Chemistry Past Paper PDF

MULTIPLE CHOICE MCQ 1 Total mass of reactants = Total mass of products Antoine Lavoisier determined that the total mass of the products of a reaction always equal the total mass of the reactants. Mass of reactant = Mass of Product...

MULTIPLE CHOICE MCQ 1 Total mass of reactants = Total mass of products Antoine Lavoisier determined that the total mass of the products of a reaction always equal the total mass of the reactants. Mass of reactant = Mass of Product H2 +O2 = H2O 10g + 5g = 15 g Example: When heated, the mercury oxide (red powder) turned into mercury metal and a gas, oxygen. When Lavoisier measured the mass of the reactants and products, he found the following: 1 Propane burns in oxygen to produce carbon dioxide and water. The reaction follows this equation: 44 g + 160g 132g + ? 204 g - 132 = Water If 44g propane react with 160g oxygen and produce 132g carbon dioxide, what is the mass of water produced? Propane + Oxygen --> Carbon dioxide + water 44 + 160 = 132 + ? 204 = 132 + ? 204 – 132 = 72g water produced 1 Balancing Equations – Watch This 1 A balanced chemical equation has the same number of atoms of each element on both sides of the arrow. In order to balance this equation, we cannot change the substances. But we can change the coefficients. Try in the table below: 1 1X2 1 2 → Mg(s) + O2(g) MgO(s) Mg(s) + O2(g è 2MgO(s) 2HgO(s è 2Hg(l) O2(g) ) ) Mg 1 1 Hg 2 2 O 2 1X2 (mercury)  4 4 1 X4 X4 4 1 1 2 In a single-displacement reaction, one element replaces another element in a compound. Example: Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s) An activity series lets you predict whether a single- displacement reaction will occur. Al A more active metal will replace any less active metal. PbSO4 Cu No3 → Fe(s) + ZnCl2(aq) Zn(s) + FeCl2(aq) 2 In a double-displacement reaction, the positive ion of one compound replaces the positive ion of the other, forming two new compounds. A double displacement reaction usually produces either water, a gas, or a solid precipitate as one product. A precipitate is an insoluble compound that comes out of solution during this type of reaction. In the example, copper(II) hydroxide is the precipitate. You can identify the precipitate by the (s). Example: CuCl2(aq) + 2NaOH(aq) → Cu(OH)2(s) + 2NaCl(aq) 3 The overall chemical equation for photosynthesis is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 Here, light energy is absorbed by chlorophyll and other pigments in the chloroplasts, driving the synthesis of glucose from carbon dioxide and water. This absorbed energy is what makes the process endergonic 3 When the energy needed to keep an endergonic reaction going is in the form of thermal energy, the reaction is called an endothermic reaction. Chemical reactions that absorb more energy than they release are called endergonic reactions.  4 A catalyst is a substance that speeds up a chemical reaction without being used up itself. Catalysts are used in industry to speed up reactions such as plastic production. Your body uses special catalysts called enzymes to break down food and keep you alive. Cars contain a catalytic converter that decomposes toxic exhaust gases. As a catalyst is not used up in a reaction, it can often be recovered and reused.  4 Sometimes a reaction needs to be slowed down. An Inhibitor is a substance that slows down reactions or prevent them from occurring. Like catalysts, they themselves are not changed in the reaction. Inhibitors are used as preservatives in food to slow down decomposition (the food spoiling). Inhibitors are also used in many body lotions and cosmetics to prevent spoilage. 4 Many reactions can happen in both directions. We call these reversible reactions. They occur in both forwards and reverse directions at the same ! time: reactants products. Equilibrium is a state in which forward and reverse reactions proceed at equal rates. A double arrow is used to indicate equilibrium. N2, 3H2, and 2NH3 are all being produced at the same rate: The equilibrium becomes temporarily unbalanced when under stress. But in time, a new equilibrium is found. Unbalanced equilibrium means one reaction happens faster than the other. Le Châtelier’s principle states that if a stress is applied to a system at equilibrium, the equilibrium shifts in the direction that opposes the stress. The