Chemical Reactions Chapter 7

Describing Chemical Reactions  What type of change is happening in the picture to the left?  Chemical Reaction  New Products being formed  Heat and Light being produced

Chemical Equations  What is present before and after a chemical change takes place?  Reactants – the substances undergoing the chemical change  Products – the new substances formed as a results of the chemical change  In the case of the burning charcoal  Reactants are carbon and oxygen and the product is carbon dioxide C + O 2 → CO 2

Using Equations to Represent Reactions  Reactants → Products  Chemical Equation - a representation of a chemical reaction in which the reactants and products are expressed as formulas

Conservation of Mass  The mass of the products is always equal to the mass of the reactants  Called the Conservation of Mass  Principal was establish by Antoine Lavoisier  Mass is neither created nor destroyed in a chemical reaction

Conservation of Mass

Balancing Equations  In rockets  Hydrazine + Oxygen yields Nitrogen + Water  N 2 H 4 + O 2 → N 2 + H 2 O  This equation is not in balance

Balancing Equations  We balance equations by adding coefficients (a whole number) before the formula itself  We never change subscripts when balancing an equations  N 2 H 4 + O 2 → N 2 + 2H 2 O

Some Practice Balancing Equations  H 2 + O 2 → H 2 O  HgO → Hg + O 2  Zn + HCl → ZnCl 2 + H 2

Counting with Moles  Chemists use a counting unit called the mole  1 mole = 6.02 X particles of that substance  Particles might be atoms, ions, molecules

Molar Mass  Molar mass – the mass of one mole of a substance  the atomic mass expressed in grams  1 mole of Na = 23 grams  1 mole of Cl = 35 grams  1 mole of NaCl = 58 grams  1 Mole of Hydrogen = 1 gram  1 Mole of Oxygen = 16 grams  1 Mole of H2O = 18 grams

Mole-Mass Conversions  Express molar mass as a conversion factor  ie: CO2 has a molar mass of 44 grams  1 mole of Carbon = 12 grams  2 moles of Oxygen = 32 grams (16 g x 2)  44.0g CO2/1 mole of CO2  1 mole of CO2/44.0 g CO2  How many moles of CO2 do I have if I have 55 grams of CO2?

The Calculation  55.0 g CO 2 X 1 mole of CO 2 /44 g CO2 = 1.25 moles of CO 2  If you know how many grams of a substance you have ~divide by the molar mass to determine how many moles you have

Types of Reactions  Synthesis Reactions  Decomposition Reactions  Single-Replacement Reactions  Double-Replacement Reactions  Combustion Reactions

Synthesis Reactions  A synthesis reaction is a reaction in which two or more substances react to form a single substance – the reactants may be either elements or compounds A + B → AB 2 Na + Cl 2 → 2 NaCl 2 H 2 + O 2 → 2 H 2 O This reaction generates electricity for satellites and spacecraft

Decomposition Reactions  A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances – The reactant in a decomposition reaction must be a compound – the products may be elements or compounds AB → A + B 2 H 2 O → 2H 2 + O 2 CaCO 3 → CaO + CO 2 Making cement 2NaN 3 → 2 Na + 3 N 2 (Air bags)

Single-Replacement Reactions  A Single-replacement reaction is a reaction in which one element takes the place of another element in a compound A + BC → B + AC (A and B have switched places) Cu + 2Ag NO 3 → Cu(NO 3 ) 2 + 2Ag 2K + 2H 2 O → H KOH

Double-Replacement Reactions  A double-replacement reaction is one in which two different compound exchange positive ions and form two new compounds AB + CD → AD + CB Pb(NO 3 ) KI → PbI 2 + 2KNO 3 CaCO HCl → CaCl 2 + H2CO 3

Combustion Reactions  A combustion reaction is one in which a substance reacts rapidly with oxygen, often (not always) producing heat and light.  One of the reactants must be oxygen CH O 2 → CO 2 + H 2 O 2H 2 + O 2 → 2H 2 O

Reactions as Electron Transfers (Redox Reactions)  Oxidation – A loss of electrons during a chemical reaction  Reduction – a gain of electrons during a chemical reaction  2 Ca + O 2 → 2 CaO  Calcium is oxidized  Oxygen is reduced  Na + Cl → NaCl  Sodium is oxidized  Chlorine is reduced

Energy Changes in Reactions  C 3 H8 + 5O 2 → 3CO H 2 O + Heat  Chemical energy is the energy stored in the chemical bonds of a substance  Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products

Breaking Bonds  Breaking bonds requires energy  This is why we need a spark to light the propane because it doesn’t happen spontaneously  Forming bonds requires energy  During a chemical reaction energy is either released or absorbed

Exothermic Reactions  A chemical reaction in which energy is released to the surroundings  Combustion reactions are extreme exothermic reactions  Ex. 1 mole of Propane produces 2220 kilojoules of heat  In an exothermic reaction the kilojoules would be written to the right of the arrow

Endothermic Reactions  A chemical reaction that absorbs energy from its surroundings is called an endothermic reaction  2HgO + Heat → 2 Hg + O 2  In an endothermic reaction heat appears on the left side of the arrow

Conservation of Energy  The total amount of energy before and after a reaction must always be the same

Reaction Rates  The progress of a chemical reaction can be measured over time  Different reactions have different durations  TNT  Leaves turning color  Any reaction that takes places over time can be expressed as a rate  A reaction rate is the rate at which reactants change into products over time – in other words how fast the reaction is going

Factors affecting Reactions Rates  Temperature  Generally an increase in temperature will increase the reaction rate  Surface area  Increasing the surface area of a reactant tends to increase the reaction rate  Stirring  Stirring the reactants will generally increase the reaction rate

Factors affecting Reactions Rates  Concentration  Increasing the concentration generally increases the reaction rate  Catalysts  A substance that affects the reaction rate without being used up in the reaction  Generally speed up a reaction

Equilibrium  Equilibrium is a state in which the forward and reverse paths of a change take place at the same rate  Physical Equilibrium  H 2 O (l) ↔ H 2 O (g)  When physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes

Equilibrium  Most chemical reactions are reversible to some extent  A reversible reaction is a reaction in which the conversion of reactants into products and the conversion of products into reactants can happen simultaneously  2 SO 2 (g) + O 2 (g) ↔ 2 SO 3 (g)  When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions

Factors Affecting Chemical Equilibrium  When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change  Discovered by Henri Le Chatlier N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) + Heat → This reaction is exothermic ← This reaction would be endothermic  Temperature  If you add heat to the system the equilibrium would shift in the direction that removes heat from the system (In the above reaction you would decrease the amount of ammonia produced

Factors Affecting Chemical Equilibrium  Pressure  Increasing the pressure would shift the equilibrium in the direction that reduces the pressure  More gas molecules = more pressure  Increasing the pressure in the ammonia production would produce more ammonia because the right side of the equation has fewer ( 2 vs. 4 ) gas molecules N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) + Heat

Factors Affecting Chemical Equilibrium  Concentration  If we removed ammonia from the system the reaction would favor producing more ammonia  If we removed either of the reactants the reaction would favor breaking down the ammonia and producing more of the reactants N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) + Heat