Section 1: Evidence for a Chemical Reaction
- Chemical reactions often give a visual signal.
- Ex. Hair changes color when it is bleached.
- Ex. A blue flame appears when natural gas reacts with oxygen.
- Chemical reactions give visual clues:
- a color changes
- a solid forms
- bubbles are produced
- a flame occurs
- HOWEVER. Reactions are not always visible.
- Sometimes the only signal that a reaction is occurring is a change in temperature as heat if produced / absorbed.
Section 2: Chemical Equations
- We represent a chemical reaction by writing a chemical equation in which the chemicals present before the reaction (reactants) are shown the left of an arrow and the chemicals formed by the reaction (products) are shown to the right of an arrow.
- The arrow indicates the direction of the change and is read as "yields" or "produces".
- Reactants > Products
Section 3: Balancing Chemical Equations
How to Write and Balance Equations:
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Summary:
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Examples:
- FeCl3 + KOH → Fe(OH)3 + KCl
Balance chlorine: FeCl3 + KOH → Fe(OH)3 + 3KCl
Balance potassium: FeCl3 + 3KOH → Fe(OH)3 + 3KCl
Balanced equation: FeCl3(aq) + 3KOH(aq) → Fe(OH)3(s) + 3KCl(aq) - P4O10 + H2O → H3PO4
Balance phosphorus: P4O10 + H2O → 4H3PO4
Balance hydrogen: P4O10 + 6H2O → 4H3PO4
Balanced equation: P4O10(s) + 6H2O(l) → 4H3PO4(aq)