Describing Chemical Reactions
l You look at your cellular phone display and read the message “U wan2 gt pza 2nite?”
l You reply “No. MaB TPM. CUL8R.”
l These messages are short for saying “Do you want to get some pizza tonight?” and “No. Maybe tomorrow afternoon (PM). See you later.”
l Cellular phone messages use ______and abbreviations to express ideas in shorter form.
l A type of shorthand is used in chemistry too.
l “Hydrogen molecules react with oxygen molecules to form water molecules” is a lengthy way to describe the reaction between hydrogen and oxygen.
l And writing it is slow. Instead, chemists often use ______in place of words.
What Are Chemical Equations?
l A chemical equation is a short, easy way to ______, using symbols instead of words.
l Although chemical equations are shorter than sentences, they contain ______information.
l Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction.
Formulas in an Equation
l All chemical equations use formulas to represent the ______involved in a reaction.
l You may recall that a chemical formula is a combination of symbols that represents the elements in a compound.
l For example, ______is the formula for carbon dioxide.
l The formula tells you that this compound is made up of the elements carbon and oxygen and each molecule has ______carbon atom and ______oxygen atoms.
l Figure 7 lists formulas of other compounds that may be familiar to you.
Formulas of Familiar Compounds
Structure of an Equation
l All chemical equations have a common structure.
l A chemical equation tells you the ______in a reaction and the substances you ______.
l The substances you have at the beginning are called the ______.
l When the reaction is complete, you have new substances called the ______..
l The formulas for the ______are written on the ______, followed by an arrow.
l You read the arrow as ______.
l The formulas for the ______are written on the ______.
l When there are two or more reactants, they are separated by ______signs.
l In a similar way, plus signs are used to separate two or more products.
l Below is the general plan for a chemical equation.
l The number of reactants and products can ______.
l Some reactions have only one reactant or product.
l Other reactions have two, three, or more reactants or products.
l In Figure 8, you can see the equation for a reaction that occurs when limestone (CaCO3) is heated.
l Count the number of reactants and products, and familiarize yourself with the parts of the equation.
l Figure 8A Chemical Equation: Like a building, a chemical equation has a basic structure. Interpreting Diagrams What does the subscript 3 in the formula for calcium carbonate tell you?______
Conservation of Mass
l Look closely at the values for mass in Figure 9.
l Iron and sulfur can react to form ______.
l The photograph represents a principle first demonstrated by the French chemist Antoine Lavoisier in 1774.
l This principle is called ______, and it states that during a chemical reaction, ______
______
l All the atoms present at the start of the reaction are present at the end.
Figure 9Conservation of Mass Mass is conserved in chemical reactions.
Modeling Conservation of Mass
l Think about what happens when classes change at your school during the day.
l A class is made of a group of students and a teacher together in a room.
l When the bell rings, people from each class ______from room to room, ending up in ______classes.
l The ______in the school has ______.
l But their arrangement has.
l Now imagine that all the students and teachers are atoms, each class is a molecule, and the changing of classes is a chemical reaction.
l At the end of the reaction, the same atoms are present, but ______
______.
l The ______does not change.
l The principle of conservation of mass states that in a chemical reaction, ______
______
Balancing Chemical Equations
l The principle of conservation of mass means that the ______of atoms exists in the ______as in the ______.
l To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.
l Chemists say an equation is balanced when it ______
______
l How can you write a balanced chemical equation?
Write the Equation
l Suppose you want to write a balanced chemical equation for the reaction between hydrogen and oxygen that forms water.
l To begin, write the correct formulas for both reactants and product.
l Place the reactants, H2 and O2, on the left side of the arrow, separated by a plus sign.
l Then write the product, H2O, on the right side of the arrow.
Count the Atoms
l Count the number of atoms of each element on each side of the equation.
l You find ______atoms of oxygen in the reactants but only ______atom of oxygen in the product.
l How can you get the number of oxygen atoms on both sides to be the same?
l You ______change the formula for water to H2O2 because H2O2 is the formula for hydrogen peroxide, a completely different compound.
l So, how can you show that mass is conserved?
Use Coefficients to Balance Atoms
l To balance the equation, use ______.
l A coefficient (koh uh fish unt) is a number placed ______of a chemical formula in an equation.
l It tells you ______atoms or molecules of a reactant or a product take part in the reaction.
l If the coefficient is 1, you don’t need to write it.
l Balance the number of oxygen atoms by writing the coefficient ______.
l That’s like saying “2 × H2O.”
l Now there are two oxygen atoms—one in each molecule of water.
l Balancing the oxygen atoms throws off the hydrogen atoms.
l There are now ______hydrogen atoms in the reactants and ______in the product.
l How can you balance the hydrogen?
l Try ______the number of hydrogen atoms on the left side of the equation by writing the coefficient 2 for hydrogen.
Look Back and Check
l The equation is balanced.
l It tells you that two molecules of hydrogen react with one molecule of oxygen to yield two molecules of water.
l Count the atoms in the balanced equation again to see that the equation is correct.
Classifying Chemical Reactions
l Substances may ______to make a more complex substance.
l They may ______to make simpler substances.
l Or, they may even ______parts.
l In each case, ______form.
l Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.
Figure 11Types of Reactions Three categories of chemical reactions are synthesis, decomposition, and replacement.
Making Models How do these different geometric shapes act as models for elements and compounds in reactions?
Synthesis
l Have you ever listened to music from a synthesizer?
l You can hear many different notes and types of sounds combined to make music.
l To synthesize is to ______.
l In chemistry, when two or more elements or compounds combine to make a more complex substance, the process is called ______-(sin thuh sis).
l The reaction of hydrogen and oxygen to make water is a synthesis reaction.
H2 + 02 à 2H2O
Decomposition
l In contrast to a synthesis reaction, a process called ______compounds into simpler products.
l You may have a bottle of hydrogen peroxide (H2O2) in your house to clean cuts.
l If you keep such a bottle for a very long time, you’ll have water instead.
l The hydrogen peroxide decomposes into water and oxygen gas.
Replacement
l When one element ______another in a compound, or when two elements in different compounds trade places, the process is called ______.
l Look at this example:
l Copper metal can be obtained by heating copper oxide with carbon.
l The carbon takes the place of copper.
l The reaction between copper oxide and carbon is called a ______ reaction because one element, carbon, replaces another element, copper, in the compound.
l In a ______ reaction, elements in one compound appear to “trade places” with elements in another compound.
l The following reaction is an example of a double replacement: