Chemistry Past Paper PDF

## Chapter 1 - Building Blocks of Matter ### Exercise 1-9 Show the covalent bonding in chlorine molecule (Cl₂) which is formed from combination of 2 chlorine atoms. Atomic number: 17 ### Exercise 1-10 Show the covalent bonding in ammonia molecule, NH₃ by drawing. Hydrogen ha...

## Chapter 1 - Building Blocks of Matter ### Exercise 1-9 Show the covalent bonding in chlorine molecule (Cl₂) which is formed from combination of 2 chlorine atoms. Atomic number: 17 ### Exercise 1-10 Show the covalent bonding in ammonia molecule, NH₃ by drawing. Hydrogen has one electron in its outer energy level. Nitrogen has two electrons in its first energy level and five electrons in its second energy level. ### 1-8 Valency The number of electrons found on the last energy level of the atoms is called valency, valence, or valence electrons, and valency determines the chemical behavior of an element. The electrons in the interior shells play a secondary role because of the effect of the existing charge in the nucleus of an atom. The valence electrons affect the behavior of an atom during the formation of a chemical bond(s) between atoms. Therefore, valence electrons can be defined as the number of lost, gained, or shared electrons of atoms obtained during the chemical reactions. Valency is beneficial in writing chemical formulas. To find the valency of an atom, the following rules must be applied: 1. The number of electrons in the outer shell and whether the shell is full or not must be known. 2. The number of electrons which would be lost, gained or shared by the atoms to fill their last shells must be known. During the formation of a chemical bond between atoms, the last shell plays a great role. Because electrons used to form the bonds(s) are transferred or shared from the last shell of the atoms. So, the electrons which are found at the last energy level in an atom are called as valence electrons. For example: lithium atom (atomic number is 3) contains one valence electron, so lithium atom tends to lose this one electron during the formation of chemical bonding. After losing that one electron from its outermost shell, it changes into lithium ion, Li⁺. Lithium is considered as a univalent element. ### Building Blocks of Matter An oxygen atom (Z=8) contains six valence electrons. Therefore, an oxygen atom tends either to gain or to share two electrons with another atom(s) to form two bonds. At the end of this, an oxygen atom completes its last energy level with eight electrons. This is what we want because all the atoms want to saturate its valence shell to have the full shell as in the noble gases. A polyatomic ion is known as a molecular ion, is a charged species (ion) composed of two or more atoms covalently bonded to each other. The polyatomic ions such as hydroxide ion, OH^-, ammonium ion, NH₄⁺, divalent carbonate ion, CO₃^2-, and sulfate ion, SO₄^2-, and trivalent phosphate ion, PO₄^3-, have valency. They can be considered to be acting as a single unit in the formation of compounds. ### 1-9 Oxidation Number and Chemical Formula The total number of natural and synthetic chemical compounds runs in the millions. For some of these substances, certain common names remain in everyday use. For example, calcium carbonate is better known as limestone, and sodium chloride is usually referred to simply as table salt. And everyone recognizes dihydrogen monoxide by its popular name, water. Unfortunately, common names usually give no information about chemical composition. To describe the atomic makeup of compounds, chemists use systematic methods for naming compounds and for writing chemical formulas. In this chapter, you will be introduced to some of the rules used to identify simple chemical compounds. ### 1-9-1 Significance of a Chemical Formula The formula for a compound is a shorthand way of writing the name for a compound. The chemical symbols that are assigned to the elements are used to write formulas. A chemical formula is a symbolic representation of: 1. The elements present in a compound. 