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Chemical Equations

Chemical Equations. Chapter 9-Section 2. Review Concepts: Chemical Formulas. A chemical formula is a combination of chemical symbols and numbers to represent a substance.

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Chemical Equations

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  1. Chemical Equations Chapter 9-Section 2

  2. Review Concepts: Chemical Formulas • A chemical formulais a combination of chemical symbols and numbers to represent a substance. • A chemical formula shows how many atoms of each kind of element are present in a molecule – the number is called the subscript.

  3. Chemical Equations • A chemical equation uses chemical formulas, chemical symbols, and coefficients to describe a reaction. • A coefficientis a number that is placed in front of a chemical symbol or formula & shows the number of each molecule needed in the reaction Ex: 2 H2 + O2 2 H2O • The starting materials in a chemical reaction are reactants. The substances formed from a reaction are products. Ex: 2Mg + O2 (reactants)  2MgO (product)

  4. Balancing Chemical Equations • In a chemical equation, the total number of atoms of each element in the reactants must equal the total number of atoms of that element in the products. This process is called balancing the equation. • WHY?? • The law of conservation of mass states that mass cannot be created or destroyed in chemical and physical changes. • This law means that the total mass of the reactants is the same as the total mass of the products.

  5. Balancing Chemical Equations • All atoms must be counted. • To count all the atoms in an equation, multiply the subscript of each element in the formula by the formula’s coefficient. • 2 Na2HCO3 has four Na atoms, two C atoms, two H atoms, & six O atoms • Coefficients are used to balance an equation. • Written before a molecule or atom – 2H2O • Shows how many sets of that matter - • Never change the subscripts!!!!!!! • Changing the subscripts in the formula of a compound changes the compound.

  6. Balancing strategy • List the chemical symbol of each of the elements present in the reactants underneath. Copy that exact list under the product side. • Count all of the atoms of each element on both sides of the equation (tally). • Is the equation balanced? If not… • Look at the most complex molecule in the equation (the one with the most atoms of the most different elements). Do you need more of them? Yes? Add a coefficient. • You need more if: • the most complex molecule has less atoms of an element than appears on the other side of the equation and/or • the most complex molecule has an odd number of atoms of an element where there is an even number of atoms of that element on the other side. • Balance all other molecules to the most complex molecule going from the next most complex to the very least (one atom of one element; “Mg”) by adding coefficients.

  7. Balancing Equations Practice (yellow = coefficients), (green = subscripts) • H2 + O2 H2O 2H2 + O2  2H2O • N2 + H2  NH3 • K + Cl2  KCl N2 + 3 H2 2NH3 2K + Cl2  2KCl

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