Unit 4. Chemical Calculations

Transcription

1 Index Unit 4. Chemical Calculations 1.- Chemical reactions What is a chemical reaction? The law of conservation of mass Balance of chemical reactions Rate of reaction Factors that affect the rate of reaction Energy Changes in Chemical Reactions Amount of substance The mole Calculating concentration Stoichiometric calculations Calculation with masses Calculations with volume Calculations with volume in different conditions than STP conditions Calculations with reactants in solution Types of chemical reactions Acid-base reactions...14 Practice exam...16 Page 1 of 16

2 1.- Chemical reactions What is a chemical reaction? A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Reactants: The substance (or substances) initially involved in a chemical reaction. Products: The substance (or substances) obtained in a chemical reaction. 1.2 The law of conservation of mass (Lavoisier, 18th century): Lavoisier was one of the first to carry out quantitatively accurate chemical measurements. He demonstrated that combustion required oxygen, and he demonstrated oxygen's role in the rusting of metals. His observations led him to deduce the following general law known as the law of conservation of mass: In every chemical transformation, an equal quantity of matter exists before and after the reaction. A direct consequence of this law is the need to balance equations 1.3 Balance of chemical reactions When you write an equation for a chemical reaction, the two sides of the equation should balance (you need the same number of each kind of element on both sides.) 1) Cl 2 + Ag AgCl 2) S + O 2 SO 3 3) Br 2 + H 2 HBr 4) C 3 H 8 + O 2 CO 2 + H 2 O 5) H 3 PO 4 + NaOH Na 3 PO 4 + H 2 O 6) HNO 3 + Fe Fe (NO 3 ) 2 + H 2 7) HCl + Ca (OH) 2 CaCl 2 + H 2 O 8) P + O 2 P 2 O 5 9) Ag + HNO 3 AgNO 3 + H 2 10) Fe + O 2 Fe 2 O 3 11) Mg + H 2 O Mg(OH) 2 + H 2 12) Pb + H 2 PbH 4 13) Na + O 2 Na 2 O 14) C 6 H 12 O 6 + O 2 CO 2 + H 2 O 15) PbS + O 2 PbO + SO 2 16) CH 4 + O 2 CO 2 + H 2 O 17) H 2 SO 4 + LiOH Li 2 SO 4 + H 2 O Page 2 of 16

3 18) Br 2 + O 2 Br 2 O 5 19) Na 2 CO 3 + Ca(OH) 2 CaCO 3 + NaOH 20) Na 3 P + H 2 O PH 3 + NaOH 2. Formulate and balance the following equations: 1) Propane burns in presence of oxygen to produce carbon dioxide and water 2) The compound dinitrogen pentoxide is unstable at room temperature. It decomposes yielding a brown gas nitrogen dioxide and oxygen. 3) When calcium chloride is mixed with silver nitrate we obtain silver chloride and calcium nitrate. 4) When potassium chlorate is heated, it decomposes to give solid potassium chloride and oxygen gas. 5) A solution containing pure barium chloride is treated with excess amounts of sulfuric acid, and the precipitate barium sulfate is collected with hydrogen chloride 6) Dihydrogen dioxide decompose to give water and oxygen. 7) Hydrochloric acid reacts with magnesium hydroxide to produce magnesium chloride and water. 8) Methane react with oxygen to produce carbon dioxide and water. 9) Calcium carbonate decompose in calcium oxide and carbon dioxide 10) Sulfur dioxide reacts with oxygen to produce sulfur trioxide. 11) hydrogen reacts with iodine to produce hydrogen iodide. 12) hydrogen react with oxygen to produce water 13) carbon reacts with oxygen to produce carbon monoxide. 14) iron reacts with oxygen to produce iron(iii) oxide. 15) hydrogen reacts with iodone to produce hydrogen iodide 16) hydrogen and oxygen prodece water 17) nitrogen and hydrogen produce ammonia 18) ethene reacts with oxygen to produce carbon dioxide and water 19) ethyne react with hydrogen to produce ethane 20) carbon reacts with oxygen to produce carbon monoxide Rate of reaction Some reactions are slow, such as rusting, and some are fast, like burning. The rate of reaction depends on the temperature and concentration of the reactants, and the surface area of any solid reactants. The rate of reaction can be found by measuring the amount of reactant used up, or the amount of product formed, in a given time. Catalysts increase the rate of a reaction without being changed themselves by the end of the reaction. Page 3 of 16

