Chapter 3: Chemical Reactions

Chemical Reactions Chemical change: when substances change into one or more different substances by changing the way atoms are arranged. Bonds between atoms are broken and new bonds are formed.

Physical Changes Physical change: a change that does not change the chemical makeup of an object; changes in state. For example, water can be found as a liquid, solid, or gas, but it always has the same chemical formula: H2O.

Chemical Changes Chemical change: a change that changes the chemical makeup of an object. Water can be separated into H and O by electrolysis – when electric current flows through water. The opposite of electrolysis is used to fuse liquid H and liquid O to launch rocket ships.

Reactants and Products Reactants: substances present at the beginning of a chem. reaction. Products: the substances formed after the chem. reaction takes place. When the bonds break in the reactants, the atoms/elements form new bonds in different ways to form the products.

Evidence of Chemical Reactions 1. Color Change: substances often change color; for example, rust. 2. Formation of a Precipitate: a precipitate is a solid product that forms when chemicals in two liquids react; for example, seashells.

Evidence of Chemical Reactions 3. Formation of a Gas: sometimes produce a gas; for example, antacids mixing with stomach acid. 4. Temperature Change: usually heat is produced in a chemical reaction; sometimes flames can be seen, other times heat can just be felt.

Classifying Chemical Reactions Synthesis: This is when a new compound is formed by the combination of simpler reactants. N2 + O2  2NO2 H2 + O  H2O

Classifying Chemical Reactions Decomposition: This is when a reactant breaks down into simpler products (elements or compounds). 2H2O  2H2 + O2

Classifying Chemical Reactions Combustion: One reactant must always be oxygen, and the other is often carbon and hydrogen. The products are carbon dioxide and water. CH4 (methane) + 2O2  CO2 + 2H2O

Rates of Chemical Reactions Three physical factors: concentration, surface area, and temperature. Chemical factor: catalyst

Concentration Measures the number of particles in a volume. High conc. = large number of particles colliding/reacting. High conc. = quicker reaction

Surface Area The larger the surface area, the more quickly a reaction can take place. If there is a large molecule, it helps to break it apart.

Temperature If particles move more quickly, then the collisions occur more frequently. To make them move more quickly, add more energy, which increases temp. The opposite is true – a reaction can be slowed by removing energy, or decreasing the temperature.

Catalysts Catalyst: substance that increases the rate of a reaction, but is not consumed in the reaction. after the reaction takes place, the catalyst is unchanged. In living organisms, such as humans, catalysts called enzymes speed up and allow certain reactions to take place.

Chapter 3: Chemical Reactions 3.3 Chemical Reactions & Thermal Energy

Bond Energy In order for a chemical reaction to take place, bonds between reactants must be broken and then reformed to create products. When this happens, bond energy is the energy that is released and absorbed. By adding up the energy in the reactants and the products, you can figure out how much bond energy is released/absorbed.

Exothermic vs. Endothermic If more energy is released when the products form than what was needed to break the bonds in the reactants, it is an exothermic reaction. The reaction releases energy and it exits. If more energy is needed to make the products than what was needed to break the bonds in the reactants, it is an endothermic reaction. The reaction absorbs energy and it enters.

Exothermic Often produce an increase in temp because the extra energy leaves as heat. More energy is released than added. Some reactions are highly exothermic: great deal of heat and much higher temps; such as powdered aluminum and iron oxide (welding)

Exothermic All common combustion reactions are exo. Some reactions release extra energy as light instead of heat. Glow sticks Fireflies and other aquatic animals Bombardier beetle – decomposition of hydrogen peroxide into water and oxygen, and a hot, toxic spray.

Endothermic Often produce decrease in temp. Energy must be taken from the surroundings, lowering the temp. Example: decomposition of water by electrolysis; the energy absorbed is in the form of electrical energy.

Endothermic The most important reaction is photosynthesis. Plants absorb sunlight, water, and CO2 and create glucose and oxygen.

Reactions work together Exothermic Reaction Reactants  Products + Energy Endothermic Reaction Reactants + Energy  Products If an exothermic reaction reverses, it becomes endothermic. If an endothermic reaction reverses, it becomes exothermic.

Chapter 3: Chemical Reactions 3.4 Chemical Reactions in Life & Industry

Cellular Respiration Living cells get energy from glucose through cell. resp. This process is basically the opposite of photosynthesis: P: CO2 + H2O + energy  glucose + O2 C.R.: glucose + O2  CO2 + H2O + energy

Cellular Respiration Glucose is stored in your body until you need energy. Then, the molecules and bonds are broken down by several steps. The steps go quickly with the use of enzymes

Technology No chemical reaction is every completely efficient. Some reactants do not complete change to products; sometimes waste products result.

Technology Gasoline combustion: hydrocarbons do not burn completely, and carbon monoxide (CO) is produced. Also, N reacts with O2 and can produce nitric oxide (NO) – creates smog and acid rain.

Technology Scientists worked to reduce the emissions from car exhaust – created the catalytic converter, which causes chemical reaction to remove waste products.

Industry Semiconductor: a material that can control the conduction of electrical signals. the most common one: silicon, which can be found in quartz. When quartz is thoroughly heated with carbon, is produced a silicon substance about 98% pure. This is still not pure enough to use in electronics.

Industry when chemicals were first used in industry, the objects made had to be quite large. Now, scientists use microchip circuits. This circuits use a photoresist, which changes properties when exposed to UV light.