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Consider a reaction that combines hydrogen gas with oxygen to make water. This reaction occurs in a hydrogen fuel cell and releases a great deal of energy. Many companies are trying to make hydrogen powered cars because the product is water and this would greatly lessen air pollution and carbon dioxide emissions that contribute to climate change. 
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The equation H2 + O2 → H2O is not quite right. If 1mole of oxygen reacts with 1 mole of hydrogen, there would be more than just 1 mole of water molecules at the end of the reaction. There would be 0.5 moles of leftover oxygen molecules along with 1 mole of water molecules. Look closely at the reaction summarized in the table below. Notice how there are 2 oxygen atoms in the reactants but there is only 1 oxygen atom in the products.
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The chemical equation H2 + O2 → H2O is an unbalanced chemical equation. The unbalanced equation identifies reactants and products but it does not correctly account for how much of each are involved. The amount of hydrogen molecules, oxygen molecules, and water molecules must be adjusted until the same type of each atom is equal on both sides of the chemical equation.
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The balanced chemical equation adds a coefficient of 2 to the hydrogen molecule on the reactant side and the water molecule on the product side. The coefficient tells you that there are two moles of hydrogen molecules reacting with one mole of oxygen molecules to produce two moles of water molecules. The chart below shows that the number of atoms of each type are now the same on the reactant and product sides of the equation.
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Chemical equations are balanced by adjusting the coefficients in front of each compound until the total number of each type of atom is the same on both sides of the equation. Only the properly balanced chemical equation tells us the correct relationship between the quantities of reactants and products.
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