10.1 Chemical Equations

Transcription

1 Section 10.1 Equations 10.1 Equations Developing the language of chemistry reactions and change Think of the element symbols as the alphabet of chemistry and the formulas as the words of chemistry. s are the sentences of chemistry and they tell the story of the material world. All the complex changes we see in the material world are the result of reactions. The big story of the world is the inter-relationships between all the countless reactions that occur. To understand that story, even in part, we need the language of s, which really are the sentences of chemistry. A reaction is any process of change. reactions convert one or more substances into new substances. In the process atoms are not created or destroyed. They are simply rearranged to form new substances. In general we represent a reaction by a as follows. s and reactions The reactants are the starting materials used in the reaction. The products are the substances that are produced by the reaction. The reactants are always written on the left side of the arrow and the products are written on the right side of the. The arrow indicates the direction of the reaction, pointing from the reactants to products. It means that the reactants give or yield or react to form the products. reaction - is the process of change. - A is an expression that describes the changes that happen in a reaction. reactants - the starting materials or substances in a reaction. products - the materials or substances resulting from a reaction. 296 A NATURAL APPROACH TO CHEMISTRY

2 Constructing a An example reaction As an example let s construct the for one of the overall reactions that takes place in a fire. In this reaction, oxygen (O 2 ) from the atmosphere reacts with carbon (C) from wood, to produce carbon dioxide gas (CO 2 ). In this reaction, O 2 and C are the reactants since they are used up by the reaction. Carbon dioxide (CO 2 ), is the product. In words, the reaction can be written Carbon reacts with oxygen to produce carbon dioxide. We can write the expression in a more compact way using the arrow. with symbols carbon + oxygen carbon dioxide However, the use of the formulas for each substance involved in the reaction is the most accurate, and least ambiguous way to describe the reaction. For our reaction we write the general relationship with the. C + O 2 CO 2 Interpreting a The diagram below summarizes the interpretation of the Why we use formulas Energy may appear on either side of a Using the formulas to write the gives us a very compact way to express reactions. It also gives a clear picture of the types of atoms that are involved in the reaction and how they are bonded together in both the products and the reactants. Does the C + O 2 CO 2 give a complete representation of the burning of wood? When we look at a wood burning fire we see that fire gives off light and we also feel that fires gives off heat. So, if the C + O 2 CO 2 does represent the phenomenon of wood burning then where is the heat and where is the light? Later in the chapter we will add energy to the and explicitly recognize the energy as either a reactant (absorbed) or a product (given off). A NATURAL APPROACH TO CHEMISTRY 297

3 Section 10.1 Equations Conservation of mass reactions conserve mass The number of atoms in a reaction does not change. One of the fundamental properties of chemistry is that of the conservation of mass. In fact this property is so important that scientists call it the law of conservation of mass. The law states that the total mass of the reactants is equal to the total mass of the products. In other words, mass is neither created or destroyed during a reaction. The fundamental building block of chemistry and reactions is the atom. Conservation of mass really means that a reaction does not change the total number and type of atoms. A reaction rearranges the atoms in the reactants into a new arrangement of the same atoms in the products. Burning hydrogen in oxygen Beginning a For example, consider burning hydrogen in oxygen. This is a reaction that is observed experimentally and gives water as the product. It is also the reaction that fuels the space shuttle. The white smoke that comes out of the shuttle engines is water vapor which is the product of the reaction. To start developing a we write down the reaction between hydrogen and oxygen with the correct formulas for participating substances. An unbalanced This correctly shows that water can be created by combining hydrogen and oxygen, but it does NOT satisfy the law of conservation of mass. There are two oxygen atoms in the reactants and only one in the products! law of conservation of mass - the total mass of reactants (starting materials) and the total mass of products (materials produced by the reaction) is the same. 298 A NATURAL APPROACH TO CHEMISTRY