4 Collision theory Different reactions can happen at different rates. Reactions that occur slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, rusting is a slow reaction: it has a low rate of reaction. Burning and explosions are very fast reactions: they have a high rate of reaction. For a chemical reaction to occur, the reactant particles must collide. But collisions with too little energy do not produce a reaction. The particles must have enough energy and right orientation for the collision to be successful in producing a reaction. The rate of reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction. 1.5 Factors that affect the rate of reaction The rate of a reaction is the speed at which a reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands, of years while others can happen in less than one second. The rate of reaction depends on the type of molecules that are combining. If you want to think of a very slow reaction, think about how long it took dinosaur bones to become fossils through breakdown. You can thank chemical processes in bacteria for most of those dinosaur bones in the museum. There is another big idea for rates of reaction called collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If there are a higher number of collisions in a system, more combinations of molecules can occur. The reaction will go faster and the rate of that reaction will be higher. Even though they are both liquids, think about how slowly molecules move in honey when compared to your soda. There are a lower number of collisions in the honey. Reactions happen - no matter what. Chemicals are always combining or breaking down. The reactions happen over and over, but not always at the same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur. a) Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will Page 4 of 16

5 be fewer collisions and the reaction will probably happen at a slower speed. Sometimes when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker. b)temperature: When you raise the temperature of a system, the molecules bounce around a lot more because they have more energy. When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction. Back to the chemistry lab! Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower. c) Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction. d) Surface area.this means that the more finely divided a solid or liquid reactant the greater its surface area per unit volume and the more contact it makes with the other reactant, thus the faster the reaction e)catalyst: Catalysis is the increase in the rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents in the chemical reaction, a catalyst is not consumed. A catalyst may participate in multiple chemical transformations. The opposite of a catalyst, a substance that reduces the rate of a reaction, is an inhibitor. Activities 3.-Why does reaction rate increase as the temperature increases? 4. What does collision theory say? 5. Give three ways of Increasing the Rate of a Reaction. 6. Why do we keep food in a fridge? 7. Explain with a drawing what happens if chlorine reacts with hydrogen and : a) we have an uneffective collisions b) we have an effective collision 2. Energy Changes in Chemical Reactions If two substances react and the temperature of the mixture decreases, the reaction is endothermic. If two substances react and the temperature of the mixture increases, the reaction is exothermic. A chemical reaction involves the breaking of bonds in the reactants and the forming of bonds in the products. It takes energy to break bonds. Energy is released when bonds are formed. If a reaction is endothermic, it takes more energy to break the bonds of the reactants than is released when the bonds of the products are formed. Page 5 of 16

