Chemical Reactions & Balancing Chemical Equations

Transcription

1 Chemical Reactions & Balancing Chemical Equations

2 Chemical Reactions A chemical reaction occurs when one or more materials turn into something different A balanced chemical equation describes what happens in a chemical reaction, including showing how many moles of each material is used and produced Chemical reactions can t be reversed Some examples: digesting, burning, rotting, rusting, tarnishing, exploding Some processes: photography, photosynthesis, light/glow sticks, baking/cooking

3 How do we know when a chemical reaction occurs? The production or release of a gas (fizzing, bubbling, foaming) The release or transfer of energy - light and/or heat (feels warm, feels cool, glows) The formation of a precipitate (gets cloudy) A distinct change in color (change from colorless to having a color)

4 X Diatomic Elements X Special group of elements. (Notice the name isn t ide ) Always combined with the same atom to form a pair - di () atomic (atoms) Diatomics separate when they take part in a reaction and become a part of a compound. Ex. Br Br - (Bromine became bromide) Hydrogen H Oxygen O Fluorine F Bromine Br Iodine I Nitrogen N Chlorine Cl

5 What does an Equation Look Like? Na + Cl NaCl Reactants: Found on Left Side of arrow A coefficient: tells how many moles are used What you start with Arrow: The change is happening Products: What is produced, What you end with Found on Right Side of arrow

6 Balancing Equations Do an element inventory - list each element symbol you see on the reactant and product sides keeping same order in inventory helps with comparing Count each element by looking at the subscripts Which elements aren t balanced? Use coefficients to balance them. Balance one element at a time! Multiply coefficients to subscripts to change each element s count. Do this ONLY on the side on which you ve just written a coefficient Continue until all elements are balanced. Never change a formula s subscripts.

7 Other Equation Balancing Tips What if a formula contains a ( )? Look for the same ( ) on the other side of the. If it s there, you can count the ( ) and not the elements inside. Balance the element that has the highest charge first. Balance (OH), H, and O last if you can. Similar compounds get similar coefficients Make odd inventories into even by multiplying by even coefficients

8 Types of Chemical Reactions Type Synthesis (Combination or Composition) A reaction that joins things together Two or more reactants producing product Reactants can be elements or compounds or both Product is always a compound Basic form: A + X AX Example: H + O H O

9 Type - Synthesis Example: H + O H O What were the reactants? What happened when the reactants were combined? Conclusion: A chemical reaction happened because: Equation:

10 Type - Decomposition A reaction that breaks things down One reactant separates into two or more products Reactant is always a compound Product can be elements, compounds or both Basic Form: AX A + X Example: K(ClO 3 ) KCl + 3 O

11 Type - Decomposition EXAMPLE: K(ClO 3 ) KCl + 3 O What are the reactants? What happened when the reactants were combined? Conclusion: A chemical reaction happened because: Equation:

12 Type 3 - Single Replacement One part of the compound switches places with the element Reactants are one element and one compound Products are one element and one compound different from the reactant element and compound Basic form #: A + BX B + AX Basic form #: Y + AZ Z + AY Example: Cu + Ag(NO 3 ) Cu(NO 3 ) + Ag Example: KI+ Br KBr+ I

13 Type 3 - Single Replacement Example: Cu + Ag(NO 3 ) Cu(NO 3 ) + Ag Example: KI+ Br KBr+ I What were the reactants? What happened when the reactants were combined? Conclusion: A chemical reaction happened because: Equation:

14 Type 4 - Double Replacement + ions switch compounds and new compounds form All reactants and products are ionic (+/-) compounds Subscripts do not carry across the arrow! Use charges to determine subscripts for new compounds on product side Basic form: AX + BY BX + AY Example: NaCl + Ag(NO 3 ) AgCl + Na(NO 3 )

15 Type 4 - Double Replacement EXAMPLE: NaCl + Ag(NO 3 ) AgCl + Na(NO 3 ) What were the reactants? What happened when the reactants were combined? Conclusion: A chemical reaction happened because: Equation:

16 Combustion Any type of carbon based fuel combining with air (burning or combustion) to produce carbon dioxide and water Bunsen burner: CH 4 + O CO + H O The fuel The air And the products of any complete combustion reaction are always carbon dioxide and water.