2. The relative numbers of atoms of each element. Consider the following formulas. The element present are denoted by their symbols and relative numbers of atoms by subscript numerals. ### Important!!! Atoms are the building blocks of elements. Molecules are the building blocks of compounds. The only difference between atoms and molecules is that chemically bonded different atoms form the molecules. But molecules of divalent compounds are formed by bonding the same atoms. As shown above, the subscripts of the formula indicate the relative numbers of atoms of each type that are combined. If a symbol carries no subscript, the number 1 is accepted. For example: the formula of sulfuric acid, H₂SO₄, indicates that two atoms of hydrogen, one atom of sulfur, and four atoms of oxygen are contained in a formula unit of sulfuric acid. Formulas used to show the atomic composition of some elements The number in front of a symbol or formula is called a **coefficient**. For example: 2H₂O 2 is the **coefficient** and the subscript is **2** ### Building Blocks of Matter In the above example, it is the formula of water which contains four hydrogen atoms and two oxygen atoms. ### Example 1-10 How many atoms are found in the following formulas? a) 2HCl b) (NH₄)₂SO₄ c) K₂SO₄ d) 5H₃PO₄ #### Solution: a) 2HCl = 2 x 1(H)+ 2 x1(Cl)= 4 atoms b) (NH₄)₂SO₄ = 2 x 1(N)+ 4 x 2(H) +1(s) + 4x1(0) = 15 atoms c) K₂SO₄ = 2 x 1(K)+1(s) + 4xl(0)= 7 atoms d) 5H₃PO₄ = 5 x 3(H)+5 x 1(P)+ 5 x 4(0)= 40 atoms By gaining or losing electrons, many elements form ions with noble gas configurations. For example, some elements lose one electron to give +1 cations, such as Na⁺. ### Exercise 1-11 How many atoms does each of the following compounds consist of? a) Ba(OH)₂ b)3Fe₂O₃ c) 5Li₂CO₃ d) 10H₂S e) (NH₄)₂CO₃ f) 5MgBr₂ ### Important!!! Oxidation number is the number of electrons gained or lost by any atom during the formation of a chemical bond. It can be either a positive integer number or a negative integer number. Some elements lose two electrons to give +2 cations, such as Mg²⁺. Ions formed from a single atom are known as **monatomic ions**. Some elements gain electrons to form anions. For example, in ionic compounds nitrogen forms the -3 anion, N^3-. The three added electrons plus the five outermost electrons in nitrogen atoms give a completed outermost shell as shown in the following figure. ### Building Blocks of Matter In writing the formulas for most simple compounds, the following rules are applied. 1. Cations are written first. If atoms tend to lose one or more electrons to form positive ions, they are called **cations**. 2. Anions are written last. If atoms tend to gain one or more electrons to form negative ions, they are called **anions**. 3. The net charge on the resulting compound must be zero. Thus, the charges indicated by the valence numbers must be balanced. To do this, subscripts are written to the right of the element or polyatomic ion. ### Important!!! XY is a compound then n and m are positive integer numbers, n and m є 1, 2, 3, 4, ...etc. 4. The valence of atomic ion must equal the algebraic sum of the charges assigned to the individual atoms making up the polyatomic ion. 5. If a polyatomic ion appears more than once in the formula, it is enclosed in parentheses, and the subscript is placed just outside to the right. The formula for the compound formed between the ions Xⁿ⁺ and Y^m-, may be written Xⁿ⁺Y^m-, X_nY_m. If |n| = |m|, then the formula is shortened to XY. ### Exercise 1-12 Write the correct formula and name of compounds formed between the following ions. a) Na⁺ and Cl^- b) Mg²⁺ and Cl^- c) Al³⁺ and O^2- d) Mg²⁺ and N^3- e) Pb²⁺ and O^2- f) NH₄⁺ and Br^- g) NH₄⁺ and SO₄^2-. i) Fe³⁺ and OH^- | Name of the Compound | Chemical Formula | Name of the Compound | Chemical Formula | |---|---|---|---| | Sodium chloride | NaCl | Calcium oxide | CaO | | Magnesium bromide | MgBr₂ | Aluminum oxide | Al₂O₃ | | Barium hydroxide | Ba(OH)₂ | Lithium carbonate | Li₂CO₃ | | Barium carbonate | BaCO₃ | Ammonium sulfate | (NH₄)₂SO₄ | | Hydrogen sulfide | H₂S | Aluminum sulfate | Al₂(SO₄)₃ | | Calcium phosphate | Ca₃(PO₄)₂ | Dihydrogen monoxide (water) | H₂O | ### 1-10 Periodic Table The periodic table is a tabular display of the chemical elements, organized on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are presented in order of increasing atomic number (number of protons). ## Chapter 2 - Chemical Reactions ### 2-1 Chemical Reactions Let's start with the idea of a reaction. In chemistry, a