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Chemical Reactions

Chapter 9

Sarah Gleason

on 21 February 2013

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Transcript of Chemical Reactions

When substances REACT
Lets take a look at what happens between some elements on the periodic table when they are combined Chemical Reactions EVERYTHING that you own is made from chemicals! Chemical Reactions And the atoms that make up those chemicals in your "stuff" are REARRANGED into new substances
Remember the Law of Conservation of Matter (Mass) CHEMICAL REACTIONS - the atoms from one or more substances are rearranged to form different substances
It is another name for a "chemical change"
They affect EVERY part of your life: break down your food when you eat, helps your cells produce energy, helps your engine work in your car, ect.... Signs to look for.... Did a Chemical Reaction Occurred? Sometimes its easier to tell,sometimes not, but here are a few signs:
1. Temperature change
2. Color Change
3. Odor - new smell
4. Gas Formation (like bubbles)
5. Formation of a solid (or a change in state of matter) What we use to represent chemical reactions Chemical Equations They have a reactant and a product
Starting substances (reactant) and ending substances (product) Arrow is known as "react" or "yields"
NEVER equals because the reactant and product are NOT equal Coefficients Chemical Equations These are the large whole numbers written in front of a chemical formula to show you the amount of a compound or molecule
Example 3H2O - 3 water molecules Make sure that when you write out a chemical equation you ALWAYS write in the lowest ration possible for the reactants and products!! How to tell if a chemical equation is balanced. Is it balanced? Step 1: Write out the skeleton equation...if its not already
Step 2: Write out the elements that are listed in the equation
Step 3: Count the atoms of each element - on the reactant side and the product side Step 1: Count the atoms in the chemical equation (previous steps) and proceed if unbalanced
Step 2: Change the Coefficients ONLY to make the number of atoms in each element EQUAL!
Remember that you can NEVER change the little subscripts, because it changes the identity of the substance
Step 3: Write the coefficients in their lowest ration possible (if they are already not)
Step 4: Check you work - by recounting the atoms to make sure the it follows the Law of Conservation of Matter How to Balance Chemical Equations **The little numbers are called subscripts and do NOT ever change!! Balancing Example Fundamental concept of why we balance equations Law of Conservation of Matter Since matter is not created nor destroyed we must balance equations to demonstrate how the reactions would occur in real life situations
Otherwise we would be incorrectly demonstrating our chemical changes There are four types: some types fall in to more than one category: synthesis, combustion, decomposition, replacement Types of Chemical Reactions Start with Synthesis Reactions:
Chemical Reaction where two substances combine to form a single product (A+B AB)
Or when two compound combine to form a single compound
Or when a 1 compound combines with 1 element to form a single product (compound) When oxygen combines with a substance and releases energy in the form of light and heat
Also Known as BURNING Combustion Reactions Oxygen can combine with MANY different substances , which makes combustion reactions very common
Some Examples:
Coal burned to produce energy and heat
Hydrocarbons - burn oxygen to yield carbon dioxide and water with coal and energy Combustion Coal is a fossil fuel that is mainly composed of carbon:
Coal can form combustion reactions with oxygen that powers electric plants in many parts of the U.S. and the world
That's why its the largest energy source in the world synthesis and combustion? Label the following Can an equation be both?? 2Na (s) + Cl2 (s) 2NaCl (s)
C (s) + O2 (g) CO2 (g)
2H2 (g) + O2 (g) 2H20 (g)
CH4 + 2 O2 → CO2 + 2 H2O
2 CH4 + 3 O2 → 2 CO + 4 H2O Synthesis
Combustion Water and dinitrogen pentaoxide react to produce aqueous hydrogen nitrate. Both pretty much the opposite of synthesis reactions Decomposition Reaction A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide.
These reactions are exothermic, meaning they produce heat
The release of heat can produce light in the form of either glowing or a flame. This is when a single compound breaks down into two or more elements/ new compounds
They require an energy source to occur (like: heat, light, electricity, ect)
AB → A + B One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:
2 H2O ---> 2 H2 + O2 involve the replacement of an
element in a compound Replacement Reactions First type is single-replacement reactions:
A reaction where the atoms of one element replace the atoms of another element in a compound
A + BX AX + B When a metal replaces hydrogen (or another metal):
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
Or metals can replace another metal:
Zn + CuCl2 ---> ZnCl2 + Cu
Or when a nonmetal replaces a nonmetal:
Cl2 + 2NaBr → 2NaCl + Br2 Single - Replacement Metals differ in their reactivity, or ability to react with another substance:
An activity series shows the order of metals by reactivity
You can use a reactivity series to predict whether or not a certain reaction will occur
A metal can replace any metal in a compound if it is MORE REACTIVE
It CANNOT replace a metal if it is less reactive
The letters NR are used to show that a reaction will not occur Predict if the reaction will occur or not, and if it will what the balanced product will be Single - Replacement Reactions Fe(s) + CuSO4(aq) --->
Br2(l) + MgCl2 (aq) --->
Mg(s) + AlCl3 (aq) ---> FeSO4(aq) + Cu(s)
NR (Br is below Mg)
Al(s) + MgCl2(aq) Exchange of ions between two compounds
also known as an exchange reaction or precipitation reaction Double - Replacement Reactions Follow this series of questions.
When you can answer "yes" to a question, then stop!

