Chemical Reactions & Equations. +. Evidence of Chemical Reactions. Release of a gas Example: bubbles formed when magnesium and hydrochloric acid were mixed Color change Example: color went from clear to yellowish orange when potassium iodide was added to hydrogen peroxide
NH4NO3(s) + H2O(l) + energy NH4NO3(aq)Endothermic or Exothermic?
Since energy is a product, this reaction is exothermic.
This reaction will generate enough heat to fry an egg!
Since energy is a reactant, this reaction is endothermic.
This is the cooling reaction of a cold pack.
“plus” or “and” indicates multiple reactants or products
“yields” or “produces” separates reactants & products, arrow points in the direction of the reaction
indicate phase of a reactant or product
“aqueous” dissolved in water (not the same as liquid (l))
indicates reaction is reversible
over the arrow, indicates heating is necessary
“no reaction”Symbols in Equations
Balance the numbers of atoms of each element on both sides of the equation by determining the coefficients of the compounds.Balancing Equations
DO NOT CHANGE SUBSCRIPTS!
2H2 (g) + O2 (g) 2H2O (l)
There are now 4 H’s and 2 O’s on each side.An Example: Forming Water
H2 (g) + O2 (g) H2O (l)
On left: 2 H and 2 O On right: 2 H and 1 OTherefore, not balanced
We could use 1/2 O2 on the left but that is not done, we always want whole number coefficients.
subscriptRules for Balancing Equations
Tips: for reactions involving oxygen, balance it last
polyatomic ions can be balanced as a unit
C2H4 + O22CO2 + 2H2O
C2H4 + 3O2 2CO2 + 2H2OBalancing Example
Write the equation for burning ethylene (C2H4) to produce carbon dioxide and water.
C and H both appear once on each side of the equation whereas O appears in both compounds on the right hand side.
Multiply CO2 by 2 to balance C
Multiply H2O by 2 to balance H
There are now 6 O’s on the right so multiply O2 by 3 to finish.
Some reactions fall into multiple categories.
Hydrocarbons (C, H, O) - heat homes, cook food, and power transportation. Always get CO2(g) and H2O(g).
hydrocarbon+ O2 (g)CO2 (g) + H2O(g)
Metals produce metal oxides (like rust and tarnishes).
2Mg(s)+ O2(g) 2MgO(s)Combustion Reactions
A substance reacts with oxygen to create products containing oxygen. Heat may be required to initiate. Exothermic.
(like FeS(s) + H2SO4(aq) --> FeSO4(aq) + H2S(g)