reaction happens when two or more molecules interact and the molecules change. That's it. What molecules are they? How do they interact? What happens? The possibilities are infinite. When you are trying to understand reactions, imagine that you are working with the atoms. Imagine the building blocks are right in front of you on the table, instead of billions of reactions in your beaker. Sometimes we do this using our chemistry toys to help us visualize the movement of the atoms. There are a few key points you should know about chemical reactions: 1. A chemical change must occur. You start with one compound and turn it into another. That's an example of a chemical change. A steel garbage can rusting is a chemical reaction. That rusting happens because the iron (Fe) in the metal combines with oxygen (O₂) in the atmosphere. When a refrigerator or air conditioner cools the air, there is no reaction between the air molecules. The change in temperature is a physical change. When you melt an ice cube, it is a physical change. When you put bleach in the washing machine to clean your clothes, a chemical change breaks up your stains. 2. A reaction could include ions, compounds, or molecules of a single element. We said molecules in the previous paragraph, but a reaction can happen with anything, just as long as a chemical change occurs (not a physical one). If you put pure hydrogen gas (H₂) and pure oxygen gas in a room, they can be involved in a reaction. The slow rate of reaction will have the atoms bonding to form water (H₂O) very slowly. If you were to add a spark, those gases would create a reaction that would result in a huge explosion. Chemists call that spark a catalyst. 3. Single reactions often happen as part of a larger series of reactions. Take something as simple as moving your arm. The contraction of that muscle requires sugars for energy. Those sugars need to be metabolized. You'll find that proteins need to move in a certain way to make the muscle contract. A whole series (hundreds) of different reactions are needed to make that simple movement happen. In the case of your arm, some are physical changes and some are chemical. In the process of making sugars in a plant, you might have as many as a dozen chemical changes to get through the Calvin cycle which makes glucose (C₆H₁₂O₆) molecules. ### Chemical Reactions & Equations Chemical reactions are occurring inside our bodies and around us at every moment. The food we eat is involved in a large number of chemical reactions to produce energy, as well as to make tissues and other substances in our bodies. Outside our bodies the number and types of chemical reactions are more abundant and obvious. From the burning of birthday candles on our cakes to the burning of gasoline in our car engines, combustion is a vital chemical reaction we use and see on a daily basis. The processing of materials to produce plastics is an example of chemical reactions. The number and types of chemical reactions are many, but in each case there is a common property: the production of new substances. In chemical reactions starting substances called reactants have their atoms rearranged to form products. The products contain the same number and types of atoms as the reactants but have rearranged to form new substances. Scientists such as John Dalton (1766-1844) developed our view of chemical reactions in terms of the rearrangement of atoms. The contributions of many scientists and many years of research have further developed our understanding of chemical reactions. We now know that in chemical reactions the electrons that surround the nuclei of atoms play a key role through their ability to form chemical bonds with other atoms. These chemical bonds can be formed in a number of ways, including the sharing of electrons to form covalent bonds or the complete transfer of electrons to other atoms to form ionic bonds. An understanding of chemical bond formation is essential to understanding and predicting chemical reactions. Energy is required to initiate all chemical reactions, though ultimately some chemical reactions release energy to the surroundings and other chemical reactions absorb energy from the surroundings. The rate at which chemical reactions occur is also a vital consideration. Chemical