Chemical Reactions Alter Arrangements of Atoms Section 7-1

Atoms interact in chemical reactions. How does a chemical reaction produce a new substance? By changing the way the atoms are arranged. Bonds between atoms are broken and new bonds form. This happens when particles collide.

Physical Changes A physical change is a change in the state (phase) of a substance. What are the three states of matter? Solid, Liquid, and Gas

What happens when a substance undergoes a physical change? Its physical form changes but the substance itself remains unchanged.

What is electrolysis? The process by which water breaks down into hydrogen and oxygen molecules. How can you tell that splitting water to form hydrogen and oxygen is a chemical change? How does this change occur? New substances form with new and different properties. Bonds break and new bonds form.

Chemical Vs. Physical How is a chemical change different from a physical change? During a chemical change the original substances turn into different substances. In a physical change, the original substances still keep their original properties.

Reactants and Products Reactant – the substances present at the beginning of a chemical reaction. Product – the substances formed by the chemical reaction.

Reactants and Products What must happen for a reactant to be changed into a product? The bonds in the reactants must be broken and new bonds must form in the products.

Evidence of a Chemical Reaction Color Change Formation of a precipitate Formation of a gas Temperature change –gives off heat or light

Evidence of a Chemical Reaction How can temperature change in a chemical reaction be observed without using a thermometer? Seeing a flame or feeling warmth (production of heat or light).

3 Types of Chemical Reactions: Synthesis Reaction – a new compound is formed by the combination of simpler reactants Decomposition Reaction – a reactant breaks down into simpler products Combustion Reaction – one reactant is always oxygen, the other often contains carbon or hydrogen.

3 Types of Chemical Reactions: Synthesis Reaction means “making a substance from simpler substances.” Decomposition Reaction means “separation into parts.” Combustion Reaction is the process of burning with oxygen.

3 Types of Chemical Reactions: How are synthesis reactions different from decomposition reactions? Synthesis reactions make more complex products from simpler reactants. Decomposition reactions break down complex reactants into simpler products.

Synthesis Reaction Two or more simple substances combine to form a new, more complex substance. A + B  C 2 reactants 1 product

Synthesis Reaction Example 2Na +Cl2  2NaCl

Decomposition Reaction A complex substance breaks down into two or more simpler substances. C  A + B 1 reactant 2 products

Decomposition Reaction Example C12H22O11  11 H2O +12C Sugar heat Water Carbon H2CO3  H2O + CO2 Carbonic Acid

The rates of chemical reactions can vary. The three physical factors that affect the rate of a chemical reaction are concentration, surface area, and temperature. The chemical factor that affects reaction rate is called a catalyst

Concentration Concentration measures the number of particles present in a certain volume.

Surface Area Why does a reaction proceed faster when the reactants have a greater surface area? More particles are available to collide and react when the surface area is greater.

Temperature The rate of a reaction can be increased by making the particles move faster. Why are collisions between particles important in chemical reactions? Particles of reactants must collide to react.

Catalysts Definition – a substance that increases the rate of a chemical reaction but is not itself consumed by the reaction Characteristics – remains unchanged Example – Enzymes Non-Example – something that slows the reaction or is changed/consumed by the reaction.

Review How might the chewing of food be related to the rate of a chemical reaction, digestion, that occurs in your body? Explain your answer.

Review How might the chewing of food be related to the rate of a chemical reaction, digestion, that occurs in your body? Explain your answer. Chewing increases the surface area of food which speeds up chemical reactions involved in digestion.

The mass of reactants and products are equal. Ch. 7.2 Notes The mass of reactants and products are equal.

The mass of reactants and products are equal. Careful observations led to the discovery of the law of conservation of mass.

Describe the Law of Conservation of Mass Lavoisier found that atoms can never be created or destroyed in a chemical reaction. Most reactions that seem to gain or lose mass actually involve reactions with gases. The mass of the reactants is always equal to the mass of the products.

Why is mass conserved in a chemical reaction? Atoms are not created or destroyed in a chemical reaction.

200g of reactant A and 100g of reactant b are combined to form a new substance- Product C. What is the mass of product C? 350 g

How did Lavoisier investigate the conservation of mass How did Lavoisier investigate the conservation of mass? Why did he use sealed containers in his experiments? By carefully observing and measuring all of the reactants and products in chemical reactions, he found that the total masses were always equal He could measure the total mass of the air and the mercury instead of just the mass of the mercury.

