REACTIONS

What are chemical reactions?

Chemical Reactions When substances undergo chemical changes to form new substances

What are the signs of a chemical reaction?

Signs of Chemical Reactions 1. Energy released a heat or light 2. Production of a gas 3. Formation of a precipitate. A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate. 4. Color change

Chemical Reactions Reactants – substances that go through the chemical change (starting parts) Product – a substance that is the result of a chemical change (ending parts)

How are chemical reactions represented?

Chemical Reactions A chemical equation represents, with symbols and formulas, the identities and amounts of the reactants and products in a chemical reaction. word equation: the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide and water vapor formula equation: (NH 4 ) 2 Cr 2 O 7 (s) → N 2 (g) + Cr 2 O 3 (s) + 4H 2 O(g)

Symbols Used in Chemical Equations

How are chemical equations written?

Writing Chemical Equations 1. Translate the compound names into formulas 2. Add the symbols for state of matter (s) (l) (aq) (g) 3. Place the reactants’ formulas on the left separated with + 4. Add the → or ↔

Writing Chemical Equations 5. Place the products’ formulas on the right separated with + 6. Add the ↑ for gas production or ↓ for precipitate products

How are chemical equations balanced?

Balancing Equations Because of the law of conservation of mass the number of reactant atoms must equal the number of product atoms. H 2 + O 2 → H 2 O This equation must be balanced by inserting coefficients in front of molecules. 2H 2 + O 2 → 2H 2 O

Balancing Chemical Equations 1. Write the formula equation for a reaction. 2. Draw boxes around each compound. 3. Make an element inventory. 4. Add coefficients in front of the boxes until the elements balance.

How is energy used in a reaction?

Energy in Reactions Energy must be added to break the original bonds apart to cause a change When bonds are formed energy is released –This energy is in the chemical bonds

Energy in Reactions Exothermic reaction – a reaction that transfers energy from the reactants to the surroundings

Energy in Reactions

Endothermic reaction – a reaction in which energy is transferred to the reactants from the surroundings

Energy in Reactions

What are the types of reactions?

Types of Reactions –Synthesis– when 2 or more substances form a new more complex compound –A + B  AB –An iron bar rusts. The iron reacts with oxygen in the air to make rust. –4 Fe + 3 O 2  2 Fe 2 O 3

SynthesisSynthesis Reaction Synthesis H H O O H H

Types of Reactions Decomposition reaction – when one compounds breaks into 2 or more products –AB  A + B –C 6 H 12 O 6 (glucose)  2 C 2 H 5 OH (ethyl alcohol) + 2 CO 2 –Add heat or Electrolysis – using electricity to break apart bonds

DecompositionDecomposition Reaction Decomposition H H O O H H

Types of Reactions Single displacement – a reaction in which atoms of one element take the place of atoms in another compound Single displacement – a reaction in which atoms of one element take the place of atoms in another compound –AB + C  AC + B Zn (s) + 2HCl (aq)  ZnCl 2 + H 2 (g) Single Displacement ReactionReaction

Types of Reactions Double displacement – a reaction in which ions exchange between 2 compounds Double displacement – a reaction in which ions exchange between 2 compoundsDouble displacement Double displacement –AB + CD  AC + BD –An antacid (calcium hydroxide) neutralizes stomach acid (hydrochloric acid). Ca(OH) HCl  CaCl H 2 O –Causes a gas, precipitate, or compound to form

Combustion Reactions In a combustion reaction, a hydrocarbon combines with oxygen, releasing a large amount of energy in the form of light and heat. The products are always CO 2 and H 2 O. example: combustion of propanecombustion C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) C x H x + O 2 CO 2 + H 2 O

Methane Combustion

Final Look At Types Of Reactions Decomposition Synthesis Single Replacement Double Replacement

What can change the rate of a reaction?

Rates of Chemical Change A reaction rate is the rate at which a chemical reaction takes place and is measured by the rate of formation of a product or disappearance of a reactant. Reaction rates depend on conditions such as temperature and pressure.

Things That Effect the Reaction Rate TemperatureTemperature - the higher the temperature the faster the rate of reaction Temperature The kinetic theory states that hotter particles contain more kinetic energy, move faster, and collide more often. - example: the hotter the stove the faster an egg cooks

Things That Effect the Reaction Rate Pressure – the higher the pressure the faster the rate of reaction Greater pressure decreases the available space for a molecule to move, increasing collisions with other molecules. - example: using a pressure cooker instead of boiling water

Things That Effect the Reaction Rate Surface area – the larger the surface area the faster the reaction  Many smaller pieces have a larger surface area than few large pieces  example: if you crush sugar it will dissolve faster than a sugar cube

Things That Effect the Reaction Rate Concentration – the higher the concentration of a substance the faster the reaction Concentration – the higher the concentration of a substance the faster the reactionConcentration Concentration is measured in molarity, moles per liter. Concentration is measured in molarity, moles per liter. example: the more bleach the whiter the clothes get example: the more bleach the whiter the clothes get

Things That Effect the Reaction Rate Presence of catalyst – a catalyst will speed the rate of the reaction Presence of catalyst – a catalyst will speed the rate of the reaction A catalyst is not a reactant or product and is not used up in the reaction. The catalyst lowers the activation energy needed for the reaction. Example – enzymes in the human body enzymes

Activation Energy The minimum energy that two colliding particles need to have before a chemical change is possible is called the activation energy, E a, of the reaction. No reaction is possible if the colliding pair has less energy than E a.