Observing and describing chemical reactions

1. Observing and describing chemical reactions Understanding how to observe and describe reactions.

2. Matter and chemistry • Review concepts: • Matter is anything that has mass and takes up space. • The study of matter and the changes in matter is called chemistry.

3. Matter and change • Changes in matter can be described in terms of physical and chemical changes. • The physical properties of matter can be observed without altering the substance. An example is the temperature which ice melts into water. • The chemical properties of matter are used to describe the ability of matter to react with other matter. An example magnesium burns in the presence of oxygen to produce magnesium oxide.

4. Changes of matter • Physical changes: alterations to the outward appearance of matter. An example is bending an object into a new form or shape. • Chemical changes: alterations to matter which create new substances. An example is lighting a substance on fire. • Reactants are chemicals which undergo chemical change. The result of the change are new substances called products.

5. Bonding and chemical change • Chemical change occurs when atoms break atomic bonds and form new atomic bonds. The result is new substances or products. • Atomic bonds are either the sharing of electrons or the transfer of electrons. • An example is Mg+O2=2MgO

6. Evidence of chemical reactions • Chemical reactions involve changes in properties and changes in energy that you can observe. • An example is a precipitate which is a solid that forms from solution during a chemical reaction. • Other evidence of chemical reactions could be the formation of bubbles, a change in color, or the release of heat as the result of a reaction.

7. Changes in energy • As matter changes it either release or absorbs energy. • Endothermic reaction: a reaction in which energy is absorbed during the reaction. An example is frying an egg. • Exothermic reaction: a reaction in which a energy is released. An example is a jet which burns fuel to move fast through the sky. Exothermic reactions release energy in the form of heat to the environment.

8. Structure of equations • Equations all have a common structure. Basically the products are on one side of the equation and the reactants are on the other. • Reactant+Reactant->Product+Product • Frequently the number of reactants and products will not be the same. • Two common reactions are acids and bases, and combustion reactions. • Combustion: • Fuel+Oxygen-> Water+Gas • Acids and bases: • Acid+Base-> Water+Salt+Gas • Example HCl+NaOH->NaCl+H2O

9. Chemical Equations • A chemical equation is a short easy way to describe what reaction occurred. • A chemical equation uses chemical formulas which are ratios of atoms in a molecule or the chemical “recipe”. • Combustion is a chemical reaction which is exothermic and uses both a fuel and oxygen as reactants. • An example is lighting rubbing alcohol on fire (combustion of fuel) • C3H8O+O->CO2+H2O • Take a minute to balance the above equation: • C3H8O+9O->3CO2+4H2O

10. Conservation of matter • In 1774 the law of the conservation of matter was established by Lavoisier. • The law states that matter is neither created nor destroyed during a chemical reaction. • The mass of the products must weigh the same as the reactants. • The number of atoms stays the same after the chemical reaction has occurred. This is the reason why we balance chemical equations. If matter is not created or destroyed, the number of atoms between the products and reactants must also be the same. • What are the products of the reaction of boiling water and is this a chemical change?

11. Open and closed systems • A open system means that when a chemical reaction occurs the matter is not contained and can leave once the reaction is complete. An example is fire which is a form of combustion which produces water vapor and carbon dioxide which appear as smoke. • A closed system does not allow the matter in a reaction to leave. All of the matter related to both the reactants and the products is contained and does not escape. An example is a chemical reaction occurring in a plastic bag or a sealed container.

12. Understanding the three types of chemical equations • Synthesis: the reaction between reactants results in a new product. An example is hydrogen gas and oxygen becoming water. 2H+O->H2O. • Decomposition: the reaction where a larger molecule breaks down into smaller molecules. An example is hydrogen peroxide breaking down into water and oxygen gas. H2O2-> H2O+O • Replacement: One or more of the molecules in the reactant gets replaced by another molecule in a different reactant. An example is the copper in copper oxide being replaced with carbon to form carbon dioxide. 2Cu2O+C->4Cu+CO2

13. Controlling chemical reactionsEnergy and reactions • Activation energy: The amount of energy required to get a chemical reaction started. All reactions need a certain amount of energy to begin. • Activation energy is similar the amount of energy required to get a boulder to start rolling down a hill. • Once a reaction has began it might require additional energy to continue. A exothermic reaction does not need additional energy to continue, while a endothermic reaction requires more energy to continue. An example is the difference between a fire burning (exothermic) and the energy required for a slow cooker to process food (endothermic).

14. Exothermic and endothermic reactions • Exothermic reactions are characterized by a high amount of activation energy to start, but once the reaction begins the reaction gives off heat. • Endothermic reactions are characterized by requiring a higher amount of activation energy than exothermic reactions, but the reaction requires the constant input of energy in order to continue.

15. Rates of chemical reactions • Chemical reactions occur at various rates which are related to several factors. These factors include: • Surface area: An increased area of a chemical allows the reaction to occur faster. An example is we eat our food is chewed into pieces which increase surface area so that saliva can mix with it and begin chemical digestion. • Temperature: An increase in the temperature of a reaction allows the reaction to occur at a faster rate. • Concentration: The higher concentration allows a reaction to continue faster. The concentration of a chemical is the amount of a substance in a given volume.

16. Catalysts and inhibitors • Catalysts: increase the rate of various reactions by lowering the required the activation energy required for a reaction to begin. An example are the enzymes which lower the energy in our bodies for a reaction to occur. Enzymes are specific to reactions meaning each enzyme will only catalyze one reaction. • Inhibitors: these are substances which reduce the rate of a chemical reaction.