Chemical Reactions Result in one or more new substances being formed from one or more original substances

Chemical Reaction Indicators Formation of a gas Emission of light or heat Formation of a precipitate Color change

Chemical Reactions Reactants: – the substances that undergo change – What you start with Products: – the new substances formed – What you end up with Reactants change to Products Carbon + Oxygen change to Carbon dioxide

Reactants Products

Chemical Energy The chemical energy of a substance is the sum of its potential energy (stored energy) and kinetic energy (energy of movement). These energies result from such things as: ◦ Attractions between electrons and protons ◦ Repulsions between nuclei ◦ Repulsions between electrons ◦ Movement of electrons ◦ Vibrations of and rotations around bonds

 All chemical reactions result in a change in the chemical energy of the reactants as they form the products Reaction Energy

During a chemical reaction, the atoms in the reactants are arranged into products with different amounts of chemical energies.

Reaction Energy Chemical changes that give off energy are called exothermic reactions. Chemical changes that absorb energy are called endothermic reactions.

Exothermic The total chemical energy of the products is less than the energy of the reactants. the difference in energy is released into the environment, often as heat energy.

Exothermic Reactions 25 o C 45 o C Gets hot Hydrochloric acid magnesium Magnesium + Hydrochloric acid

Energy Level Diagram: Exothermic Reaction Energy / kJ) Progress of reaction (time) reactants Reactants have more chemical energy. Some of this is lost as heat which spreads out into the room. products Products now have less chemical energy than reactants.

Endothermic The chemical energy of the products is greater than the energy of the reactants. Energy must be absorbed from the surrounds in order for the reaction to occur.

Endothermic Reactions Ammonium nitrate Water Cools Heat energy taken in as the mixture returns back to room temp.

products Energy / kJ) Progress of reaction reactants HH This is how much energy is taken in Energy Level Diagram: Endothermic Process

Reaction Energy Why doesn’t natural gas burst immediately into flame when it comes into contact with air? Why must matches be struck for them to light? To answer these questions you need to think about what happens to the chemical bonds of the substances during a reaction.

Reaction Energy The bonds between the atoms in the reactants must first be broken. – For this to occur energy must always be absorbed New bonds form as the products are created – Energy is always released as this happens

Reaction Energy Step 1: Energy must be SUPPLIED to break bonds: Step 2: Energy is RELEASED when new bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC

Burning Methane CH 4 + 2O 2 2H 2 O + CO 2 These bonds must be broken : These bonds must form

Activation Energy The energy required to break the bonds of reactants is called the activation energy. A diagram showing this is called an energy profile.

Why is Switzerland like a chemical reaction?

Activation Energy Chemical Reactions must go over an energy hill like a car over the mountains (Swiss Alps).

Homework Read 7.3

Collision Theory For a chemical reaction to occur, the particles involved must collide with each other. The collisions must be with sufficient energy to overcome the activation energy ‘barrier’. The rate of reaction (how quickly the reaction occurs) depends on the number of energy sufficient collisions per time.

Collision Model Molecules must collide in order for a reaction to occur. What factors affect the rate of collisions?

Factors That Affect Reaction Rates There are four main ways in which reaction rates can be increases: – Increase the surface area of solid reactants (crush) – Increase the concentration of the reactants – Increasing the temperature of the reactants – Adding a catalyst Explain how each of these factors can increase the rate of reaction.

Surface Area Only the reactants that are on the surface of a cube are able to react. They are the only molecules exposed to other reactants. The greater the number of exposed molecules the larger the number of possible collisions. Take a cube 2 X 2 X 2 cm. Find the total surface area. Brake the cube into 8 separate cubes of 1 X 1 X 1 cm. Find the total surface area.

Concentration As the concentration of the reactants goes up the distance between the molecules becomes less. With less distance to travel the time between collisions becomes less

Temperature The faster the molecules move the less time it take for them to reach another molecule

Catalyst Many reactions occur more rapidly in the presence of particular elements or compounds. These substances, known as catalysts, are not consumed during the reactions, they just help it along They often work by lowering the activation energy. Enzymes are biological catalysts.

Catalyst

Catalytic converters are used to convert the polluting exhaust gases of burned lead-free gasoline into harmless gases. Platinum (Pt) is the catalysts used. Only a small amount is needed.

Homework Read 7.4

Physical Equilibrium A dynamic state where the concentrations of all reactants and products remain constant.

Physical Equilibrium You can change the equilibrium point by changing the physical conditions. – Take the top off a soda bottle. What happens to the dissolved carbon dioxide?

Chemical Equilibrium In chemical equilibrium, the products break back down into the reactants. The forward and reverse reactions occur at the same rate; the concentration of the reactants and products remain constant. You can cause the equilibrium to shift by changing the conditions such as temperature, pressure and concentration

Homework Read page 216, 217

Describing Chemical Reactions In a sentence Copper reacts with chlorine to form copper (I) chloride. In a word equation Copper + chlorine  copper (I) chloride In a symbol equation Cu (s) + Cl 2 (g)  CuCl (aq)

The seventeenth century French chemist Antoine Lavoisier performed experiments that indicated conservation of mass in chemical reactions. Lavoisier's experiments suggested that when tin is heated, the white powder formed results from the tin combining with a gas from the air. The increase in mass of the powder over the tin was the mass of the gas. Lavoisier revolutionized chemistry by basing his ideas on accurate measurements. Antoine Lavoisier ( )

The English chemist Joseph Priestley discovered a gas that Lavoisier later named oxygen When oxygen combines chemically with another substance, the process is called oxidation, and the substance is said to be oxidized. Joseph Priestley ( )

