Chapter 17: Chemical Reactions Section 1: Chemical Formulas and Equations Chemical Equations Balancing Chemical Equations Law of Conservation of Mass Evidences of Chemical Reactions
Symbols to represent elements the symbols are letters Chemical formulas to represent compounds letters and numbers And also: Chemical equations to represent chemical reactions
Chemical Reactions form a new substance: A + B = AB AB is the result of a chemical reaction *AB has physical and chemical properties that are totally different than A and B EX: H 2 + O H 2 O hydrogen and oxygen are gases form water which is a liquid
EX of Chemical Equations Reactants are the substances that exist before the reaction begins Products are the substances that form as a result of the reaction
2H 2 + 2O 2 2H 2 O 2 Coefficients and subscripts will show the amount of atoms participating in the reaction H one atom of hydrogen H 2 2 atoms of hydrogen forming a molecule (united by a chemical bond) 2H two atoms of Hydrogen, not united by a chemical bond 2H 2 two molecules of hydrogen (4 atoms of hydrogen in total) 2(H 2 O) 4 atoms of hydrogen and 2 of oxygen
1. 2H 2 + 2O 2 2H 2 O 2 Coefficients and subscripts will show the amount of atoms participating in the reaction How many atoms of hydrogen you have in the beginning and at end of this reaction? Oxygen?
CO CO2 2CO 2CO2 H2SO H2SO4 2H2SO4 H2(SO4)3 2H2(SO4)3
Chemical Equations can show the state of matter of the reactants and the products 2H 2(g) + 2O 2(g) 2H 2 O 2(l)
Law of Conservation of Mass According to the law of conservation of mass, the mass of the products must be the same as the mass of the reactants in a chemical reaction. This principle was first stated by the French chemist Antoine Lavoisier (1743-1794).
How can we prove the Law of Conservation of Mass????
**Chemical Reactions in Closed Systems and Open system Lab ** The Law of Conservation of mass is better observed in a closed system. ** In an open system, you will not be able to measure the mass that is lost if gases are released during the reaction
chemical reactions are much like mathematical equations. In chemical equations the number and kind of atoms are equal on the two sides of the equation Chemical Equations illustrate the Law of Conservation of Mass
Balancing Chemical Equations: 1. Count the number of atoms on each side of the equation. 2. Add coefficients to make the number of atoms equal on both sides of the equation
Balancing Chemical Equations: same number of all types of atoms in both sides
Balancing Equations GOLDEN RULE: never add subscripts when balancing a chemical equation, only add coefficients
Energy in Chemical Reactions
Energy Endothermic reactions absorb heat energy from the environment. We include the word energy with the reactants in the chemical equation
Exothermic reactions release heat energy. This energy comes from the bonds between the atoms. We include the word energy with the products in the chemical equation
Physical or Chemical Change? Physical changes in a substance affect only physical properties Physical changes are reversible and do not change the chemical composition of the substance. Ex: Cut a log Fold a piece of paper Bend a piece of metal
Physical or Chemical Change? Chemical changes produce new substances The new substances have properties different from those of the original substances Chemical Changes affect the physical and chemical properties of the reactants. Not reversible because the composition of the substances will change.
Examples of Ch. Reactions around us rusting burning of any material metabolism of food in the body Cooking Explosion of fireworks
**Evidences of Chemical reaction Signs that a chemical reaction happened and that new substances were formed: 1) a gas is being formed - bubbles 2) color change 3) precipitate a solid is being formed 4) change in temperature the reaction releases or absorbs heat 5) Others: Light, sound, smell, smoke
Examples of Physical Properties *color and shape *length and mass *volume and density *melting point and boiling point *state of matter *malleability and ductility - metals *magnetic properties metals *Physical Prop can help you identify substances
Examples of Chemical Properties Flammability - Ability to burn, same as ability to react with Oxygen Reactivity - ability to react with other substances
Chapter: Chemical Reactions Section 2: Rates of Chemical Reactions What is reaction rate?
Reaction Rate The rate of reaction tells how fast a reaction occurs after it has started. It is possible to alter the speed of chemical reaction or totally avoid a chemical reaction to happen. Important in the industry: Faster production Conservation of products
To measure 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.
Why reactions happen - starting a reaction *****Reactions occur because of the random motion of particles which collide into each other. **Any factor that increases the possibility of collision between molecules will increase the rate of reaction
Factors that affect Reaction Rate can increase or decrease the collision between molecules of reactants. Temperature Concentration of the reactants The surface area of the molecules of the reactants or particle size
Temperature Most chemical reactions speed up when temperature increases. Molecules will move faster and collide more frequently at higher temperatures and are more likely to react.
Temperature x Action of bacteria Food spoiling happens due to chemical reaction or action of microorganisms. How does temperature affect the action of bacteria?
Temperature x the action of bacteria Lowering the temperature of the food slows down the action of most bacteria. Increasing the temperature have the opposite effect. Why does this happen? Bacteria grow faster at higher temperatures. reaching dangerous levels sooner and producing toxins.
Concentration The amount of substance present in a certain volume is called the concentration of that substance.
Concentration 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 Reaction Rate The exposed surface area of reactant particles also affects how fast the reaction can occur. the higher the exposed surface area, the higher the reaction rate. If the particles are smaller, the higher will be the surface area.
Important vocabulary Section 2 Activation Energy Inhibitor Catalyst Enzyme
Activation Energy Starting a Reaction Activation energy is the minimum amount of energy needed to start a chemical reaction. This energy breaks the chemical bonds of the reactants
Inhibitor A substance that slows down a chemical reaction is called an inhibitor. Inhibitors are frequently added to processed food to increase the shelf-life. ( = to make the food last longer)
Catalysts A catalyst is a substance that speeds up a chemical reaction. Catalysts do not appear in chemical equations because they DO NOT PARTICIPATE IN THE REACTION Catalysts will only facilitate the reaction
How do catalysts work: 1)By increasing the exposed surface area catalysts position the molecules in a certain way that facilitates the reaction. 2)By lowering the activation energy
Lowering activation energy Ea = activation energy
Enzymes Are Catalysts Enzymes are large protein molecules that speed up many reactions inside organisms. Enzymes are also used in the industry An enzyme will position the reacting molecules so that their structures fit together and react.
**Enzymes have an active site that is molded so the reactants will fit properly. The active site facilitates the reaction by bringing the reactants closer to one another.
Factors that affect Enzymes temperature, ph, enzyme concentration, substrate concentration, ( substract is the substance the enzyme works on) Temperature and ph will alter the shape of the active site and the enzyme will not function properly.