Chapter: Chemical Reactions Table of Contents Section 1: Chemical Formulas and Equations What you will learn and why is it important: page 492

Physical changes in a substance affect only physical properties, such as its size and shape, or state of matter. **Physical changes are reversible and do not change the chemical composition of the substance. Physical or Chemical Change? Chemical Formulas and Equations 1 1

Chemical changes produce new substances that have properties different from those of the original substances * Not reversible because the composition of the substances will change. A process that produces chemical change is a chemical reaction. Physical or Chemical Change? Chemical Formulas and Equations 1 1

Symbols to represent elements Chemical formula to represent compounds And also: Chemical equations to represent chemical reactions We use…….

A chemical equation tells chemists the reactants, products, physical state, and the proportions of each substance present the equation. Chemical Formulas and Equations 1 1

Reactants are the substances that exist before the reaction begins. The substances that form as a result of the reaction are called the products. EX of Chemical Equations Chemical Formulas and Equations 1 1

Subscripts, coefficients, arrows and plus signs EX of Chemical Equations Chemical Formulas and Equations 1 1

chemical reactions are much like mathematical equations. 1 1 In chemical equations the number and kind of atoms are equal on the two sides – law of conservation of mass Balancing Chemical Equations

Count the number of atoms of each type in the reactants and in the products. The number of atoms of each type has to be the same in the reactants and in the products Count the Atoms to balance an equation Chemical Formulas and Equations 1 1

Balancing Chemical Equations (this equation is not balanced) Chemical Formulas and Equations 1 1 ADD COEFFICIENTS BUT NEVER ADD SUBSCRIPTS

Place a 2 ( coefficient) in front of the reactant Ag and check to see if the equation is balanced. Recount the number of atoms of each type. Count the Atoms Chemical Formulas and Equations 1 1 The equation is now balanced.

Energy in Chemical Reactions During a chemical reaction: Energy can be absorbed or released This energy comes from the bonds between the atoms.

Endothermic reactions absorb energy in the form of heat. During a chemical reaction: 1) Energy can be absorbed or released 2) This energy comes from the bonds between the atoms. ENERGY 1 1 Exothermic reactions release energy in the form of heat.

Chapter: Chemical Reactions Table of Contents Section 2: Rates of Chemical ReactionsRates of Chemical Reactions

Rate of Chemical Reactions It is possible to speed up, slow down chemical reactions or totally avoid a chemical reaction to happen

2) Reaction Rate Rates of Chemical Reactions 2 2 The rate of reaction tells how fast a reaction occurs after it has started. To find the rate of a reaction, you can measure either how quickly one of the reactants is consumed or how quickly one of the products is created.

Starting a Reaction Before a reaction can start, atoms and molecules of the reactants have to collide. reactions occur because of the random motion of atoms and molecules that collide into each other. Rates of Chemical Reactions 2 2

Activation energy is the minimum amount of energy needed to start a chemical reaction. ( this energy breaks the chemical bonds of the reactants) Rates of Chemical Reactions 2 2 Activation Energy— Starting a Reaction

3)Factors affecting Reaction Rate Rates of Chemical Reactions 2 2 Temperature Concentration of the reactants Surface area

4)Temperature 2 2 Food spoiling happens due to chemical reaction or action of microorganisms ((bacteria and fungi).

4)Temperature 2 2 Lowering the temperature of the food slows the rate of chemical reactions and the action of most bacteria. Increasing the temperature will increase chem. Reactions – more collision of molecules Increasing the temperature will increase the action of bacteria.

Concentration Rates of Chemical Reactions 2 2 The amount of substance present in a certain volume is called the concentration of that substance.

5) Concentration Rates of Chemical Reactions 2 2 If you increase the concentration, you increase the number of atoms of a substance. More atoms - more collisions of atoms – more chem. reactions

Surface Area Affects Rate The exposed surface area of reactant particles also affects how fast the reaction can occur. 2 2 the higher the exposed surface area, the higher the reaction rate.

Slowing Down Reaction A substance that slows down a chemical reaction is called an inhibitor. Rates of Chemical Reactions 2 2 Inhibitors are frequently added to processed food to increase the shelf-life

6) Catalysts A catalyst is a substance that speeds up a chemical reaction. Rates of Chemical Reactions 2 2 Catalysts do not appear in chemical equations, because they are not changed permanently or used up. THEY DO NOT PARTICIPATE IN THE REACTION

Speeding Up Reactions Catalysts work by: 2 2 1)Reducing the activation energy needed to start the reaction – the catalysts will provide energy for the reaction to start

Ea = activation energy

Speeding Up Reactions Catalysts work by: 2 2 2) Increasing the exposed surface area – - the higher the exposed surface area of the reactants, the higher the reaction rate. - catalysts position the molecules in a certain way that facilitates the reaction

Enzymes Are Catalysts Enzymes are large protein molecules that speed up reactions needed for your cells to work properly. Like other catalysts, an enzyme will position the reacting molecules so that their structures fit together and react. Rates of Chemical Reactions 2 2

**Enzymes have an active site that is molded so the reactants will fit properly, increasing the exposed surface area of the reactants

End of Chapter Summary File