Presentation on theme: "Writing Chemical Reactions. Chemical reactions, or chemical changes, happen when the atoms in one or more chemicals split up and join together in new."— Presentation transcript:
Writing Chemical Reactions
Chemical reactions, or chemical changes, happen when the atoms in one or more chemicals split up and join together in new ways. For example, hydrogen can react with oxygen : hydrogen + oxygen water Before the reaction the oxygen atoms go round in pairs, and the hydrogen atoms go round in pairs. When they react we get two new molecules, of the compound water. Each of these molecules has three atoms, two hydrogen atoms joined to one oxygen atom. We can write this as H 2 O.
Many chemical reactions have defining characteristics which allow them to be classified as to type.
The five types of chemical reactions in this unit are: Combination Decomposition Single Replacement Double Replacement Combustion The reaction of sugar with concentrated sulphuric acid.
Two or more substances combine to form one substance. The general form is A + X AX Example: Magnesium + oxygen magnesium oxide 2Mg + O 2 2MgO
Combination reactions may also be called composition or synthesis reactions. Some types of combination reactions: Combination of elements K + Cl 2 One product will be formed
K + Cl 2 Write the ions: K + Cl - Balance the charges: KCl Balance the equation: 2K + Cl 2 2KCl
Some types of combination reactions: Oxide + water Nonmetal oxide + water acid SO 2 + H 2 O H 2 SO 3 Metal oxide + water base BaO + H 2 O Ba(OH) 2
Some types of combination reactions: Metal oxides + nonmetal oxides Na 2 O + CO 2 Na 2 CO 3 CaO + SO 2 CaSO 3
One substance reacts to form two or more substances. The general form is AX A + X Example: Water can be decomposed by electrolysis. 2H 2 O 2H 2 + O 2
Types of Decomposition Reactions: Decomposition of carbonates When heated, some carbonates break down to form an oxide and carbon dioxide. CaCO 3 CaO + CO 2 H 2 CO 3 H 2 O + CO 2
Types of decomposition reactions: Some metal hydroxides decompose into oxides and water when heated. Ca(OH) 2 CaO + H 2 O Note that this is the reverse of a similar combination reaction.
Types of decomposition reactions: Metal chlorates decompose into chlorides and oxygen when heated. 2KClO 3 2KCl + 3O 2 Zn(ClO 3 ) 2 ZnCl 2 + 3O 2 Some of these reactions are used in explosives.
Some substances can easily decompose: Ammonium hydroxide is actually ammonia gas dissolved in water. NH 4 OH NH 3 + H 2 O Some acids decompose into water and an oxide. H 2 SO 3 H 2 O + SO 2
Some decomposition reactions are difficult to predict. The decomposition of nitrogen triiodide, NI 3, is an example of an interesting decomposition reaction.
A metal will replace a metal ion in a compound. The general form is A + BX AX + B A nonmetal will replace a nonmetal ion in a compound. The general form is Y + BX BY + X
Examples: Ni + AgNO 3 Nickel replaces the metallic ion Ag +. The silver becomes free silver and the nickel becomes the nickel(II) ion. Ni + AgNO 3 Ag + Ni(NO 3 ) 2 Balance the equation: Ni + 2AgNO 3 2Ag + Ni(NO 3 )
Not all single replacement reactions that can be written actually happen. The metal must be more active than the metal ion. Aluminum is more active than iron in Al + Fe 2 O 3 in the following reaction:
Al + Fe 2 O 3 Aluminum will replace iron(III) as was seen in the video. Iron(III) becomes Fe and aluminum metal becomes Al 3+. 2Al + Fe 2 O 3 2Fe + Al 2 O 3
An active nonmetal can replace a less active nonmetal. The halogen (F 2, Cl 2, Br 2, I 2 ) reactions are good examples. F 2 is the most active and I 2 is the least. Cl 2 +2 NaI 2 NaCl + I 2
Ions of two compounds exchange places with each other. The general form is AX + BY AY + BX Metathesis is an alternate name for double replacement reactions.
NaOH + CuSO 4 The Na + and Cu 2+ switch places. Na + combines with SO 4 2- to form Na 2 SO 4. Cu 2+ combines with OH - to form Cu(OH) 2 NaOH + CuSO 4 Na 2 SO 4 + Cu(OH) 2 2NaOH + CuSO 4 Na 2 SO 4 + Cu(OH) 2
CuSO 4 + Na 2 CO 3 Cu 2+ combines with CO 3 2- to form CuCO 3. Na + combines with SO 4 2- to form Na 2 SO 4. CuSO 4 + Na 2 CO 3 CuCO 3 + Na 2 SO 4
Na 2 CO 3 + HCl Notice that gas bubbles were produced rather than a precipitate. What was the gas? Write the double replacement reaction first.
Na 2 CO 3 + HCl Na + combines with Cl - to form NaCl. H + combines with CO 3 2- to form H 2 CO 3. Na 2 CO 3 + 2HCl 2NaCl + H 2 CO 3 H 2 CO 3 breaks up into H 2 O and CO 2.
The gas formed was carbon dioxide. The final balanced reaction is: Na 2 CO 3 + HCl NaCl + H 2 O + CO 2. Balance the equation. Na 2 CO 3 + 2HCl 2NaCl + H 2 O + CO 2
When a substance combines with oxygen, a combustion reaction results. The combustion reaction may also be an example of an earlier type such as 2Mg + O 2 2MgO. The combustion reaction may be burning of a fuel.
Methane, CH 4, is natural gas. When hydrocarbon compounds are burned in oxygen, the products are water and carbon dioxide. CH 4 + O 2 CO 2 + H 2 O CH 4 + 2O 2 CO 2 + 2H 2 O
Combustion reactions involve light and heat energy released. Natural gas, propane, gasoline, etc. are burned to produce heat energy. Most of these organic reactions produce water and carbon dioxide.
Classify each of the following as to type: H 2 + Cl 2 2HCl Combination Ca + 2H 2 O Ca(OH) 2 + H 2 Single replacement
2CO + O 2 2CO 2 Combination and combustion 2KClO 3 2KCl + 3O 2 Decomposition
FeS + 2HCl FeCl 2 + H 2 S Double replacement Zn + HCl ? Single replacement Zn + 2HCl ZnCl 2 + H 2