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If you add two substances together sometimes you get a chemical reaction. This is when a new substance is made.
It is very difficult to reverse the reaction and get the original substances back.
Signs that a chemical reaction has taken place include a change in colour, heat being given off or bubbles of gas being made.
Adding acid to alkali
Fireworks Driving a car
Ice cream melting
Boiling the kettle frying an egg
Burning a candle Melting candle wax
Dissolving sugar in tea
Lighting a matchMaking ice cubes
Ironing clothes
Printing Baking a cake Launching a space rocket
Discuss in pairs which of these are chemical reactions
x x
x x
xx
x
The changes that are not chemical changes are called physical changes
4
Objectives
Define and give general equations for synthesis, decomposition, single-replacement, and double-replacement reactions.
Classify a reaction as synthesis, decomposition, single-replacement, single-replacement, double-replacement, or combustion.
5
Objectives List three types of synthesis reactions
and six types of decomposition reactions.
List four types of single-replacement reactions and three types of double-replacement reactions.
Predict the products of single reactions given the reactants.
6
Background Thousands of known chemical reactions
occur in various systems. Memorizing the equations for so many chemical reactions would be difficult. It is more useful and realistic to classify reactions according to various similarities and regularities.
8
Synthesis Reactions In a synthesis reaction, also known as
composition reaction, two or more substances combine to form a new compound.– General equation: A + X AX.
Types of synthesis reactions:– Reactions of elements with oxygen and sulfur.– Reactions of metals with halogens.– Synthesis reaction with oxides.
+Synthesis Reaction
Synthesis reaction – 2 substances combine to form a single product
A + B AB+
2Mg(s) + O2(g) 2MgO(s)Magnesium and oxygen combine to form magnesium
oxide.
2H2(g)+ O2(g) 2H2O(l)Hydrogen and oxygen combine to form dihydrogen
monoxide
Look at the reactants represented below, which reaction involves elements as reactants? Which reaction involves compounds as reactants?
Synthesis reaction Decomposition reaction
A synthesis reaction involves the combination smaller molecules
A decomposition reaction involves the breaking apart of larger molecules
LETS US LOOK AT TWO OF THE REACTION TYPES:SYNTHESIS AND DECOMPOSITION
Recognizing the types of reactants is key to identifying the reaction type
Synthesis Reactions
• Involve the combination of smaller atoms and /or molecules into larger molecules.
• They are also called combination reactions
• General formula
• A + B AB
• If you see two elements as reactants, you know the reaction has to be a sysnthesis reaction
• Synthesis reactions can also involve combinations of small molecules.
• For example, when ammonia and hydrogen chloride vapours combine, they form a white smoke as solid particles of ammonium chloride are formed.
Hydrogen chloride + ammonia ammonium chlorideHCl + NH3 NH4Cl
TYPICAL EXAMPLE , RAINWATER
Similarly, combination of water and carbon dioxide molecules Synthesis reaction
CO2 + H2O H2CO3(aq)
15
Examples of Reaction of Elements with Oxygen and Sulfur:– Forming Oxides and sulfides:
2Mg(s) + O2(g) 2MgO(s)
16Rb(s) + S8(s) 8Rb2S(s)
8Ba(s) + S8(s) 8BaS(s)
S8(s) + 8O2(g) 8SO2(g)
C(s) + O2(g) CO2(g)
2C(s) + O2(g) CO(g)
2H2(g) + O2(g) 2H2O(l)
17
Reactions of Metals with Halogens (most metals react with the halogens (group 17) to form either ionic or covalent compounds.– Group 1 metals with Group 17 elements
2Na(s) + Cl2(g) 2NaCl(s)
2K(s) + I2(g) 2KI(s)
– Group 2 metals with Group 17 elements Mg(s) + F2(g) MgF2(s)
Sr(s) + Br2(l) SrBr2(s)
18
– Fluorine is so reactive that it combines with almost all metals:
2Na(s) + F2(g) 2NaF(s)
2Co(s) + F2(g) 2CoF3(s)
U(s) +3F2(g) UF6(g) Practical application with fluorine:
– Sodium fluorine added to municipal water supplies.– Cobalt(III) fluoride is a strong fluorinating agent.– Uranium(VI) fluoride is the first step in the production
of uranium for use in nuclear power plants.
19
Synthesis Reactions with Oxides– Oxides of active metals react with water to
produce metal hydroxides - example CaO(s) + H2O(l) Ca(OH)2(s)
– Many oxides of nonmetals (upper right portion of the periodic table) react with water to produce oxyacids - example
SO2(g) + H2O(l) H2SO3(aq) this reacts with oxygen to produce sulfuric acid
2H2SO3(aq) + O2(g) 2H2SO4(aq)
– Certain metal oxides and nonmetal oxides react with each other in synthesis reaction to form salts. CaO(s) + SO2(g) CaSO3(s)
20
Decomposition Reactions
In a decomposition reaction, a single compound undergoes a reaction the produces two or more products.– General equation AX A + X
Decomposition Reactions
• It involve the splitting of a large molecule into elements or smaller molecules.
• General formulaAB A + B
• Example, electrolysis of water uses electricity to split water molecules into their elements
water hydrogen + oxygenH2O H2 + O2
2H2O 2H2 + O2
+Decomposition ReactionDecomposition reaction – A single compound breaks
down into 2 or more products.