6 If a reaction is exothermic, more energy is released when the bonds of the products are formed than it takes to break the bonds of the reactants. The heat flow for a reaction at constant pressure, q p, is called enthalpy, ΔH. Exothermic reactions have negative enthalpy values (-ΔH). Endothermic reactions have positive enthalpy values (+ΔH). 8. What type of reactions are the following, exothermic or endothermic? a) 2 H 2(g) + O 2(g) ----> 2 H 2 O (l) ΔH = kj b) 2 C 2 H 6 (g) + 7 O 2 (g) ---> 4 CO 2 (g) + 6 H 2 O(l) ΔH = kj c) N 2(g) + 2 O 2(g) ----> 2 NO 2(g) ΔH = kj d) 4 NO(g) + 6 H 2 O(l) ---> 4 NH 3 (g) + 5 O 2 (g) ΔH = kj 3. - Amount of substance 3.1- The mole A mole is defined as the amount of matter that contains as many objects as the number of atoms in exactly 12 grams of 12C. Various experiments have determined that this number is x This is usually abbreviated as x for simplification, and is known as Avogadro's number. One mole of atoms, cars, people, etc. contains 6.02 x of these objects. Molar mass is the weight of one mole (or x molecules) of any chemical compound. It is the same number as molecular mass but with different units. Molar mass is expressed in grams. How can we calculate the molar mass? The molar mass can be calculated as the sum of the individual masses (as found in the periodic table) of all the atoms in any molecule, and expressed in grams. Calculate the molar mass of CO 2 m = 1 m C + 2 m O = (1 12) + (2 16) = = 44 g The relation between moles and molar mass Exercise with moles: m = n. M where m is mass n are moles M is molar mass Page 6 of 16

7 9.- Calculate the molar mass of: a) H 2 CO 3 b) Mg (OH) 2 c) Al 2 (SO 4 ) 3 d) Cl 2 O 5 e) O 2 f) CuSO 3 Data of atomic mass in u: H = 1; C = 12; O = 16; Mg = 24; Al = 27 ; S = 32 ; Cl = 35.5 ; Cu = 63.5 Solutions a) 62 g b) 58 g c) 342 g d) 151 g e) 32 g f) g 10.- How many atoms are there in 3 moles of potassium? Sol: atoms 11.- If a glass has 3.8 moles of water. How many molecules are there in the glass? Sol: molecules 12.- How many atoms are there in 3 moles of Ag? Sol: atoms 13.- How many molecules are there in 3 moles of H 2 SO 4? How many atoms? Sol: molecules atoms 14.- How many atoms are there in 2 moles of AgNO 3? 15.-How many moles are in atoms of Li? Sol: atoms Sol: 0.5 moles 16.-In a laboratory glass bottle there are 300 g of CuSO 4. Work out how many moles of that substance are contained in the glass. Sol 1.88 moles of CuSO How many moles are in 10 g of Na OH? (Na = 23 u ; O = 16 u ; H = 1 u) 18.- How many grams are in 2 moles of H 3 PO 4? ( H = 1 u; P = 31 u; O = 16 u) 19.- How many grams are in molecules of water? ( H = 1 u ; O = 16 u) 20.- How many molecules are in 100 g of P 2 O 5? (P = 31 u ; O = 16 u) Sol: 0.25 moles Sol: 196 g Sol: 53.8 g Sol: molecules Page 7 of 16

8 3.2.- Calculating concentration Percent Composition by Mass (%) Volume Percent (% v/v) Molarity (M) mass of sol u t e p er c en t composition by mass= mass of solution 100 %V = volume percent = v ol u m e of sol u t e v o lu m e of solution 100 m o l e s of so l ut e M o la r ity= li t r e s of solution grams/litres(g/l) mass ( g ) grams/ litres = volume of solution 21.- Determine the percent composition by mass of 20 g of salt mixed with 80 g of water. Sol: 20% 22.- What is the molarity of a solution made when water is added to 11 g CaCl 2 to make 100 ml of solution? (Ca = 40 u ; Cl = 35.5 u) Sol: 1.0 M 23.- Calculate percent composition by mass of a solution containing 45 g of salt in 500 g of water Calculate the grams of solute contained in a 250 g solution that is 6 % by mass. Sol: 8.26 % 25.- Calculate percent composition by mass of a solution containing 25 g of glucose in 100 g of water Calculate the volume of alcohol contained in a 150 ml solution that is 60 % by volume. Sol: 15 g Sol: 20 % Sol: 90 ml ml of pure ethyl alcohol is mixed with 250 ml of water. Calculate the volume percent of the solution Calculate the molarity of a solution containing 58.8 g of CaI 2 in 1 litre solution. m Ca = 40 u m I = 127 u 29.- Calculate the mass of NaOH contained in 50 ml of a solution 0.6 M. m Na = 23 u m O = 16 u m H = 1 u Sol: 23.08% Sol: 0.2 M Page 8 of 16