reactions show a great deal of variation, from the slow rusting of a car to the rapid combustion of gasoline in the engine. Scientists have studied reaction rates in detail, since controlling reaction rates in the production of materials is very important. There are many variables that can affect chemical reaction rates, and these variables include temperature, pressure, and the presence of special substances called catalysts, which accelerate the reaction rate without themselves being consumed. ### 2-2 Chemical Equations A chemical equation is a type of relation you will encounter every day in chemistry. Here's a look at what a chemical equation is and some examples of chemical equations. A chemical equation is a written representation of the process that occurs in a chemical reaction. A chemical equation is written with the reactants on the left side of an arrow and the products of the chemical reaction on the right side of the equation. The head of the arrow typically points toward the right or toward the product side of the equation, although reactions may indicate equilibrium with the reaction proceeding in both directions simultaneously. ### Important!!! Coefficients are used to balance an equation. In a balanced equation, the number of each kind of atom is the same on both sides of the arrow. You can change the coefficients to balance an equation. You should never change the subscripts in the formula of a compound in order to balance the equation. This kind of reactions are called as the balanced chemical equation. The elements in an equation are denoted using their symbols. Coefficients next to the symbols indicate the stoichiometric numbers. Subscripts are used to indicate the number of atoms of an element present in a chemical species. An example of a chemical equation may be seen in the combustion of methane: CH₄ +20₂ → CO₂ + 2H₂O ### Important!!! You'll need to know the symbols for the elements to understand what is taking place in a chemical reaction. In this reaction, C is carbon, H is hydrogen and O is oxygen. #### Left Side of Reaction: Reactants The reactants in this chemical reaction are methane and oxygen: CH₄ and O₂. #### Right Side of Reaction: Products The products of this reaction are carbon dioxide and water: CO₂ and H₂O. #### Direction of Reaction: Arrow It is the convention to right the reactants on the lefthand side of the chemical equation and the products on the righthand side of the chemical equation. The arrow between the reactants and products should point from left to right. ### Example 2-1 Magnesium and sulfur elements are chemically combined to form compound magnesium sulfide. Write the word equation and the formula equation for this reaction. Magnesium + sulfur → magnesium sulfide (word equation) Mg + S → MgS (formula equation) ### Balancing chemical equations; A chemical equation describes what happens in a chemical reaction. The equation identifies the reactants (starting materials) and products (resulting substance). Balancing a chemical equation refers to establishing the mathematical relationship between same elements of reactants and products. #### Important!!! Coefficients are used to balance an equation. In a balanced equation, the number of each kind of atom is the same on both sides of the arrow. You can change the coefficients to balance an equation. You should never change the subscripts in the formula of a compound in order to balance the equation. What coefficients must be written and where should they be placed, in order to balance the following equation: Fe + O₂ → Fe₂O₃ There is only 1 iron atom on the left and 2 iron atoms on the right. Then you write 2 as the coefficient of Fe. There are 2 oxygen atoms on the left and 3 oxygen atoms on the right. You may write 3/2 in front of O₂. 2 Fe + 3/2 O₂ → Fe₂O₃ You can multiply both sides of the equation by 2, if you want to see the whole number coefficients. 4 Fe + 3O₂ → 2 Fe₂O₃ This equation shows that four atoms of (4Fe) iron combine with three molecules of oxygen (3O₂) to form two molecules of rust (2 Fe₂O₃). ### Exercise 2-1 Express the following chemical reactions both in word and formula equations. 1- Methane gas burns with oxygen gas to produce carbon dioxide and water. 2- After dissociation of calcium carbonate, calcium oxide and carbon dioxide compounds are produced. 