equilibrium shifts to oppose  5 When gases dissolve in gases or when liquids and gases dissolve in liquids, particle movement spreads solutes throughout the mixture. Liquids and gases move much more freely than solids, so this process happens relatively fast. Solids do not move freely. Therefore, solid solutions are made by first melting the solids, then mixing them together while they are still molten. Once evenly  5 Dissolving of a solid in a liquid occurs at the surface of the solid. Recall: water molecules are constantly moving, and water is a polar molecule. (polar mean: having positive Step 1 : At the and negative surface of a sugar crystal, ions) oppositely charged parts of the sugar and water molecules attract each other. Step 2 : Because water molecules are moving in the liquid, they pull sugar molecules away from the Stepcrystal. 3 : Water molecules and sugar molecules continue to spread until a homogeneous mixture forms. 5 1 2 3  6 Concentration The concentration of a solution is the amount of solute dissolved in a given amount of solvent. 17 Volume of solute ___ X100 __________________ Volume of solvent X100 25 A dilute substance has low concentration, a concentrated substance has high concentration. One way to express concentration is percentage by volume. 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 E.g. adding 25ml orange 𝑡𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 juice ∗ 100 to 75ml water = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 % 𝑣/𝑣 gives ___% juice.∗ 25𝑚𝑙 100 = 25 % 𝑣/𝑣 25𝑚𝑙 + 75𝑚𝑙  6 Solubility Solubility is the maximum amount of a solute that can be dissolved in a given amount of solvent at a given temperature. Solubility is often expressed as grams of solute per 100 g of water. E.g. 1g of solute A can dissolve in 100ml of water. Add any more and it just sinks to the bottom without dissolving. But 3g of solute B can be added before it stops dissolving. 6 I can Add more 7 8 A combustion reaction occurs when a substance reacts with oxygen to produce energy in the form of heat and light. Example: C(s) + O2(g)→ CO2(g) Some combustion reactions also fit into other categories. E.g. the example above is also a synthesis reaction… 8 When the energy of an exergonic reaction is given off mostly in the form of thermal energy, the reaction is called an exothermic reaction. The burning of coal and other fossil fuels, or even rocket fuel, are examples of exothermic reactions. 9 Answer in the next slide 9 10 The collision model states that atoms, ions, and molecules must collide with enough energy to react. Temperature generally increases reaction rates. At higher temperatures, particles have more energy and move faster. As a result, they collide more frequently. They also collide with greater energy, increasing the percentage of collisions resulting in a reaction. 10 To calculate a rate of reaction, divide the change in amount of substance by the number of seconds the reaction took. E.g. The concentration of CH3CHO increases from 0.0300 mol/L to 0.0900 mol/L in 25 seconds in the following reaction. What is the average rate of the reaction? Concentration increased from 0.03 à 0.09, therefore by 0.06 mol/L Change in concentration / time taken = Rate of a chemical reaction 0.06 mol/L ÷ 25 s = 0.0024 mol CH3CHO/L·s 11 At the surface of a sugar crystal, oppositely charged parts of the sugar and water molecules attract each other.  11 The rate at which a solute dissolves into a solvent can be increased by: stirring increasing the surface area of the solute increasing the temperature of the solvent Stirring moves the solvent around, bringing fresh solvent into contact with the solute. This solvent can then pull the solute away. Increasing temperature brings more solvent into contact with the solute, and increases the energy of these contacts, making them more likely to break loose chunks of the solute Increased surface area means more places for the solvent to pull away molecules or atoms of solute. 