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Chemical Reactions

Chemical Reactions. Signs of a Chemical Reaction. Evolution of heat and light Formation of a gas Formation of a precipitate Color change. Symbols in Chemical Reactions. Many symbols are used when writing a chemical reaction. A Balancing Act. Law of Conservation of Mass.

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Chemical Reactions

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  1. Chemical Reactions

  2. Signs of a Chemical Reaction • Evolution of heat and light • Formation of a gas • Formation of a precipitate • Color change

  3. Symbols in Chemical Reactions • Many symbols are used when writing a chemical reaction

  4. A Balancing Act

  5. Law of Conservation of Mass • mass is neither created nor destroyed in a chemical reaction • total mass stays the same • atoms can only rearrange 4 H 2 O 4 H 2 O 36 g 32 g 4 g

  6. Chemical ReactionVocabulary • Chemical Reaction: One or more substances, the reactants change into one or more new substances, the products • Reactant: A substance present at the start of a reaction • Product: A substance produced in a chemical reaction • Chemical Equation: An expression representing a chemical reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas for the products (on the right). • Example: • Reactants Products

  7. Practice: • Write the equation for… • Copper can be produced by heating copper (II) sulfide with oxygen gas. Sulfur dioxide is also produced. • Water is formed by the explosive reaction between hydrogen and oxygen

  8. Answers • CuS + O2→ Cu + SO2 • 2 H2 + O2 → 2 H2O

  9. Meaning of Chemical Formula Chemical Symbol Meaning Composition H2OOne molecule of water: Two H atoms and one O atom 2 H2OTwo molecules of water: Four H atoms and two O atoms H2O2One molecule of hydrogen peroxide: Two H atoms and two O atoms

  10. Meaning of Chemical Formula subscript • When a __________is written after the chemical symbol for an element it indicates the___________________. • When a coefficient is written in front of an atom or molecule it indicates the ____________ of that atom or molecule that is present. • If there are subscripts in that atom/molecule they need to be ____________ by the coefficient. # of that element number multiplied

  11. Unbalanced and Balanced Equations H Cl Cl H H Cl H Cl Cl Cl H Cl H H H2 + Cl2 2 HCl (balanced) H2 + Cl2 HCl (unbalanced) reactants products reactants products H H 2 1 2 2 Cl Cl 2 1 22 Timberlake, Chemistry 7th Edition, page 167

  12. How do we go from Unbalanced to Balanced Equations? MAY NOT • You ____________ change the formula of your atom/ion/molecule! • This means you may not change the ___________ ! • Instead, you may put a coefficient __________ of your atom/ion/molecule to balance the number of atoms after you have ___________ it by the subscript subscript in front multiplied

  13. Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Add coefficients as needed to make #s equal on both sides of the equation. Coefficient  subscript = # of atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

  14. Helpful Tips • Balance one element at a time. • Update ALL atom counts after adding a coefficient. • If an element appears more than once per side, balance it last. • Balance polyatomic ions as single units. • “1 SO4” instead of “1 S” and “4 O”

  15. Balancing Examples P + O2→ P4O10 H2 + O2→ H2O AgNO3 + CuSO4 Ag2SO4 + Cu(NO3)2

  16. Al +CuCl2Cu +AlCl3 Al Cu Cl Balancing Example Aluminum and copper(II)chloride react to form copper and aluminum chloride. 2 3 3 2 1 Al 1 1 Cu 1 2 Cl 3 2  6 2  3  6   3

  17. Balancing Example • Aluminum and copper(II)chloride react to form copper and aluminum chloride. • Step 1. Determine the Correct Formula • Aluminum = Al • Copper(II)chloride = CuCl2 • Copper = Cu • Aluminum chloride = AlCl3 • Step 2. Write the skeleton equation • Al + CuCl2 Cu + AlCl3 • Step 3. Determine the # of atoms in reactants and products • Reactants: 1 Al, 1 Cu, 2 Cl Products: 1 Al, 1 Cu, 3 Cl • Step 4. Balance the elements one at a time by using coefficients. • Start with Cl since it is the only one that isn’t balanced. To make both sides match we use coefficients to make both sides match. When we make that change it then effects the others so we have to use coefficients to balance those as well. • 2Al + 3CuCl2 3Cu + 2AlCl3 • Step 5.Check each atom or polyatomic ion to be sure they are equal on both sides of the equation. • Reactants: 2 Al, 3 Cu, 6 Cl Products: 2 Al, 3 Cu, 6 Cl • Step 6: Make sure all the coefficients are in the lowest possible ratio.

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