3- The reaction between nitrogen gas and hydrogen produce ammonia. ### Exercise 2-2 1- Write a balanced chemical equation for each reaction. a) water → hydrogen + oxygen b) calcium hydroxide + hydrochloric acid → calcium chloride + water c) magnesium + sulfuric acid → magnesium sulfate + hydrogen d) calcium carbonate → calcium oxide + carbon dioxide 2- Balance the following equations. a) KCIO₃ → KCI +O₂ b) Fe + H₂O → Fe₃O₄ + H₂ ### 2-3 Types of Chemical Reactions A chemical reaction is a process that is usually characterized by a chemical change in which the starting materials (reactants) are different from the products. Chemical reactions tend to involve the motion of electrons, leading to the formation and breaking of chemical bonds. There are several different types of chemical reactions and more than one way of classifying them. Here are some common reaction types. ### 2-3-1 Combination Reactions (Synthesis Reactions) In a combination (or synthesis) reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form: A+BAB Two or more reactants yielding one product is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide: Fe + S → FeS Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water: H₂ + 1/2 O₂ → H₂O ### 2-3-2 Decomposition Reactions In a decomposition reaction, a compound is broken into smaller chemical species. Therefore, a decomposition reaction is the opposite of a synthesis reaction These reactions are in the general form: AB→A+B The electrolysis of water into oxygen and hydrogen gas is an example of a decomposition reaction: 2H₂O → 2H₂↑+O₂↑ 2HgO → 2Hg + O₂ ### 2-3-3 Single Displacement Reactions In a single replacement reaction, a single uncombined element replaces another in a compound; in other words, a substitution or single displacement reaction is characterized by one element being displaced from a compound by another element. These reactions come in the general form of: AB + C → A + BC One example of a single displacement reaction is when soduim replaces hydrogen in water to make soduim hydroxide and hydrogen gas: 2Na + 2H₂O → 2NaOH + H₂↑ Another example of a single displacement reaction occurs when zinc combines with hydrochloric acid. The zinc replaces the hydrogen: Zn + 2HCl → ZnCl₂+H₂↑ ### 2-3-4 Double Displacement Reactions In a double replacement reaction, the anions and cations of two compounds switch places and form two entirely different compounds. These reactions are in the general form: AB + CD → AD + CB For example, when barium chloride, BaCl₂, and magnesium sulfate, MgSO₄, react, the sulfate, SO₄^2- anion switches places with the chloride, Cl^- anion, giving the compounds barium sulfate, BaSO₄, and magnesium chloride, MgCl₂. BaCl₂ + MgSO₄ → BaSO₄ + MgCl₂ Another example of a double displacement reaction is the reaction of lead(II) nitrate, Pb(NO₃)₂ with potassium iodide, KI to form lead(II) iodide, PbI₂ and potassium nitrate, KNO₃: Pb(NO₃)₂ + 2 KI → PbI₂+ 2 KNO₃ One more example of a double displacement reaction occurs between sodium chloride, NaCl and silver nitrate, AgNO₃ to form sodium nitrate, NaNO₃ and silver chloride, AgCl. NaCl + AgNO₃ → NaNO₃ + AgCl ### 2-3-5 Combustion Reactions A combustion reaction is a major class of chemical reactions. Combustion usually occurs when a hydrocarbon reacts with water to produce carbon dioxide and water. In the more general sense, combustion involves a reaction between any combustible material and an oxidizer to form an oxidized product. Combustion is an exothermic reaction, so it releases heat, but sometimes the reaction proceeds so slowly that a temperature change is not noticeable. Good signs that you are dealing with a combustion reaction include the presence of oxygen as a reactant and carbon dioxide, water and heat as products. #### General Form of a Combustion Reaction Hydrocarbon + Oxygen → Carbon dioxide + Water #### Examples of Combustion Reactions ##### Combustion of methane CH₄ + 2O₂ → CO₂ + 2H₂O ##### Burning of naphthalene C₁₀H₈ + 12O₂ → 10CO₂ + 4H₂O ##### Combustion of ethane 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O ### Exercise 2-3 Determine the type of each of the following reactions :- H₂+I₂2HI 2HBr → H₂ + Br₂ Cl₂+ KBr → 2KCl + Br₂ 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O