AB A + B +
2H2O2(aq) O2(g)+ 2H2O(l)Hydrogen peroxide decomposes into oxygen gas and dihydrogen
monoxide.
2NaCl(s) 2Na(s) + Cl2(g)Sodium chloride decomposes into sodium and chlorine gas.
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Types of decomposition reactions– Decomposition of Binary Compounds– Decomposition of Metal Carbonates– Decomposition of Metal Hydroxides– Decomposition of Metal Chlorates– Decomposition of Acids
Decomposition of Binary Compounds electricity
– 2H2O(l) 2H2(g) + O2(g) (called electrolysis)
2HgO(s) 2Hg(l) + O2(g)
24
Decomposition of Metal Carbonates– CaCO3(s) CaO(s) + CO2(g)
Decomposition of Metal Hydroxides– Ca(OH)2(s) CaO(s) + H2O(g)
Decomposition of Metal Chlorates– 2KClO3(s) 2KCl(s) + 3O2(g)
Decomposition of Acids– H2CO3(aq) CO2(g) + H2O(l)
– H2SO4(aq) SO3(g) + H2O(l)
25
Single-Replacement Reactions
In a single-replacement reaction, also know as a displacement reaction, one element replaces a similar element in a compound.– A + BX AX + B or– Y + BX BY + X
Single Displacement Reactions• Are chemical changes
that involve an element and a compound as reactants.
• One element displaces or replaces another element from the compound.
• Example, when magnesium ribbon is placed in a solution of silver nitrate
Mg + AgNo3 Ag + Mg(NO3)2
Mg + 2AgNO3 2Ag + Mg(NO3)2
+Single Replacement Reaction
Single Replacement reaction – A single element takes the place of another element in a compound.
A + BC C + AB
+ +
AgNO3(aq) + Cu(s) Ag(s) + CuNO3(aq)Copper replaces silver in silver nitrate to copper (I) nitrate and silver.
Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)Zinc replaces hydrogen in hydrogen chloride to yield hydrogen gas and zinc
chloride.
28
Types of single-replacement reactions– Replacement of a Metal in a Compound by
Another Metal– Replacement of Hydrogen in Water by a
Metal– Replacement of Hydrogen in Acid by a Metal– Replacement of Halogens
Replacement of a Metal in a Compound by Another Metal. – A more active metal will replace a less active metal.– 2Al(s) + 3Pb(NO3)2(aq) 3Pb(s) + 2Al(NO3)3(aq)
29
Replacement of Hydrogen in Water by a Metal – The most-active metals such as those in Group 1,
react vigorously with water to produce metal hydroxides and hydrogen.
– example 2Na(s) + 2H2O(l) 2NaOH(aq) + 4H2(g)
– example 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) (Less-active metals react with steam to form a metal oxide and hydrogen gas)
Replacement of Hydrogen in an Acid by a Metal
– The more-active metals react with certain acidic
solutions replacing the hydrogen in the acid. – example Mg(s) + 2HCl(aq) H2(g) + MgCl2(ag)
30
Replacement of Halogens – One halogen replaces another halogen in a
compound. Fluorine is the most-active halogen and can replace any other halogen in their compounds.
– Cl2(g) + 2KBr(aq) 2KCl(aq) + Br2(l)
– F2(g) + 2NaCl(aq) 2NaF(aq) + Cl2(s)
31
Double-Replacement Reactions
In double-replacement reactions, the ions of two compounds exchange places in an aqueous solution to form two new compounds.– General equation AX + BY AY + BX
Formation of a Precipitate– The formation of a precipitate occurs when the
cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound.
– 2KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2KNO3(aq)
32
Formation of a Gas– In some double-replacement reactions, one of the
products in an insoluble gas that bubbles out of the mixture.
– FeS(s) + 2HCl(aq) H2S(g) + FeCl2(aq)
Formation of Water– In some double-replacement reactions, a very
stable molecular compound, such as water, is one of the products.
– HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
+Double Replacement Reaction
Double Replacement reaction – Elements in 2 compounds switch places to form 2 new compounds.
AB + CD AD + CB+ +
HCl(aq) + NaOH(aq) HOH(l) + NaCl(aq)Hydroxide and chlorine switch places to hydrogen hydroxide (water) and sodium
chloride
Na2S(aq) + Zn(NO3)2(aq) 2NaNO3(aq) + ZnS (s)Sulfur and nitrate switch places to form sodium nitrate and zinc sulfide.
34
Combustion Reactions In a combustion reaction, a substance
combines with oxygen, releasing a large amount of energy in the form of light and heat.– 2H2(g) + O2(g) 2H2O(g)
– C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
+Combustion Reaction
Combustion reaction – A hydrocarbon (H and C) burn in oxygen to produce water and carbon dioxide. Heat is given off as energy.
CxHx + O2 H2O + CO2 + heatCH4(g) + 2O2(g) 2H2O (l) + CO2(g)
Methane burns in oxygen to produce water and carbon dioxide.
C12H22O11(s) + 12O2(g) 11H2O(l) + 12CO2(g)Sucrose burns in oxygen to produce water and carbon dioxide.
36
Section Review List five types of chemical reactions. Complete and balance each of the following
reactions identified by type:– synthesis: ______ Li2O
– decomposition: Mg(ClO3)2 ______
– single-replacement: Na + H2O ______
– double-replacement: HNO3 +Ca(OH)2
– combustion: C5H12 + O2 ______