17 So what type of chemical reaction is a human cell using when it reacts the sugars in your food with the oxygen delivered by your blood to your cells? C 6 H O 6 + O The fuel from food

18 ? H +? O? H O H H O O By looking at the subscripts, OXYGEN is not balanced. What number should the RIGHT side be multiplied with to make the OXYGEN on the two sides equal?

19 ? H +? O H O H H 4 O O When the RIGHT side is multiplied by a coefficient, both HYDROGEN and OXYGEN are multiplied by and increase in count. What must the LEFT side HYDROGEN be multiplied by to make HYDROGEN on both sides equal?

20 H +? O H O H 4 H 4 O O The HYDROGEN and OXYGEN are now equal on BOTH sides. What can the OXYGEN on the LEFT side be multiplied by that won t change any of the element counts?

21 H + O H O The equation is fully balanced. Multiplying by one will not change the element count.

22 ? NaBr +? Cl? NaCl +? Br Na Na Br Br Cl Cl The BROMINE and CHLORINE both need to be balanced. What must the LEFT side BROMINE be multiplied by?

23 NaBr +? Cl? NaCl +? Br Na Na Br Br Cl Cl BROMINE is now balanced. SODIUM increased to on the LEFT side. What must the RIGHT side CHLORINE be multiplied by to balance?

24 NaBr +? Cl NaCl +? Br Na Na Br Br Cl Cl When CHLORINE on the RIGHT side is multiplied by, so is the SODIUM. So now both are balanced. What can the LEFT CHLORINE and RIGHT BROMINE be multiplied with that won t change the element count?

25 NaBr + Cl NaCl + Br The equation is now balanced. Multiplying by does not change the element count.

26 _ Ag(NO 3 ) + _ BaCl _ AgCl + _ Ba(NO 3 ) Ag Ag (NO 3 ) (NO 3 ) Ba Ba Cl Cl

27 _ Ag(NO 3 ) + _ BaCl AgCl + _ Ba(NO 3 ) Ag Ag (NO 3 ) (NO 3 ) Ba Ba Cl Cl

28 Ag(NO 3 ) +? BaCl AgCl + _ Ba(NO 3 ) Ag Ag (NO 3 ) (NO 3 ) Ba Ba Cl Cl The balanced equation is: Ag(NO 3 ) + BaCl AgCl + Ba(NO 3 )

29 Special Equation Symbols pg Symbols that indicate physical states follow the chemical formula in an equation. They re always in ( ): (s) = solid and (c) = crystal (l) = liquid (g) = gas (aq) = aqueous solution (dissolved in water) ( ) = precipitate formed on product side. A precipitate is a solid formed from solutions. It can t dissolve in water (insoluble) ( ) = gas formed on product side. Fizzing, bubbling, or foaming can be seen when once the reaction starts and products are being formed. Until then there are no bubbles

30 Special Equation Symbols pg Symbols that indicate energy is being used as a reactant (endothermic) or released as a product (exothermic) during a reaction Endothermic: Reactants + Energy (kj) Products (Note the energy is on the reactant side) Exothermic: Reactants Products + Energy (kj) (Note the energy is on the product side) Catalysts: Help a reaction occur at a faster pace. Catalysts are neither reactants nor products Reactants catalyst Products The catalyst sits on top of the arrow. Neither reactant nor product

31 Ag(NO 3 ) (aq) + K (CrO 4 ) (aq) Ag (CrO 4 ) ( ) + K(NO 3 ) (aq) H (g) + O (g) H O (l) + 86 kj (NH 4 ) (SO 4 ) (aq) + Na(OH) (aq) NH 3 ( ) + H O (l) + Na (SO 4 ) (aq)