9 Sol: 1.2 g 30.- We have a 0.7 M solution of hydrochloric acid, HCl. To make a chemical reaction we need 0.05 moles of HCl. What is the volume needed? m Cl = 35.5 u m H = 1 u 31.- Calculate the molarity of a solution containing 2.8 g of KOH in 200 ml of solution. m K = 39 u m O = 16 u m H = 1 u 32.- Calculate the mass contained in 500 ml 1 M solution of hydrochloric acid, HCl. m Cl = 35.5 u m H = 1 u 33.- Calculate the molarity of a solution containing 7.9 g of H 2 SO 4 in 284 ml of solution. m S = 32 u m O = 16 u m H = 1 u Sol: L Sol:0.25 M Sol: g Sol: 0.28 M 34.- Calculate the volume of a solution 0.05 M of nitric acid, HNO 3 containing 0.08 moles of HNO 3. m N = 14 u m O = 16 u m H = 1 u Sol: 1.6 L grams of sodium hydroxide, NaOH, are dissolved in methanol until the volume of solution is 20 ml. Express the concentration in grams/litre sol: 190 g/l 4.- Stoichiometric calculations Stoichiometry is the study of quantitative relationships involved in chemical reactions. To make stoichiometric calculations you have to follow 4 steps: 1.- Balance the equation. 2.- Convert units of a given substance to moles. 3.- Using the mole ratio, calculate the moles of substance yielded by the reaction. 4.- Convert moles of wanted substance to desired units. 6.1 Calculation with masses Example: What will be the mass of iodine needed to react completely with 7 grams of aluminium? Al + I 2 AlI 3 Data: m Al = 27 u m I = 127 u sol: g Activities: Page 9 of 16

10 36.- What will be the mass of KCl obtained when we have 5 g of KClO 3? KClO 3 KCl + O 2 Data: m K = 39 u m Cl = 35.5 u m O = 16 u sol: 3 g 37.-Given the reaction Mg + O 2 MgO, work out the mass of oxygen which is consumed and the mass of magnesium oxide produced if we get 8.1 g of magnesium to react. m Mg = 24 u m O = 16 u Sol: 5.3g O g MgO 38. The following reaction is one of the methods for obtaining nitric acid: NO 2 + H 2 O HNO 3 + NO Calculate how many grammes of nitrogen dioxide must react in order to produce 8.8 g of nitric acid. m N = 14 u m O = 16 u m H = 1 u Sol: 9.6 g NO Carbon disulphide reacts with oxygen to produce carbon dioxide and sulphur dioxide, both in a gaseous state. Write the equation and calculate the mass of oxygen that is to be consumed so that 100g of carbon dioxide are produced. m C = 12 u m O = 16 u m S = 32 u Sol: 218 g of O Ethanol C 2 H 5 OH burns in the presence of oxygen O 2 in the air thus producing water and carbon dioxide. Calculate the mass of oxygen needed to burn 10.5 g of ethanol. Calculate the masses of water and carbon dioxide obtained. m C = 12 u m O = 16 u m H = 1 u Sol: 21.9g; 12.3g and 20.1g 41. The process of roasting of zinc sulphide is represented by the reaction ZnS (s) + O 2 (g) ZnO (s) + SO 2 (g) a. What amount of oxygen is needed to react with 974 g of zinc sulphide? b. What amount of sulphur dioxide is emitted in the process? m Zn = 65.4 u m O = 16 u m S = 32 u Sol:480g ; 640g 42. Hydrochloric acid HCl reacts with calcium carbonate, CaCO 3, and produces calcium chloride, CaCl 2, carbon dioxide CO 2 and water. What mass of water is obtained when 18 g of calcium carbonate react fully? m Ca = 40 u m C = 12 u m Cl = 35.5 u m O = 16 u Sol:3.24 g 6.2 Calculations with volume Example:.- What will be the volume of oxygen measured at STP, obtained when we have 1 kg of KClO 3? KClO 3 KCl + O 2 Page 10 of 16

