Representing chemical change: State symbols *

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1 OpenStax-CNX module: m Representing chemical change: State symbols * Free High School Science Texts Project This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License State symbols and other information The state (phase) of the compounds can be expressed in the chemical equation. This is done by placing the correct label on the right hand side of the formula. There are only four labels that can be used: 1. (g) for gaseous compounds 2. (l) for liquids 3. (s) for solid compounds 4. (aq) for an aqueous (water) solution To show that heat is needed for a reaction, a Greek delta ( ) is placed above the arrow. tip: You may remember from Physical and chemical change that energy cannot be created or destroyed during a chemical reaction but it may change form. In an exothermic reaction, H is less than zero and in an endothermic reaction, H is greater than zero. This value is often written at the end of a chemical equation. Exercise 1: Balancing chemical equations 5 (Solution on p. 5.) Solid zinc metal reacts with aqueous hydrochloric acid to form an aqueous solution of zinc chloride (ZnCl 2 )and hydrogen gas. Write a balanced equation for this reaction. Exercise 2: Balancing chemical equations 5 (advanced) (Solution on p. 5.) Balance the following equation: (NH 4 ) 2 SO 4 + NaOH NH 3 + H 2 O + Na 2 SO 4 (1) In this example, the rst two steps are not necessary because the reactants and products have already been given. The following video explains some of the concepts of balancing chemical equations. * Version 1.2: Sep 29, :59 am

2 OpenStax-CNX module: m Khan Academy video on balancing chemical equations This media object is a Flash object. Please view or download it at < Figure Balancing more advanced chemical equations Write balanced equations for each of the following reactions: 1. Al 2 O 3 (s) + H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 3H 2 O (l) 2. Mg (OH) 2 (aq) + HNO 3 (aq) Mg (NO 3 ) 2 (aq) + 2H 2 O (l) 3. Lead (II) nitrate solution reacts with potassium iodide solution. 4. When heated, aluminium reacts with solid copper oxide to produce copper metal and aluminium oxide (Al 2 O 3 ). 5. When calcium chloride solution is mixed with silver nitrate solution, a white precipitate (solid) of silver chloride appears. Calcium nitrate (Ca (NO 3 ) 2 ) is also produced in the solution. Click here for the solution 1 Balanced equations are very important in chemistry. It is only by working with the balanced equations that chemists can perform many dierent calculations that tell them what quantity of something reacts. In a later chapter we will learn how to work with some of these calculations. We can interpret balanced chemical equations in terms of the conservation of matter, the conservation of mass or the conservation of energy. This media object is a Flash object. Please view or download it at < phpapp02&stripped_title=reactions > Figure 1 2 Summary A chemical equation uses symbols to describe a chemical reaction. In a chemical equation, reactants are written on the left hand side of the equation and the products on the right. The arrow is used to show the direction of the reaction. When representing chemical change, it is important to be able to write the chemical formula of a compound. In any chemical reaction, the law of conservation of mass applies. This means that the total atomic mass of the reactants must be the same as the total atomic mass of the products. This also means that the number of atoms of each element in the reactants must be the same as the number of atoms of each element in the product. 1

