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# 3.5 chemical equations

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### 3.5 chemical equations

1. 1. Chemical Equations Section 3.5
2. 2. Chemical Equations • A chemical equation is a way of representing a chemical change. • It shows reactants and products. • To balance an equation means to change the numbers of each molecule involved, so that the same number of atoms of each element appear on the reactants side and on the products side. • Chemical equations balance on an atomic level, not molecular. • You cannot change the formula of a substance, i.e. if the equation has NH3 you cannot change this you can only put a number in front of it, 2NH3, increasing the number of N’s and the number of H’s. • Never change the subscripts (small numbers). • It is possible to write balanced equations for lots of reactions but that does not mean that the reaction actually takes place.
3. 3. Formation of Water (Oxygen does not exist as single atoms) (This reaction does not take place)
4. 4. Balancing an Equation Methane reacts with oxygen, forming carbon dioxide and water vapour only. Write a balanced chemical equation for this reaction. Solution CH4 O2 CO2 H2 O Examine each element in turn, to have the same no. of atoms of each on either side. (Leave the O until last as it appears the most often) Examining C: 1 on LHS and 1 on RHS, leave as they are. Examining H: 4 on LHS and 2 on RHS, put a 2 in front of the H2 O, on the RHS. CH4 O2 CO2 2H2 O Examining O: 2 on LHS and 4 on RHS, put a 2 in front of the O2 , on the LHS. CH4 2O2 CO2 2H2 O
5. 5. Balance the following 1. Fe HCl FeCl2 H2 Fe 2HCl 2. O2 H2 3. H2 1 2 FeCl2 H2 H2 O O2 H2 O O2 C 4 H10 4. C 4 H10 13 2 Al O2 2Al 3 2 CO2 O2 4CO2 Al2 O3 O2 H2 O Al2 O3 5H2 O
6. 6. Balancing Redox Equations • In working out what is oxidised and what is reduced in a reaction, it is important to remember that oxidation numbers are not a charge. • Write the oxidation numbers below each atom to which it applies, as shown in the examples. H2 O 1 2 Cr2 O7 2 6 2 OH 2 1 • Oxidation is an increase in oxidation number. • Reduction is a decrease in oxidation number.
7. 7. Balance the following redox equation Cr2 O72 1. Fe2 H Cr 3 Fe3 H2O Cr2 O7 2 Fe2 H Cr 3 Fe3 3 3 H2 O 6 2. 2 2 Cr2 6e 2Cr 3 Fe2 Fe3 6e 6e 2Cr 3 6Fe3 3 e Cr2 2 Cr2 6Fe2 e 3 Balance the half equations. 2Cr 3 4. Cr2 O72 6Fe2 H Cr2 O72 6Fe2 14H Write down half equations of what is oxidised and reduced. Attach subscripts to atoms oxidised and reduced and balance the half equations. 3 6 3 6Fe2 5. 1 Fe3 6 2 1 Cr 3 Cr 3e Fe2 3. 2 Assign oxidation numbers to all the atoms in the equation. 6Fe3 2Cr 3 6Fe3 2Cr 3 6Fe3 H2 O 7H2 O Attach species that were attached to the oxidised and reduced atoms. Include all the original species and complete the balancing by inspection.
8. 8. Calculations using Chemical Equations • A balanced equation for a chemical reaction gives the relative amounts of each reactant and each product involved in the reaction. • If the amount of one substance is known, based on the molar ratios in the equation, the amounts (masses, particles or volume)of other substances can be calculated. • Make sure to always work in moles.
9. 9. A dry - cleaning fluid composed of carbon and chlorine was found to have the composition 14.5% C, 85.5% Cl. What is the empirical formula of this compound? Element Percentage C 14.5% Cl 85.5% CCl2 Percentage Ar 14.5 1.208 12 85.5 2.408 35.5 Simplest Ratio 1 2