12 12 140 100 95 60 39g 13  14 The law of conservation of energy states that energy can change from one form to another, but the total amount does not change. Fire woks (10 J -------- 7 J light + 3J Sound ) In a chemical reaction, the energy released or absorbed is equal to the difference between the chemical energy of the reactants and products. The following equation expresses this relationship for an exergonic reaction: chemical chemical energy energy of energy of + released reactants products 14 Chemical reactions that release more energy than they absorb are called exergonic reactions. The glowing abdomen of a firefly is due to light energy given off by exergonic reactions. In an exergonic reaction, the energy required to break the chemical bonds is less than the energy released when new  ion combine to form Compound 15 Na+ Cl- = NaCl Charged Partical ===== Loss electron (+) Gains electron (-) Dissociation is a process in which Ionization is a process in which positive and negative ions of an neutral molecular compounds ionic solid mix with solvent to form a dissolve in water and separate solution. into charged particles. Dissociation happens with substances that are already made of ions, not Example: H2O surrounds neutral neutral molecules. HCl molecules and pulls them Example: NaCl dissociates as H2O apart to form H+ and Cl- ions. The molecules attract and pull the Na+ H2O molecules and H+ ions and Cl- ions from the crystal. The H2O attract each other and form molecules then surround and 15 Oils on human skin and hair are important for holding in moisture, but they can also attract and hold dirt. Dissolve in water As oil and dirt is a nonpolar mixture, trying to wash them Polar in with water alone will not properly clean away the dirt. polar That’s why we need soap. Soaps are salts of fatty acids. A fatty acid is a long, nonpolar hydrocarbon chain with a polar –COOH Non polar group at the end. dissolve in Oil  15 To turn it into a salt, the H from –COOH is replaced with a positive sodium or potassium ion (-COONa) this forms a highly polar ionic bond. The long nonpolar tail interacts with dirt and oils, while the polar head interacts with water. When water binds to the polar head and dissolves it, any nonpolar material stuck to the tail is also dissolved!  Precise Means – Exact value Relative means – Not exact value An unsaturated solution is a solution that can dissolve more solute at a particular Unsaturated solution means a temperature. solution with little solute, you can add more solute to make it saturated. If you heat a saturated solution, it often When we say precise means, the becomes unsaturated and can dissolve more exact value of solute that can solute. dissolve in a given amount of solvent. For example, solubility of salt is 35.9 which is an exact value. A supersaturated solution is one that contains But unsaturated can be any amount more solute than a saturated solution. of solute, so the value is not exact. So it is not precise. WRITING PART FRQ WRITING QUESTION 1  → NiCl2(aq) + 2NaOH(aq) Ni(OH)2(s) + 2NaCl(aq). The numbers in front of the formulas in a chemical equation are called coefficients. When no coefficient appears, a coefficient of 1 is assumed. How many molecules of oxygen (O2) are in the reactants of the following equations? How many molecules of ammonia (NH3) are produced in the following equation? Answer: 2 There are 3 molecules of sulphuric acid (H2SO4) reacting with 2 molecules of Aluminiun (Al) Giving 1 molecule of Aluminium sulphate (Al2 (SO4 )) and 3 molecules of Hydrogen  A chemical reaction is a change in which one or more substances are converted into new substances. The substances at the start, the substances that react, are called reactants. The new substances produced are called products. Reactant à Product Reactant + Reactant à Product + Product The reactants react to produce the products Reactants Products  Mass No of Molecule 1 X2 + 16 = 18g One mole is the amount of substance that contains 6.02 × 1023 particles of that substance. Molecule = Coefficient Equation C(s) + O2(g) è CO2(s) Number of moles 1 1 1 Mass 12.01 g 32.00 g 44.01 g Number of particles 6.02 × 1023 6.02 × 1023 6.02 × 1023 atoms molecules molecules Substance amu Molar Mass Carbon, C(s) 12.01 12.01 g/mol Oxygen, O2(g) 32 2 × 16.00 g/mol = 32.00 g/mol Nitrogen dioxide, NO2(g) 46.01 14.01 g/mol + (2 × 16.00 g/mol) = 46.01 g/mol a. Synthesis Reaction - two reactants react to give a new substance a. Single displacement reaction - Fe (Iron) being more reactive than Cu (Copper) displaces copper a. Combustion reaction - 12 O2 in the reactant side shows how the oxygen reacts and helps to burn a. Double displacement