3 OpenStax-CNX module: m If the number of atoms of each element in the reactants is the same as the number of atoms of each element in the product, then the equation is balanced. If the number of atoms of each element in the reactants is not the same as the number of atoms of each element in the product, then the equation is not balanced. In order to balance an equation, coecients can be placed in front of the reactants and products until the number of atoms of each element is the same on both sides of the equation. 2.1 End of chapter exercises 1. Propane is a fuel that is commonly used as a heat source for engines and homes. Balance the following equation for the combustion of propane: C 3 H 8 (l) + O 2 (g) CO 2 (g) + H 2 O (l)click here for the solution 2 2. Aspartame, an articial sweetener, has the formula C 14 H 18 N 2 O 2. Write the balanced equation for its combustion (reaction with O 2 ) to form CO 2 gas, liquid H 2 O, and N 2 gas. Click here for the solution 3 3. F e 2 (SO 4 ) 3 + K (SCN) K 3 Fe (SCN) 6 + K 2 SO 4 Click here for the solution 4 4. Chemical weapons were banned by the Geneva Protocol in According to this protocol, all chemicals that release suocating and poisonous gases are not to be used as weapons. White phosphorus, a very reactive allotrope of phosphorus, was recently used during a military attack. Phosphorus burns vigorously in oxygen. Many people got severe burns and some died as a result. The equation for this spontaneous reaction is: P 4 (s) + O 2 (g) P 2 O 5 (s) a. Balance the chemical equation. b. Prove that the law of conservation of mass is obeyed during this chemical reaction. c. Name the product formed during this reaction. d. Classify the reaction as endothermic or exothermic. Give a reason for your answer. e. Classify the reaction as a synthesis or decomposition reaction. Give a reason for your answer. Click here for the solution 5 (DoE Exemplar Paper ) 5. Mixing bleach (NaOCl) and ammonia (two common household cleaners) is very dangerous. When these two substances are mixed they produce toxic chloaramine (NH 2 Cl) fumes. Balance the following equations that occur when bleach and ammonia are mixed: a. NaOCl (aq) + NH 3 (aq) NaONH 3 (aq) + Cl 2 (g) b. If there is more bleach than ammonia the following may occur: NaOCl + NH 3 NaOH + NCl 3 Nitrogen trichloride (NCl 3 ) is highly explosive. c. If there is more ammonia than bleach the following may occur: NH 3 + NaOCl NaOH + NH 2 Cl These two products then react with ammonia as follows: NH 3 + NH 2 Cl + NaOH N 2 H 4 + NaCl + H 2 O One last reaction occurs to stabilise the hydrazine and chloramine: NH 2 Cl+N 2 H 4 NH 4 Cl+N 2 This reaction is highly exothermic and will explode. Click here for the solution 6 6. Balance the following chemical equation: N 2 O 5 NO 2 + O 2 Click here for the solution 7 7. Sulphur can be produced by the Claus process. This two-step process involves reacting hydrogen sulphide with oxygen and then reacting the sulphur dioxide that is produced with more hydrogen

4 OpenStax-CNX module: m sulphide. The equations for these two reactions are: H 2 S + O 2 SO 2 + H 2 O H 2 S + SO 2 S + H 2 O (2) Balance these two equations. Click here for the solution 8 8

5 OpenStax-CNX module: m Solutions to Exercises in this Module Solution to Exercise (p. 1) Step 1. The reactants are zinc (Zn) and hydrochloric acid (HCl). The products are zinc chloride (ZnCl 2 ) and hydrogen (H 2 ). Step 2. Zn + HCl ZnCl 2 + H 2 (3) Step 3. You will notice that the zinc atoms balance but the chlorine and hydrogen atoms don't. Since there are two chlorine atoms on the right and only one on the left, we will give HCl a coecient of 2 so that there will be two chlorine atoms on each side of the equation. Zn + 2HCl ZnCl 2 + H 2 (4) Step 4. When you look at the equation again, you will see that all the atoms are now balanced. Step 5. In the initial description, you were told that zinc was a metal, hydrochloric acid and zinc chloride were in aqueous solutions and hydrogen was a gas. Zn (s) + 2HCl (aq) ZnCl 2 (aq) + H 2 (g) (5) Solution to Exercise (p. 1) Step 1. With a complex equation, it is always best to start with atoms that appear only once on each side i.e. Na, N and S atoms. Since the S atoms already balance, we will start with Na and N atoms. There are two Na atoms on the right and one on the left. We will add a second Na atom by giving NaOH a coecient of two. There are two N atoms on the left and one on the right. To balance the N atoms, NH 3 will be given a coecient of two. The equation now looks as follows: (NH 4 ) 2 SO 4 + 2NaOH 2NH 3 + H 2 O + Na 2 SO 4 (6) Step 2. N, Na and S atoms balance, but O and H atoms do not. There are six O atoms and ten H atoms on the left, and ve O atoms and eight H atoms on the right. We need to add one O atom and two H atoms on the right to balance the equation. This is done by adding another H 2 O molecule on the right hand side. We now need to check the equation again: (NH 4 ) 2 SO 4 + 2NaOH 2NH 3 + 2H 2 O + Na 2 SO 4 (7) The equation is now balanced.