Describing Chemical Reactions In a sentence Copper reacts with chlorine to form copper (I) chloride. In a word equation Copper + chlorine  copper (I) chloride In a symbol equation Cu (s) + Cl 2 (g)  CuCl (aq) In a balanced symbol equation 2Cu (s) + Cl 2 (g)  2CuCl (aq)

Homework Read pages 192 – 194

Symbols used in equations (s) after the formula –solid Cu (s) (g) after the formula –gas H 2 (g) (l) after the formula -liquid H 2 O (l) (aq) after the formula - dissolved in water, an aqueous solution. CaCl 2 (aq)  used after a product indicates a gas (same as (g)) O 2   used after a product indicates a solid (same as (s)) CaCo 3 

Summary of Symbols

Diatomic elements There are 8 elements that never want to be alone. They form diatomic molecules. H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2, and At 2 The –gens and the –ines pattern on the periodic table

Balancing Equations ___ H 2 (g) + ___ O 2 (g) ---> ___ H 2 O(l) 22 What Happened to the Other Oxygen Atom? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule.

Balancing Site Practice Game ion/balancing-chemical-equations ion/balancing-chemical-equations Homework Checker

Convert this sentence to a balanced symbol equation Solid iron (III) sulfide reacts with hydrogen chloride to form iron (III) chloride and hydrogen sulfide. Fe 2 S 3 + HCl  FeCl 3 + H 2 S

Grouping Reactions (Types) 1.Synthesis or Composition Reactions 2.Decomposition Reactions 3.Single Replacement Reactions 4.Double Replacement reactions 5.Combustion reactions You need to be able to identify and work with each type.

1. Synthesis C + O 2  CO 2 OO C +  OO C A + B  AB

Synthesis Reactions Also called combination reactions 2 elements, or compounds combine to make one compound. A + B  AB Na (s) + Cl 2 (g)  NaCl (s) Ca (s) +O 2 (g)  CaO (s) SO 3 (s) + H 2 O (l)  H 2 SO 4 (s) We can predict the products if they are two elements. Mg (s) + N 2 (g)  Mg 3 N 2 (s)

A simulation of the reaction: 2H 2 + O 2  2H 2 O

2. Decomposition NaCl  Na + Cl 2 AB  A + B  Cl Na Cl + Na Cl

Decomposition Reactions decompose = fall apart one compound (reactant) falls apart into two or more elements or compounds. Usually requires energy AB  A + B NaCl  Na + Cl 2 CaCO 3  CaO + CO 2

Ex. Decomposition Reaction

3. Single Replacement CuCl 2 + Zn  ZnCl 2 + Cu  Zn Cl Cu + AB + C  CB + A Cl Zn Cu +

Single Replacement Just because you can write a chemical equation does not mean it will actually occur. The element metal will replace the compound metal only if it is more “active” An Activity Series shows the metals arranged according to their ability to undergo reactions If the element metal is above the compound metal it will replace

Single Replacement One element replaces another Reactants must be an element and a compound. Products will be a different element and a different compound. A + BC  AC + B 2Na + SrCl 2  Sr + 2NaCl F 2 + LiCl  LiF + Cl 2

Single Replacement We can predict if a reaction will occur Some elements are more active than other More active replaces less active

Ex. Single Replacement Reaction

4. Double Replacement MgO + CaS  MgS + CaO AB + CD  AD + CB S O  Mg Ca + O S Mg Ca +

Double Replacement Two things replace each other. Reactants must be two ionic compounds or acids. Usually in aqueous solution AB + CD  AD + CB AgNO 3 + NaCl  AgCl + NaNO 3 ZnS + 2HCl  ZnCl + H 2 S

5. Combustion CH 4 + 2O 2  CO 2 + 2H 2 O H O  C + O C + H H H O O H O H C + AB  AC + BC OO O H H

Combustion A reaction in which a compound (often carbon) reacts with oxygen C 3 H 8 + O 2  CO 2 + H 2 O C 6 H 12 O 6 + O 2  CO 2 + H 2 O

A silver spoon tarnishes. The solid silver reacts with sulfur in the air to make solid silver sulfide, the black material we call tarnish. Ag (s) + H 2 S (g) + O 2 (g)  Ag 2 S (s) + H 2 O

Explain how to balance equations Many ADDS for math class only practice game

Aluminum metal reacts with liquid bromine to form solid aluminum bromide Translate Equation ___ Al(s) + ___ Br 2 (l) →___ AlBr 3 (s) 2 32

The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction is C + O 2  CO 2

Acid/Base Reaction An acid and a base react to form a salt and water. Always in aqueous solution Acid (H + ) + Base (OH - ) → Salt + H 2 O NaOH + HCl → NaCl + H 2 O NH 4 OH + H 2 SO 4 → (NH 4 ) 2 SO 4 + H 2 O

How to recognize which type Look at the reactants Element(E), Compound(C) E + E C E + C C + C Acid + Base Look at the Products CO 2 + H 2 O Redox Synthesis Decomposition Single replacement Double replacement Acid/Base reaction Combustion

Examples Synthesis Decomposition Single replacement Double replacement H 2 + O 2  H 2 O  AgNO 3 + NaCl  Zn + H 2 SO 4  HgO  KBr +Cl 2  Mg(OH) 2 + H 2 SO 3 

Examples Acid/Base Decomposition Single replacement Synthesis Acid/Base Single replacement Double replacement HNO 3 + KOH  CaPO 4  AgBr + Cl 2  Zn + O 2  HgO + Pb  HBr + NH 4 OH  Cu(OH) 2 + KClO 3 

Summary An equation: Describes a reaction Must be balanced to follow The Law of Conservation of Matter Can only be balanced by changing the coefficients. Can describe different types of reactions.

Homework Read 7.2