11 Data: m K = 39 u m Cl = 35.5 u m O = 16 u sol: L 43. When zinc is attacked by hydrochloric acid HCl, a dissolution of zinc chloride, ZnCl 2 takes place, and hydrogen gas, H 2 is released. Calculate the volume of this gas as measured in normal conditions, which is obtained by allowing 20 g of Zn to react. m Zn = 65.4 u m H = 1 u m Cl = 35.5 u Sol: 6.72 L 44. Calculate the volume of ammonia NH 3 which is obtained from 15 L of hydrogen H 2, both at 1 atm and 0ºC, according to the reaction: N 2 (g) + H 2 (g) NH 3 (g) Sol:10 L 45. Iron in contact with the oxygen O 2 in the air is oxidised and iron oxide(iii) is obtained. Calculate the volume of oxygen which is consumed when 8 g of iron come in contact with oxygen, measured in normal conditions of pressure and temperature. m Fe = 56 u m O = 16 u Sol:2.4 L 46. What volume of oxygen at STP is needed for the complete combustion of 3,3 L of propane (C 3 H 8 )? (Hint: CO 2 and H 2 O are the products as in all combustion reactions) Sol: 16.5 L 6.3 Calculations with volume in different conditions than STP conditions Example: Hydrochloric acid HCl reacts with calcium carbonate, CaCO 3, and produces calcium chloride, CaCl 2, carbon dioxide CO 2 and water. What volume of carbon dioxide, measured at 20 ºC and 0.98 atm is obtained when 18 g of calcium carbonate react fully? m Ca = 40 u m C = 12 u m Cl = 35.5 u m O = 16 u Sol:4.41 L 47. What volume of hydrogen (H 2 ) measured at 30ºC and 780 mm Hg is obtained when 130 g of zinc reacts with an excess of sulphuric acid (H 2 SO 4 ). ZnSO 4 is also produced.? Data: m O = 16; m S = 32: m H = 1; m Zn = 65,4 Solution: 48,18 L g of marble (CaCO 3 ) reacts completely with hydrochloric acid. a) How many grams of carbon dioxide are obtained? b) What volume will it occupy at STP? c) And if it is at 25ºC and 2 atmospheres of pressure? In the chemical reaction also we obtain calcium dichloride and water. Data: m O = 16; m C = 12: m Ca = 40 Solution: a)4,4 g of CO 2 ; b) 2,24 L; c) 1,22 L 49. Calculate the mass of pure iron which will be dissolved in H 2 SO 4 solution if we obtain 89 L of H 2 measured at 90 º C and 710 mm Hg, according to the following chemical reaction: Fe (s) + H 2 SO 4 (aq) FeSO 4 (s) + H 2 (g) Datum: m Fe = 56 Solution: g of Fe Page 11 of 16

12 g of magnesium are added to HCl solution. Magnesium chloride and hydrogen are produced. Calculate the volume of hydrogen released at 25ºC and 700 mm Hg of pressure. Datum: m Mg = 24 Solution: 1,66 L of H 2 51) A determined amount of calcium carbonate (CaCO 3 ) has been broken down, by means of heat, in calcium oxide and carbon dioxide. 500 kg of calcium oxide has been obtained. a) Write the corresponding chemical equation. b) Calculate the amount of calcium carbonate before it had broken down. c) What volume of CO 2, at STP, has been obtained? Data: m O = 16; m C = 12: m Ca = 40 Solution: ; b) 892,86 kg; c) L of CO Sulphur trioxide reacts with water to give sulphuric acid. If 100 g of this acid is wanted, what volume of sulphur trioxide is necessary, measured at 40 ºC and 780 mm Hg? Data: m O = 16; m S = 32: m H = 1 Solution: 25.5 L of SO Calculations with reactants in solution Example: Given the equation: NaOH + HCl NaCl + H 2 O 25 ml of a 0,2 M hydrochloric acid solution was pipetted into a conical flask and titrated with sodium hydroxide. It was found that 15 ml of the sodium hydroxide was needed to neutralise the acid. Calculate the concentration of the sodium hydroxide. Exercises: 53.- Given the equation: KOH + HNO 3 KNO 3 + H 2 O Sol: 0,33 M 20 ml of a 1,3 M potassium hydroxide solution was pipetted into a conical flask and titrated with nitric acid. It was found that 17 ml of the nitric acid was needed to neutralise the base. Calculate the concentration of the nitric acid Given the equation: Ca(OH) 2 + H 3 PO 4 Ca 3 (PO 4 ) 2 + H 2 O Sol: 1.53 M 10 ml of a 0,4 M calcium hydroxide solution was pipetted into a conical flask and titrated with phosphoric acid. It was found that 11 ml of the phosphoric acid was needed to neutralise the base. Calculate the concentration of the phosphoric acid. Sol: 0.24 M 55.- A 3,7 g sample of calcium carbonate is dissolved in water. The final solution has a volume of 500 ml. 25 ml of this solution is then pipetted into a conical flask and titrated with hydrochloric acid. It is found that 20 ml of the hydrochloric acid completely neutralises the calcium carbonate solution. Page 12 of 16

13 a) What is the molarity of the calcium carbonate solution? b) What is the concentration of the hydrochloric acid? The equation for this reaction is: Data: Ca = 40 u C = 12 u O = 16 u CaCO 3 + HCl CaCl 2 + H 2 O+ CO 2 Sol: M ,9 g of sulfuric acid is dissolved in water and the final solution has a volume of 220 cm3. Using an acid-base titration, it was found that 20 ml of this solution was able to completely neutralise 10 ml of a sodium hydroxide solution. Calculate the concentration of the sodium hydroxide. The equation is: Data: S = 32u H = 1 u O = 16 u 5. Types of chemical reactions H 2 SO 4 + NaOH Na 2 SO 4 + H 2 O Depending on the mechanism of the reaction. We have: Sol: M Acid-Base Reaction An acid-base reaction is type of double displacement reaction that occurs between an acid and a base. The H + ion in the acid reacts with the OH - ion in the base to form water and an ionic salt: HA + BOH H 2 O + BA The reaction between hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction: HBr + NaOH NaBr + H 2 O Oxidation-Reduction or Redox Reaction In a redox reaction the oxidation numbers of atoms are changed. Redox reactions may involve the transfer of electrons between chemical species. Ag 2 O(aq) + H 2 (aq) 2 Ag(s) + H 2 O(l) Precipitation Reactions If a reaction taking place in aqueous solution produces an insoluble product, that product will precipitate, forming a solid product at the bottom of the reaction vessel. Precipitation often happens with double replacement reactions, and one of the most dramatic of such reactions is the reaction between lead nitrate and potassium iodide: 2KI(aq) + Pb(NO 3 ) 2 (aq) -> 2KNO 3 (aq) + PbI(s) Both of the reactants form colorless solutions, but if these solutions are poured together the Page 13 of 16

