Chapter 6 Chemical Reactions Matter and Change Changes in matter can be described in terms of physical changes and chemical changes. A physical property is a characteristic of a substance that can be observed without changing the substance into another substance. Color, hardness, texture, and flexibility are some physical properties of matter. Matter and Change Continued A chemical property is a characteristic of a substance that describes its ability to change into other substances. Flammability, rust, and changing into another substance are all chemical properties. Changes of Matter A physical change is any change that alters the form or appearance of a substance but does not make the substance into another substance. Chemical change is a change in matter that produces one or more new substances. The substances that undergo change in a chemical reaction are called reactants. The new substances formed by the reaction are called products. Bonding and Chemical Change Chemical changes occur when bonds break and new bonds form. Atoms form bonds when they share or transfer electrons. Chemical reactions involve changes in properties and changes in energy that you can observe. One way to detect chemical reactions is to observe changes in the properties of the materials involved. Color change, precipitate, formation of a gas are examples of chemical changes. Changes in Energy Endothermic reaction the total making and breaking of bonds is a net absorption of energy. The energy is absorbed as heat from nearby matter, which cools. In an exothermic reaction the total making and breaking of bonds results in a net release of energy. The energy is typically released as heat into nearby matter. What are Chemical Equations A chemical equation is a short, easy way to show a chemical reaction, using symbols instead of words. Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction. All chemical equations use formulas to represent substances involved in a reaction. Reactant + Reactant Product + Product Conservation of Matter It states that, during a chemical reaction, matter is neither created nor destroyed. The total mass of the reactants must equal the total mass of the products. In chemical reactions, the number of atoms stays the same no matter how they are arranged. So, their total mass will remain the same. Open and Closed Systems In an open system, matter can enter from or escape to the surroundings. A burning match is an example of an open system. In a closed system, matter is not allowed to enter or leave. Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atoms on both sides of the equation. 1. Write the equation. 2. Count the atoms. 3. Use coefficients to balance atoms. 4. Look back and check. A coefficient is a number placed in front of a chemical formula in an equation. Classifying Chemical Reactions Three general types of chemical reactions are synthesis, decomposition, and replacement. Synthesis when 2 or more elements combine to make a more complex substance. 1.A + B AB 2.Example : 2H 2 + O 2 2H 2 0 Decomposition When a compound is broken down into simpler products. 1.AB A + B 2.Example: 2H 2 O 2 2H 2 O + O 2 Replacement When one element replaces another in a compound, or when two elements in different compounds trade places. 1. AB + CD AD + CB 2. AB + C A + CB Examples 1. FeS + 2HCl FeCl 2 + H 2 S 2. 2Cu 2 O + C 4Cu + CO 2 Energy and Reactions Activation energy is the minimum amount of energy needed to start a chemical reaction. All chemical reactions require a certain amount of activation energy to get started. Factors that affect rates of reaction include surface area, temperature, concentration, and the presence of catalysts or inhibitors. Continued Larger surface area means a slower rate of reaction. Increasing temperature increases the rate of reaction by having particles coming in contact with one another with a greater frequency. Continued Concentration is the amount of a substance in a given volume. So, by increasing the concentration of reactants the rate of reaction also increases. A catalyst is a material that increases the rate of reaction by lowering the activation energy. ENZYMES The cells in your body contain biological catalysts called enzymes. Your body has thousands of different enzymes. Each enzyme is specific in nature and affects only one chemical reaction. Inhibitors are materials used to decrease the rate of reactions, most inhibitors work by preventing reactants from coming together.