reaction - Na replace Ag and Ag replace Na (check the activity series) a. Decomposition reaction - One reactant splits or breaks down When the energy of an exergonic reaction is given off mostly in the form of thermal energy, Activation energy: the the reaction is called an energy required to break the bonds in the exothermic reaction. reactants Chemical reactions that Energy released: the release more energy than difference in energy they absorb are called between reactants and exergonic reactions products WRITING QUESTION 2  Increasing the concentration of a reactant in a For chemical reactions involving gases, volume solution increases the rate of a reaction. The and pressure of the gas are important because more reactant particles per unit volume, the they relate to the concentration of the gases. greater the chance that a reaction will occur. Increasing the pressure and decreasing the Consider the reaction: Mg (s) + HCl (aq) à volume of a gas increases the concentration of MgCl2 (aq) + H2 (g) the gas. The bubbles indicate H2 gas being produced. This increases the number of successful The reaction with higher concentration HCl is collisions, and therefore increases reaction rate. much faster than the reaction with low concentration HCl You need to produce more ethanal which is the product, that means you need to shift the equilibrium to the ri 1. Increase the concentration of the reactants so the system will try to decrease it by initiating forward reacti Equilibrium will shift to the right. 2. Forward reaction is exothermic and backward reaction is endothermic, So decreasing the temperature, the Will try to increase it, to increase the temperature we need exothermic reaction and so it initiates forward rea The equilibrium will be shifted to the right. 3. Number of molecules (coefficient) in the reactant side is 2 and in the product side it is 1, so more pressure y The reactant side. So if you increase the pressure, system will try to decrease it by moving to the side where th Is less which is the product side, so forward reaction happens, and the equilibrium is shifted to the right. WRITING QUESTION 3  A solution is a homogeneous mixture that remains constantly and uniformly mixed and has particles so small they cannot be seen with a microscope. The individual molecules are evenly spread out through the mixture. A solute is a substance being dissolved. A solvent is a substance in which a solute is dissolved. Example: In salt water, salt is the solute and water is the solvent.  A solution can be Air with solute and solvent as gas, or solution can be solid with solute and solvent being solid Solutions can also be gaseous or even solid. Example: Air is a solution. An alloy is a mixture of elements that has metallic properties. Alloys are made by melting a solid solute and solvent together. Example: Sterling silver is an alloy of 92.5 percent silver (solvent) and 7.5 percent copper (solute). For the rectangle : Width 1.5cm, Length 3cm, Height 3cm 2(1.5cm x 3cm) + 2(3cm x 3cm) + 2(3cm x 1.5cm) 2 x 4.5 + 2 x 9 + 2 x 4.5 9 + 18 + 9 = 36 cm3 WRITING QUESTION 4 Solubility is often expressed as grams of solute per 100 g of water. E.g. 1g of solute A can dissolve in 100ml of water. Add any more and it just sinks to the bottom without dissolving. But 3g of solute B can be added before it stops dissolving.  Solubility is the maximum amount of a solute that can be dissolved in a given amount of solvent at a given temperature. Solubility is often expressed as grams of solute per 100 g of water. Solubility = solute /solvent x100 As per the table, 1. Table sugar is most soluble with 203.9 solubility 2. Salt is in the middle with 35.9 solubility 3. Washing soda is the least soluble with 21.4 solubility  Increasing pressure or decreasing temperature allows more gas to be dissolved in a liquid (a hotter solution will encourage more gas particles to escape out of solution). Opening a soft drink bottle reduces pressure at the surface, so dissolved CO2 gas bubbles out. A warm soft drink produces more bubbles than a cold soft drink.  A saturated solution is a solution that contains all of the solute that it can hold at a given temperature. If you try to add any more solute, it will not dissolve. As the temperature of a liquid solvent increases, the amount of solid solute that it can dissolve increases.