Lecture5sf

Lecture5sf - Writing Chemical Equations Magnesium burns in...

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Writing Chemical Equations Magnesium burns in air. Burning is a reaction with oxygen, forming an oxide. Magnesium + oxygen → Magnesium oxide Mg + O 2 → MgO (unbalanced) 2Mg + O 2 → 2MgO (balanced Balancing uses coefficients to change the amounts of different substances reacting. The purpose of balancing is to conserve atoms. In a chemical reaction, atoms are rearranged, but not created or destroyed. Butane, C 4 H 10 , undergoes combustion. When hydrocarbons burn, they react with oxygen, forming carbon dioxide and water. C 4 H 10 + O 2 → CO 2 + H 2 O (unbalanced) 2 C 4 H 10 + 13O 2 → 8CO 2 + 10H 2 O (balanced) Ammonium dichromate decomposes to yield chromium (III) oxide, nitrogen and water (NH 4 ) 2 Cr 2 O 7 → Cr 2 O 3 + N 2 + H 2 O (unbalanced) (NH 4 ) 2 Cr 2 O 7 → Cr 2 O 3 + N 2 + 4H 2 O (balanced)
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Stoichiometry Stoichiometry deals with calculations involving quantities of substances reacting and forming in a chemical reaction. The underlying principle of stoichiometry is mole ratio , which relates amounts of reactants and products in a balanced chemical equation. Consider the equation for the combustion of magnesium: 2Mg + O 2 → 2MgO The coefficients are 2, 1, 2 (the one in front of O 2 is understood). This means that 2 Mg atoms react with 1 O 2 molecule to form 2 MgO molecules. Or, in terms of moles, 2 mol of Mg react with 1 mol of O 2 to form 2 mol of MgO. But the equation can also be balanced with coefficients 6,3,6 12,6,12, 50,25,50 Indeed, any 2:1:2 ratio balances the equation. We should consider the coefficients as ratios of amounts (moles). If we had 0.80 mol Mg react, then 0.40 mol O 2 react, forming 0.80 mol MgO (keeping 2:1:2 ratio). If we had 1.80 mol O 2 react, then 3.60 mol Mg would be needed, forming 3.60 mol MgO (keeping 2:1:2 ratio). The mole ratio relates amounts of reactants and products in the equation. A general approach to solving any stoichiometry problem: 1. Write a balanced equation for the reaction. 2. Take what is known and convert to moles. 3. Use mole ratio in the balanced equation to calculate moles of unknown. 4. Convert moles of unknown into whatever the problem requires.
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A sample of 10.0 g of magnesium reacts with oxygen, forming magnesium oxide. What mass of oxygen reacts; what mass of magnesium oxide forms?
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Lecture5sf - Writing Chemical Equations Magnesium burns in...

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