14 lead iodide product is a dramatic yellow solid which precipitates out of the solution. Combustion A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat (exothermic reaction). Usually in a combustion reaction oxygen combines with another compound to form carbon dioxide and water. An example of a combustion reaction is the burning of naphthalene: C 10 H O 2 10 CO H 2 O 57.- Classify the following reactions : a) 2 Mg + O 2 2 MgO b) Zn(OH) 2 + H 2 SO 4 ZnSO H 2 O c) Cu + S CuS d) H 2 CO NaOH 2 H 2 O + Na 2 CO 3 e) Na 2 CO 3 + Ca(OH) 2 CaCO 3 (S) + 2 NaOH f) 2 NaCl 2 Na + Cl 2 g) CH O 2 CO 2 + 2H 2 O h) 2 HCl + Zn(OH) 2 ZnCl 2 + H 2 O 58.- Balance and classify the following reactions : a) Cl 2 + KBr KCl + Br 2 b) C 3 H 8 + O 2 CO 2 + H 2 O c) HNO 3 + Ca(OH) 2 Ca(NO 3 ) 2 + H 2 O Acid-base reactions When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt. The H(+) cation of the acid combines with the OH(-) anion of the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt. The combination of hydrochloric acid and sodium hydroxide produces common table salt, NaCl: The word salt is a general term which applies to the products of all such acid-base reactions. Acids properties. When dissolved in water, acids 1.-Conduct electricity 2.-Have a sour taste 3.-React with bases to neutralize their properties 4.- React with active metals to liberate hydrogen. Page 14 of 16

15 Base Properties When dissolved in water, bases 1. Conduct electricity 2. Have a slippery feeling 3. React with acids to neutralize their properties. ph Scale It is a measure of acidity or alkalinity of water soluble substances (ph stands for 'potential of Hydrogen'). A ph value is a number from 1 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity which increases as the number decreases, 1 being the most acidic. Values above 7 indicate alkalinity which increases as the number increases, 14 being the most alkaline. Page 15 of 16

16 Practice exam 1.- a) Define: reactant, catalyst, chemical reaction, acid-base reaction and endothermic reaction b) Write down the ph scale and give 3 examples. c) Using the collision theory explain why temperature and concentration affect the rate of a reaction 2. Balance the following reactions: a) N 2 + O 2 N 2 O 3 b) FeS + HCl FeCl 2 + H 2 S c) C 5 H 12 + O 2 CO 2 + H 2 O d) H 2 O H 2 + O 2 e) HCl + Al AlCl 3 + H Balance the following chemical reactions and write if they are exothermic or endothermic: a) N 2(g) + O 2(g) ----> NO 2(g) ΔH = kj b) C 2 H 6 (g) + O 2 (g) ---> CO 2 (g) + H 2 O(l) ΔH = kj c) H 2(g) + O 2(g) ----> H 2 O (l) ΔH = kj d) NO(g) + H 2 O(l) ---> NH 3 (g) + O 2 (g) ΔH = kj 4.- Sodium carbonate (Na 2 CO 3 ) reacts with hydrochloric acid (HCl) as seen in the following: Na 2 CO 3 + HCl NaCl + CO 2 + H 2 O Calculate the mass of CO 2 and water that result from a complete reaction of 15 g of sodium carbonate in enough hydrochloric acid. Data: m C = 12 u m Na = 23 u m O = 16 u m H = 1 u Sol: a) 6.23 g b) 2.55 g 5.- Calculate the volume of C 4 H 10 that we need to burn to produce 5 L of carbon dioxide. All the gases are measured at STP. The chemical reaction is: C 4 H 10 + O 2 CO 2 + H 2 O 6.- If it takes 54 ml of 0.1 M NaOH to neutralize 125 ml of an HCl solution, what is the concentration of the HCl? 7.- Find the molarity of the following solutions: 1) 0.5 moles of sodium chloride is dissolved to make 0.05 L of solution. 2) 0.5 grams of sodium chloride is dissolved to make 0.05 L of solution. Data: Cl = 35.5 and Na = 23 u Sol: 1.25 L Sol: M Sol: 10 M, 0.17 M 8.- a) work out how many molecules and how many atoms are contained in 34 g of phosphorus trichloride b) What mass of this gas is contained in 98 L measured at 1.12 atm and 20 ºC? R = atm.l/mol.k P =31 u Cl = 35,5 u Sol: a) , b) 628,15g Page 16 of 16