1) Does your reaction have oxygen as one of it's reactants and carbon dioxide and water as products? If yes, then it's a combustion reaction
2) Does your reaction have two (or more) chemicals combining to form one chemical? If yes, then it's a synthesis reaction
3) Does your reaction have one large molecule falling apart to make several small ones? If yes, then it's a decomposition reaction
4) Does your reaction have any molecules that contain only one element? If yes, then it's a single displacement reaction
5) If you haven't answered "yes" to any of the questions above, then you've got a double displacement reaction This will hopefully help?! Exchange between positive charged ions (cations) and negatively charged ions (anions):
AB + XY ---> AY + XB
A and B represent the positively charged ions
X and Y represent the negatively charged ions
A positive ion always switches places to bond with negative ion Example: BaCl2(Aq) + Na2SO4(Aq) --> BaSO4(s) + 2NaCl(Aq)
Notice that our positive ions are Sodium (Na) and Barium (Ba)
Our negative ions are Chlorine (Cl) and Sulfate (SO4)
The two positive cations have switched places with the two negative anions All double replacement reactions:
Produce either water, a precipitate or a gas as a product
A precipitate is a solid produced during a chemical reaction in a solution - when a solid is produced in a chemical reaction in a liquid Practice List what type the following reactions are:

1) NaOH + KNO3 --> NaNO3 + KOH

2) CH4 + 2 O2 --> CO2 + 2 H2O

3) 2 Fe + 6 NaBr --> 2 FeBr3 + 6 Na

4) CaSO4 + Mg(OH)2 --> Ca(OH)2 + MgSO4

5) Pb + O2 --> PbO2

6) Na2CO3 --> Na2O + CO2 1) double displacement
2) combustion
3) single displacement
4) double displacement
5) synthesis
6) decomposition How to Write a Double Replacement Reaction The steps 1. Write the components of the reactant in skeleton equation
Al(NO3)3 + H2SO4
2. Identify the cation (+) and anions (-) in each compound
Al(NO3)3 has Al (3+) and NO3 (1-)
H2SO4 has H (+) and SO4 (2-)
3. Pair up each cation with an anion in the other compound
Al with SO4
H with NO3
4. Write the formulas using the pairs from the previous step
Al2(SO4)3 and HNO3
5. Write the complete equation out - reactant and product
Al(NO3)3 + H2SO4 ----> Al2(SO4)3 + HNO3
6. Make sure the equation is balanced
2Al (NO3)3 + 3H2SO4 ----> Al2(SO4)3 + 6HNO3 Lets Practice K(CN)(aq) + HBr ---->
Copper (II)Bromide + Aluminum Chloride ----> Answers:
---> HCN + KBr
CuBr2 + AlCl3 ---> AlBr3 + CuCl2
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