Chemical Reactions Three signs of a chemical reaction – A gas is given off. – A color change. – Energy is released as heat or light. In a reaction the substance that undergoes the chemical change is called the reactant, and the substance that is a result of the chemical change is called the product.

In a chemical reaction chemical bonds in the reactants are broken and the elements are rearranged to form new bonds with different elements to produce the products of the reaction.

Balancing Chemical Equations All chemical reactions are described using chemical equations. These equations use chemical formulas and symbols to show the reactants and products in a chemical reaction. The equations look very similar to a math problem. A + B  AB – A and B are the reactants, the arrow means yield or give, and the AB is the product of the reaction.

Chemical equations need to be balanced to account for the conservation of mass. – This means that there needs to be the same number of each atom on both sides of the equation. Balancing chemical equations is done by changing the coefficient in front of the compound. – This is best done by setting up a chart and balancing the equation one element at a time. – Always go back when you think you are done and check one more time.

Balance the following equations by setting up a chart and balancing one element at a time. – __CH 4 + __O 2 => __CO 2 + __H 2 O – __Mg + __O 2 => __MgO – __CuSO 4 + __Al => __Al 2 (SO 4 ) 3 + __Cu – __H 2 S + __O 2 => __H 2 SO 4 ElementReactantsProductsBalanced

The Mole (mol) is an SI unit that measures the amount of a substance. Avogadro's number equals the number of representative particles in a mole of a substance. – 6.02 x Conversions: The Mole: A Measurement of Matter

How many atoms are in 2.00 moles of Silicon? How many moles is 2.17 x molecules of Br 2 ? Examples

Molar mass is the mass in grams of one mole of a representative particle – The atomic mass in grams is the mass of one mole of an element – For a compound, add the atomic masses in grams of each of the elements together. This is the mass in grams of one mole of a compound. Mass of a Mole of an Element

Write the molar mass of the following: – Na – C – H 2 O – PCl 3 – H 2 O 2 What is the mass of 2.00 moles of sodium hydrogen carbonate? Examples

Find the mass in grams of the following: – 9.45 mol of Al 2 O 3 – 4.52 x mol C 20 H 42 – 2.50 mol of iron (II) hydroxide Mole-Mass Relationship

Find the amount in moles of the following: – 92.2 g of Fe 2 O 3 – 3.70 x g of boron – 75.0 g of dinitrogen trioxide Mole-Mass Relationship

Avogadro’s hypothesis states that equal volumes of gas contain equal numbers of particles. At Standard Temperature and Pressure (STP = 0ºC and kPa or 1 atm): 1 mol or 6.02 x particles = 22.4 L of gas – This is called the molar volume of a gas Mole-Volume Relationship

The Mole Road Map

Reaction types The simplest type of reaction is a synthesis reaction. In a synthesis reaction two substances join to produce a more complex product. – Synthesis reactions follow the general form: A + B => AB – For example 2Na + Cl 2 => 2 NaCl For example 2Na + Cl 2 => 2 NaCl

The opposite of a synthesis reaction is a decomposition reaction, in which a compound breaks down into simpler products. – Decomposition reactions follow the general form: AB => A + B – For example the electrolysis of water breaks water down into it components Hydrogen and Oxygen 2H 2 O => 2H 2 + O 2

Any substance that burns in the presence of oxygen is combustible. A combustion reaction is a reaction in which a compound and O 2 burn. – One example of a combustion reaction is when an organic gas like methane, propane, or natural gas burns in the presence of oxygen 2CH 4 +4O 2 => 2CO 2 + 4H 2 O – Often water is a product of a combustion reaction

In single-displacement reactions elements trade places to form new compounds. This is like when someone new moves in and steals your boyfriend or girlfriend. – Single-displacement reactions follow the general form: XA +B => BA +X. As you can see in the example the B and the X change places, and the B displaces the X. – One example is the reaction between copper (II) chloride and aluminum. 3CuCl 2 + 2Al => 2AlCl 3 + 3Cu Here the aluminum atoms displace the copper atoms in copper (II) chloride.

In the last type of reaction the ions seem to change places. This is called a double- displacement reaction. This is like when two couples trade partners. These compounds are swingers and swap partners. – Double displacement reactions follow the general form: AX + BY => AY + BX. As you can see the A and B swap partners. – Example: Pb(NO 3 ) 2 + K 2 CrO 4 => PbCrO 4 + 2KNO 3 Example Here the lead and potassium swap partners.

Energy Changes in Reactions Often energy must be added to break bonds to produce a chemical reaction. This energy is referred to as the activation energy. Throughout all chemical reactions energy is conserved. Some reactions give off heat to the surroundings, while other reactions take in heat from the surroundings.

Exothermic reactions give off heat. – Think of heat as a product of the reaction. – The heat comes from the energy stored in the chemical bonds. This is called chemical energy. Endothermic reactions absorb heat. – Think of heat as a reactant. – Often the products have a lower temperature than the reactants, because heat is absorbed from the surroundings.

Rates of Change Chemical reactions can occur at different speeds or rates. There are many factors that can affect the reaction rate.

Most reactions go faster at higher temperatures – According to kinetic theory at higher temperatures particles move faster and are more likely to collide with each other. – This means the particles have more of a chance of reacting with each other. – Compare the following reaction of calcium metal with ice water, water at room temperature, and boiling water. Ca + H 2 O => CaO + H 2 – Which reaction goes the fastest?

Larger surface area speeds up a reaction. – The more surface of a compound that is exposed increases the likelihood of that substance reacting. – For example compare burning an iron nail with burning iron powder. Which one has more surface area? Which one reacts the fastest? – Also compare burning an bar of aluminum, aluminum foil, and aluminum powder. Which one has more surface area? Which one reacts the fastest?

Concentrated solutions react faster. – A higher concentration means that there are more particles and they are more likely to interact with each other. – Therefore higher concentrations speed up the rate of the reaction. – Compare the following reaction of zinc with various concentrations of hydrochloric acid. 2HCl + Zn => ZnCl 2 + H 2 At which concentration of HCl does the reaction proceed the fastest?

Reactions are faster at higher pressures. – Pressure is a force that is exerted on the walls of a container by the gas particles in side the container. – Higher pressure means that there are more particles in a container or a higher concentration of particles. – Higher concentrations increase the likelihood of particles reacting and speed up the rate of the reaction. – What are some ways of increasing pressure?

Large, Bulky molecules react slower – The size and shape of a molecule can effect the rate of a reaction – Larger particles move slower than smaller particles according to the kinetic theory of matter. – The slower the particles move the less likely they are to collide with one another and react.

Catalysts change the rates of reactions – Catalysts are added to make a reaction go faster – Catalysts are not one of the reactants they are not used up in the reaction. – Hydrogen peroxide naturally decomposes to produce water and oxygen gas, however this is a very slow process. If you add the catalyst manganese dioxide to the reaction you can get the reaction to speed up. 2H 2 O 2 => 2H 2 O + O 2 Does the catalyst get used up in the reaction? Other catalysts like KI or yeast can be used to speed up this reaction.

– Some Catalyst are used to slow down reactions. These are called inhibitors. – Enzymes are biological catalysts. They are used to speed up reactions just like other inorganic catalysts. Catalase is an enzyme which, acts as a catalyst to help break down hydrogen peroxide into water and oxygen gas.