Chemical Reactions

Chemical Reactions Change Substances Chemical reactions occur when substances undergo chemical changes to form new substances. Common, every day chemical reactions include: growing, ripening, decomposing, & burning (combustion) Growing Ripening Decomposing Burning

Signs of Chemical Reactions 1. Bubbles appear. 2. A precipitate (solid) forms in the bottom of the test tube. 3. A color change occurs. 4. The temperature changes (either increases or decreases). 5. Light is given off – often happens in combustion reactions. 6. A change in smell or taste occurs. Bubbles forming in a reaction Glowing jellyfish: give off light -- bioluminescence

Chemical Reactions Rearrange Atoms Photosynthesis Equation Reactants Products 6 CO2 + 6 H2O + sunlight  C6H12O6 + 6 O2 carbon dioxide + water glucose oxygen

Energy is Conserved in Chemical Reactions 1. Energy is neither created nor destroyed. 2. Energy is transformed from one for into another. 3. The total amount of energy is the same at the beginning of the reaction as at the end of the reaction.

Energy is Transformed in Chemical Reactions In photosynthesis, light energy is transformed into chemical energy for the plant. Energy is transformed from one form into another.

Energy and Chemical Reactions 1. Exothermic Reactions: reactions that release energy (energy exits to the environment) – they will heat up! 2. Temperature of surroundings increases because energy is released. 3. Examples of exothermic reactions: a. digesting food b. decomposition c. burning fuel 4. Exothermic reactions may give off light A burning match is an exothermic reaction – it gives off light & heat.

Energy and Chemical Reactions 1. Endothermic reactions: absorb energy from the environment 2. Requires energy to form chemical bonds. 3. Energy used may be in the form of light or heat. 4. May (but not always) see a drop in temperature Photosynthesis is an endothermic reaction.

Synthesis 2 or more elements or compounds combine to make one product A + B  AB

Decomposition One compound is broken down into 2 or more elements or compounds AB  A + B

Single Replacement Replacing of one element in a compound with another element AB + C  AC + B

Double Replacement Two compounds exchange elements AB + CD  AD + CB

Combustion Fuel burns in the presence of oxygen to produce carbon dioxide and water (along with other by-products) ALWAYS EXOTHERMIC Fuel + O2  CO2 + H2O

Reactants  Products Reactants (always go on the left): What you want to react What you mix together What you start with Products (always go on the right): What is produced by the reaction The chemicals created by the reaction What you end up with

Identify the reactants and products! H2 + O2  H2O Na + Cl2  NaCl KClO3  KCl + O2 (Disclaimer: The above reactions are not balanced!)

What’s wrong with this picture? H2 + O2  H2O  Answer: It is not balanced. The number of atoms on each side are not equal

A Closer Look: H2 + O2  H2O  The Law of Conservation of Matter says that matter cannot be created or destroyed. Is matter being created or destroyed above?

Balancing Chemical Equations YOU CANNOT CHANGE SUBSCRIPTS! Changing the subscript in a chemical formula changes the chemical. Example: Methane = CH4 Ethane = C2H6 Subscripts

Balancing Chemical Reactions You CAN Change COEFFICIENTS. Changing a coefficient changes the NUMBER of particles of the substance that you have! CH4 (one assumed) = 1C and 4H 2CH4 = 2C and 8H 3CH4 = 3C and 12H

Counting Atoms You ALWAYS multiply coefficients by subscripts to get total of a particular element How many atoms are represented on each side? 4Al + 3Cl2  2Al2Cl3 Al=4 Cl=6

A Closer Look Lets Balance the creation of Water… our original reaction: H2 + O2  H2O First… List all elements in the same order on both sides 2 2 Next, go down the list and balance one at a time Whenever you make a change, you should go through the list again to make sure! We are BALANCED! Balanced Balanced Unbalanced H=4 H=2 H=2 H=4 Unbalanced Balanced O=2 O=2 O=1

Lets Do another! Na + Cl2  NaCl 2 2 1st: List all the elements you have in the same order 2nd: Go down list and balance as you go. 4th: We are BALANCED!. 3rd: You made changes. Balance list again. Balanced Balanced Unbalanced Na=1 Na=2 Na=1 Na=2 Balanced Unbalanced Cl=2 Cl=2 Cl=1

X One More! KClO3  KCl + 2 2 3 O2 O3 O2 PLEASE NOTE: Since 2 does not divide into 3, you will need to change two at once to balance oxygen. Both 2 and 3 divide into 6 so this should be your new total! You must do all chemical reactions as shown before. You can ONLY change coefficients! So lets get started… You might be tempted to… change the subscript Add coefficients that will give you 6 oxygens on each side. Step 3: Since you made changes, do the list again! Finally: We are BALANCED! Step 2: Balance the list. Step 1: List all elements in the same order But as you can see, doing this changes regular oxygen into another substance altogether… Ozone! Since this reaction NEVER creates ozone, doing this is WRONG! K=1 K=2 K=2 K=1 Balanced Unbalanced Balanced Notice we balanced this too! Cl=1 Cl=2 Cl=2 Cl=1 Balanced O=3 O=6 O=6 O=2 Unbalanced Balanced You CAN NEVER change subscripts!

Classify & Balance Each Chemical Equation Warm-Up 1. Mg + O2  MgO 2. H2O  H2 + O2 3. Al + CuCl2  AlCl3 + Cu 4. FeCl2 + Na2CO3  NaCl + FeCO3

Warm-Up: Check Your Answers Synthesis Decomposition Single replacement Double replacement 1. 2 Mg + O2  2 MgO 2. 2 H2O  2 H2 + O2 3. 2 Al + 3 CuCl2  2 AlCl3 + 3 Cu 4. FeCl2 + Na2CO3  2 NaCl + FeCO3