Describing Chemical Reactions Chemical reaction process by which

Describing Chemical Reactions

• Chemical reaction – process by which one or more substances (reactants) are changed into one or more different substances (products). • Chemical equation – symbols and formulas that identify relative molecular or molar amounts of reactants and products.

• To know for sure if a chemical reaction has taken place, you need proof that one or more substances have undergone a change in identity. That requires chemical analysis. • Some easily observed changes are used as indicators that a chemical reaction has taken place. – Evolution of light/heat – Gas production – Precipitate formation – Color change

• Chemical equations have to contain correct chemical formulas for all reactants and products – So you still have to know all your ions and the rules for writing correct formulas • The law of conservation of mass must be satisfied – The same number of atoms of each element must appear on each side of the equation – To balance an equation, you adjust coefficients as necessary

• Word Equation – has only qualitative meaning • Formula Equation – replaces words with correct formulas. Also uses other symbols which give more information about the reaction • Chemical Equation – a balanced formula equation.

• Hydrogen gas reacts with oxygen gas to form water vapor. • H 2 (g) + O 2 (g) H 2 O (g) • 2 H 2 (g) + O 2 (g) 2 H 2 O (g)

• Properly written chemical equations often include a variety of symbols that give more information about the state of each reactant and product as well as conditions under which the reaction takes place. – The conditions are indicated by placing information above or below the reaction arrow. – Symbols that you should know and use are in Table 2 on p. 266

• In many reactions, as soon as the products begin to form, they immediately begin to react with each other and re-form the reactants. • These are called reversible reactions. They are indicated with two arrows pointing in opposite directions. • These reactions tend to move in one direction more than the other, and eventually reach an equilibrium.

• We will use chemical equations to do quantitative chemical work. • If we know the amount (in mass or in moles) or any one reactant or product, we can determine the amount of all other reactants and products. • The coefficients provide relative amounts of each reactant and product in either numbers of particles or numbers of moles.

• To obtain the proper coefficients, the equation must be balanced – making sure that the same number of atoms appear on each side of the equation. • Balance atoms one at a time, unless a polyatomic ion appears on both sides of the equation – balance them as single units. • Balance atoms that appear in only one compound on each side of the equation first. • Usually balance H and O last • Always double check when you think you’re done balancing.

• The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write a balanced chemical equation for the reaction.