. has
3. is
4. is
5. allows
6. occurs
7. take
8. assume
9. come
10. are
11. have
12. suggest
13. stem (remind students that verbs after modals remain in base form – no SVA needed)
14. have
15. rely
16. has
17. become
18. provide
19. reduce
20. played
21. is/are (it is now acceptable in Standard English for ‘media’ to take either a singular or plural verb – 2005 Cambridge Markers’ Report)
22. are
23. determine
24. reveal
25. compete
26. engenders
27. plague
28. is

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A-Level Physics Tuition Singapore/H2 Physics Tuition/JC Physics Tutor

Hi A-level/H2/JC Physics Tuition students

Mastering Qualitative Questions

Chapter 16 Electromagnetic induction

1. State Lenz’s Law and explain how it is directly related to the law of conservation of energy.

2. State Faraday’s law of electromagnetic induction and hence explain why when the current in the coil A is switched on, the millivoltmeter connected to the neighbouring coil B indicates an induced e.m.f. for a short period of time.

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A-Level Biology Tuition Singapore/H2 Biology Tuition/JC Biology Tutor

TOPIC 1: CELLULAR FUNCTIONS – Part 13

LEARNING OUTCOME

(m)Explain the need for the production of genetically identical cells and fine control of replication.

ESSAY ANSWER

The production of genetically identical cells is needed for growth, repair and asexual reproduction [refers to LO (l)].
Fine control of replication is needed for the following reasons:

1. Ensure that all the genetic information is retained during the doubling of the DNA, i.e. the integrity of the genetic information is maintained. Genetic stability for growth, repair, and asexual reproduction results from the semi-conservative replication of DNA during the S phase.

2. Ensure that no genetic variation (not including spontaneous mutations) occurs in mitosis so that the chromosomes in each daughter cell carry the same hereditary content as the parental cells in terms of number and type.

3. Prevent the formation of cancerous cells. Cell division in normal cells is regulated by a cell cycle control system. Cancerous cells have
mutations in genes that regulate cell growth and division protooncogenes
and tumor-supressor genes), leading to changes in regulation of the cell cycle control, thus dividing excessively to form a mass of cells – tumor.

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