4 Reading s Why an unbalanced is inaccurate balanced means the same number of atoms appear on both sides The unbalanced Coefficients One problem with the H 2 + O 2 H 2 O is that if you mixed one mole of oxygen and one mole of hydrogen you would NOT get one mole of water molecules. You would actually get one mole of water molecules and a 0.5 moles of leftover oxygen molecules. You get leftover oxygen molecules because there are more oxygen atoms in the reactants than there are in the products. The chart below summarizes the reaction by counting the type of each atom on the reactant and product side. The number of hydrogen atoms is the same in both the reactants and the products. We therefore can say that hydrogen atoms are balanced. The H 2 + O 2 H 2 O is an unbalanced. The unbalanced tells you what substances are involved in the reaction but it does not tell you how much of each are involved. In order to make it balanced, we need to adjust the number of hydrogen molecules, oxygen molecules, and water molecules until there are the same type and number of each atom on both sides of the. The number of molecules in a is given by the coefficient in front of the formula. For example, the following might appear in a reaction to describe 2 hydrogen molecules, 1 oxygen molecule or 3 water molecules. Coefficients versus subscripts The subscript tells you how many atoms are in the molecule. For example the subscript 2 in H 2 tells you there are 2 hydrogen atoms in a hydrogen molecule. The coefficient tells you how many molecules there are in the reaction. If 2H 2 appears in a you know there are two molecules of hydrogen that include a total of 4 atoms of hydrogen (2 molecules 2 atoms per molecule). unbalanced - a that does not satisfy the law of conservation of mass. The number of each type of atom on the reactant side of the does not equal the same number for each atom on the product side. A NATURAL APPROACH TO CHEMISTRY 299

5 Section 10.1 Equations A balanced The balanced Multiplying the hydrogen molecule by a factor of 2 and the water molecule by a factor of 2 we obtain a that satisfies mass conservation. balanced conserves mass The balanced correctly tells you amounts This is balanced. There are two rules for balancing s. 1. DO NOT CHANGE the subscripts in the formulas because this would change the substances involved in the reaction, in essence making a different reaction. For example if we change H 2 O to H 2 O 2 we have changed water to hydrogen peroxide. 2. DO CHANGE the coefficients to adjust the number of each molecule in the reaction The balanced tells us that when we combine 2 molecules of hydrogen with one molecule of oxygen we can obtain 2 molecules of water. This is the complete recipe for making water. It correctly represents the amount of each substance in the reaction and it satisfies mass conservation because the same number of each type of atom appear on the reactant and product sides of the. balanced - a that satisfies the law of conservation of mass. 300 A NATURAL APPROACH TO CHEMISTRY

6 Balancing s Why balance s? Coefficients of one Coefficients apply to whole molecules Balance pure elements last Balance one element at a time The goal of balancing a is to get the numbers of each type of atom to be the same on both the reactant and product sides of the. This is really just counting atoms on either side then adjusting the coefficients until the counts match. At this point the process of balancing a is trial and error. You guess and try different coefficients until it works out. For many reactions this approach works fine. However, for reactions that involve a number of reactants and products a more structured approach is needed and will develop it in the next few pages. When a single molecule is in a the coefficient of 1 not written explicitly. For example, O 2 in a means 1 molecule of oxygen (which contains 2 atoms). The coefficient of 2 in front of a molecule applies to the entire molecule. For example, 2H 2 O means that we have two water molecules. Since each water molecule has 2 hydrogen atoms, the total number of hydrogen atoms is 4. It is always easiest to balance pure elements last. For example if O 2 appears in a, you can change the coefficient to make 2O 2 or 3O 2 and change only the number of oxygen atoms. Making CH 4 into 2CH 4 changes both the number of carbon atoms and the number of hydrogen atoms. Another good strategy is to start with one element, often carbon. Then check oxygen, and so on, checking and balancing one element at a time. However, every time you change a coefficient, you should make sure you have not unbalanced an element that you already balanced! The glucose (C 6 H 12 O 6 ) contained in biomass is used as a biofuel to produce ethanol (C 2 H 6 O) and carbon dioxide (CO 2 ). Write the balanced for this reaction. Given: The unbalanced C 6 H 12 O 6 C 2 H 6 O + CO 2 Asked: Find the coefficients in order to balance the. Relationships: The same number of each type of atom must appear on each side. Solve: All atoms involved in the reaction are unbalanced. There are 6 C atoms in the reactants and 3 C atoms in the products There are 12 H atoms in the reactants and 6 H atoms in the products There are 6 O atoms in the reactants and 3 O atoms in the products From this accounting we notice that the number of atoms in the products is half the number of the reactants. By multiplying the products by a factor of 2 we obtain a balanced of the reaction. Answer: The balanced is C 6 H 12 O 6 2 C 2 H 6 O + 2 CO 2 A NATURAL APPROACH TO CHEMISTRY 301