Chemical reactions can be described by ____________ __________________.

How does a chemical equation show the conservation of mass? By showing on each side of the equation the same number of atoms of each element involved in the reaction.

In order to write a chemical equation, what information do you need to know? The reactants and products in the reaction The atomic symbols and chemical formulas of the reactants and products in the reaction The direction of the reaction

C + O2  CO2 Reactants Product Write the chemical equation represented by the following sentence. Carbon (C) reacts with oxygen (O2) to yield carbon dioxide (CO2). Label the reactants and products. C + O2  CO2 Reactants Product

Chemical equations must be balanced.

Is the equation below balanced? Why or Why not? H2 + O2  H2O No, the equation has only one oxygen atom on the right side and two oxygen atoms on the left side.

4 steps to balancing equations: Write chem. Equation with correct symbols & formulas Write the number of atoms of each element on each side of the equation. (Write the # very small under the symbol) Balance atoms by using coefficients(large # in front of symbol or formula, then change # under symbol) Check by counting the numbers of atoms of each element.

4 steps to balancing equations: Cont. *DO NOT CHANGE SUBSCRIPTS!!!!* *Oxidation numbers in a compound must add up to equal zero. *When adding a subscript to a polyatomic ion, you must put the polyatomic ion in parenthesis first. *The total number of atoms on both sides of the equation must be the same [(Law of conservation of mass: mass remains constant in a chemical reaction) (mass is neither created nor destroyed; it only changes form.]

Using Coefficients to Balance Equations Why can you change coefficients to balance an equation but not subscripts? Subscripts are part of a chemical formula, while coefficients just show the number of molecules involved.

Chemical reactions involve energy changes. Ch. 7.3 Notes Chemical reactions involve energy changes.

I. Chemical reactions release or absorb energy.

The amount of energy in a chemical bond between atoms Bond energy The amount of energy in a chemical bond between atoms

For each reaction, write absorbed or released. a. Breaking bonds: energy is absorbed b. Forming bonds; energy is released c. Endothermic: energy is absorbed d. Exothermic: energy is released

Exothermic Reactions Definition: a chemical reaction that releases energy Characteristics: often causes an increase in temperature; energy is released as heat Example: rocket blasting off Nonexample: photosynthesis

Endothermic Reaction Definition: a reaction in which energy is absorbed Characteristics: energy must be added to the reaction, often causes a drop in temperature Example: photosynthesis Nonexample: rocket blasting off

II. Exothermic reactions release energy.

Describe exothermic reactions Exothermic reactions release energy Bond energies of the reactants are less than the bond energies of the products Some reactions produce a lot of heat All common combustion reactions are exothermic Exothermic chemical reactions also occur in living things.

The picture below shows a common chemical reaction. Is the reaction exothermic or endothermic? How do you know?

The picture below shows a common chemical reaction. Exothermic; It is releasing energy (heat)

Describe the bond energies in an exothermic reaction. The bond energies of the reactants are less than the bond energies of the products.

III. Endothermic reactions absorb energy

Describe Endothermic reactions Endothermic reactions absorb energy. The energy difference is usually absorbed as heat; Photosynthesis is probably the most important series of endothermic reactions

IV. Exothermic and endothermic reactions work together to supply energy.

How does the combination of endothermic and exothermic reactions help us use energy from the sun? Energy from the Sun is stored by endothermic reactions (photosynthesis) and released by exothermic reactions (combustion)

Complete 7.3 Review Questions Pg. 219: questions 1-6

Answers to review questions: Exothermic reactions; products have higher bond energy than reactants and energy is released; endothermic reactions: reactants have higher bond energy than products and energy is absorbed. Exothermic; energy is released as heat and light

Answers to review questions: Endothermic: energy from light is absorbed in order for the reaction to occur. They are approximately opposite reactions in terms of energy as well as reactants and products

Answers to review questions: Fossil fuels contain the carbon stored in plants by photosynthesis hundreds of millions of years ago. So, energy in fossil fuels started as energy from the Sun. Bond energy is greater in the reactants; when the reaction is reversed, the bond energy is greater in the products and energy is released.