Chemical Reactions and Equations

What is a chemical reaction? – The process by which the atoms of one or more substances are rearranged to form different substances. A chemical reaction is another name for a chemical change. How can you tell when a chemical reaction has taken place? – Temperature change – releasing energy in the form of heat and light. Absorbing heat. – Color change – rust – Odor – Gas bubbles – The appearance of a solid

What is a chemical equation? – A statement used to represent a chemical reaction. – Reactants – starting substances – Products – substances formed during the reaction – An arrow is used instead of an equal sign. It means “reacts to produce” or “yield” Reactants are on the left and products are on the right – A plus sign is used when there are more than one reactant or more than product reactant 1 + reactant 2 product 1 + product 2 – Reactants and products can exist as solids, liquids, and gases. When dissolved in water, they are called aqueous

WORD EQUATIONS ex. Iron (s) + chlorine (g) iron (III) chloride (s) SKELETON EQUATIONS ex. Fe(s) + Cl 2 (g) FeCl 3 (s) – You do one. Carbon and sulfur to form carbon disulfide. Carbon and sulfur are solids. Remember the Law of Conservation of Mass. – In a chemical reaction, matter is neither created nor is it destroyed, it is changed. Matter is conserved.

Balancing chemical equation – shows the conservation of mass. – 2Fe(s) + 3Cl 2 (g) 2FeCl 3 (s) 2 iron atoms and 6 chlorine atoms = 2 iron atoms and 6 chlorine atoms – To balance an equation, you must find the correct coefficients for the chemical formulas in the skeleton equation. – Coefficient – the number written in front of a reactant or product. Usually a whole number and usually not written if the value is one. The smallest number of particles of the substance involved in the reaction. The lowest whole number ratio of the amounts of the reactants and products

Steps for balancing equations 1.Write the skeleton equation for the reaction. 2.Count the atoms of the elements in the reactants. 3.Count the atoms of the elements in the products. 4.Change the coefficients to make the number of atoms of each element equal on both sides of the equation. 5.Write the coefficients in the lowest possible ratio. 6.Check your work. H 2 and Cl 2 to form HCl

CLASSIFYING CHEMICAL REACTIONS – Synthesis reactions – a chemical reaction in which 2 or more substances react to produce a single product. 2Na (s) + 2Cl (g) 2NaCl (s) CaO(s) + H 2 O (l) Ca(OH) 2 (s) 2SO 2 (g) + O 2 (g) 2SO 3 (g) – Combustion reactions – oxygen combines with a substance and releases energy in the form of heat and light. 2H 2 (g) + O 2 (g) 2H 2 O (l) C (s) + O 2 (g) CO 2 (g) CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g)

– Decomposition reactions – a single compound breaks down into 2 or more elements or compounds. NH 4 NO 3 (s) N 2 O (g) + 2H 2 O (g) 2NaN 3 (s) 2Na (s) + 3N 2 (g) – Replacement reactions – the replacement of an element in a reaction. A +BX AX + B Single replacement – a reaction in which the atoms of one element replace the atoms of another element in a compound. – 2Li (s) + 2H 2 O (l) 2LiOH (aq) + H 2 (g) – Cu(s) + 2AgNO 3 (aq) 2Ag(s) + Cu(NO 3 ) 2 (aq) – Ag(s) + Cu(NO 3 ) 2 (aq) NR (no reaction) Double replacement – an exchange of ions between 2 compounds. AX + BY AY + BX – Ca(OH) 2 (aq) +2HCL (aq) CaCl 2 + 2H 2 O (l) – 2NaOH (aq) + CuCl 2 (aq) 2NaCl (aq) + Cu(OH) 2 (s)

– Cu(OH) 2 (s) is a precipitate. A solid produced during a chemical reaction in a solution. All double replacement reactions produce either a precipitate, a gas, or a water. – KCN (aq) + HBr (aq) KBr (aq) + HCN (g)

Reactions in an aqueous solution – Solution – when a substance dissolves in water. A solution is a homogeneous mixture. – A solution contains one or more substances called solutes dissolved in water. – The water is the solvent, the most plentiful subtance in the solution. – An aqueous solution is a solution in which the solvent is water. – Some solutes are molecular compounds that exist as molecules in aqueous solutions. – Others are molecular compounds that form ions when they dissolve in water. HCl(g) H + (aq) + Cl - (aq)

When 2 aqueous solutions that contain ions as solutes are combined, the ions may react with one another. ALWAYS double-replacement reactions. – Double replacement can produce precipitate, water, or gas. Reactions that form precipitates Experiment - NaOH and epsom salt(NaHCO 3 ) – 2NaOH(aq) + CuCl 2 (aq) 2NaCl(aq) + Cu(OH) 2 (s) – As separate solutions, they exist as ions. – When combined, they form a solution and a precipitate

An Ionic equation shows this. 2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq) 2Na+(aq) + 2Cl-(aq) + Cu(OH) 2 (s) – An ionic equation that shows all of the particles in a solution as they realistically exist – complete ionic equation – Ions that do not participate in the reaction are called spectator ions and are usually NOT shown in ionic equations. – A net ionic equation will only include the particles that participate in the reaction. 2Na + (aq) + 2OH - (aq) + Cu 2+ (aq) + 2Cl - (aq) 2Na + (aq) + 2Cl - (aq) + Cu(OH) 2 (s) 2OH - (aq) + Cu 2+ (aq) Cu(OH) 2 (s) This is the net ionic equation.

Reactions that form water – The water molecules produced increase the solvent particles. HBr(aq) + NaOH(aq) H 2 O(l) + NaBr(aq) – Write the ionic equation and the net ionic equation. Reactions that form gas Experiment - Vinegar (acetic acid and water) and baking soda (sodium hydrogen carbonate) – Some commonly produced gases are carbon dioxide, hydrogen cyanide, hydrogen sulfide. Hydroiodic acid and lithium sulfide produce hydrogen sulfide And lithium iodide 2HI(aq) + Li2S(aq) H2S(g) + 2LiI (aq)