Chemical Reactions Ch 2.1 Observing Chemical Change Ch 2.2 Describing Chemical Reactions Table of Contents – Book L

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Properties and Changes of Matter Chemistry- the study of matter and how matter changes Matter- anything that has mass and takes up space Matter can be described in terms of two kinds of properties: 1. Physical properties- a characteristic of a substance that can be observed without changing the substance into another substance. Ex. Melting point, freezing point, color, ability to dissolve in water, conducting heat and electricity

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Properties and Changes of Matter Chemical properties- a characteristic of a substance that describes its ability to change into other substances To observe the chemical properties of a substance, you must change it to another substance. Ex: Burning, tarnishing, rusting

Chemical Reactions - Observing Chemical Change ProperBook L Ch 2.1 pgs(46-53)- Properties and Changes of Matter Matter can undergo both physical change and chemical change.

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Properties and Changes of Matter Physical change - any change that alters the form or appearance of a substance but that does not make the substance into another substance Ex: Squashing a marshmallow, freezing or melting water Still the same substance, but in a changed form

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Properties and Changes of Matter Chemical change (chemical reaction) - a change in matter that produces one or more new substances Ex: Burning of gasoline  car exhaust Burning wood  ashes

Chemical Reactions - Observing Chemical Change Book L Ch 2.1 pgs(46-53)- Properties and Changes of Matter Chemical changes occur when bonds break and new bonds form. As a result, new substances are produced.

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Evidence for Chemical Reactions Chemical reactions involve two main kinds of changes that you can observe 1. Formation of new substances - color changes - a solid may appear - a gas might be produced from solids or liquids - other observable changes Ex. soft dough baking to form bread Precipitate- a solid that forms from solution during a chemical reaction

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Evidence for Chemical Reactions 2. Changes in Energy As matter changes, it can either absorb or release energy. One common indication of this energy transfer is a change in temperature. Endothermic reaction - a reaction in which energy is absorbed Ex: an egg absorbing heat on a frying pan baking soda absorbing heat from vinegar making the solution feel cooler

Chemical Reactions Book L Ch 2.1 pgs(46-53)- Evidence for Chemical Reactions Exothermic reaction- a reaction that releases energy in the form of heat Ex: reaction between fuel and oxygen in an airplane engine releasing energy

Chemical Reactions Energy in Chemical Changes A student places two substances in a flask and measures the temperature once per minute while the substances react. The student plots the time and temperature data and creates the graph at left. - Observing Chemical Change

Chemical Reactions Energy in Chemical Changes At 4 minutes the temperature in the flask was about 23ºC. The first time the temperature was 6ºC was at about 7 minutes. Reading Graphs: What was the temperature in the flask at 4 minutes? When was the first time the temperature was at 6ºC? - Observing Chemical Change

Chemical Reactions Energy in Chemical Changes 20ºC Calculating: How many degrees did the temperature drop between 2 minutes and 5 minutes? - Observing Chemical Change

Chemical Reactions Energy in Chemical Changes The reaction was endothermic; it absorbed thermal energy from the reaction mixture, causing the temperature to drop. Interpreting Data: Is the reaction endothermic or exothermic? Explain. - Observing Chemical Change

Chemical Reactions Energy in Chemical Changes The reaction stopped at about 2ºC. You can tell because that is the lowest temperature reached. Inferring: At what temperature did the reaction stop? How can you tell? - Observing Chemical Change

Chemical Reactions Energy in Chemical Changes If the temperature increased, the reaction would be exothermic; an exothermic reaction is one in which energy is released. Drawing Conclusions: Suppose the temperature in the flask increased instead of decreased as the reaction occurred. In terms of energy, what kind of reaction would it be? Explain. - Observing Chemical Change

Chemical Reactions Comparing Properties

Chemical Reactions Links on Chemical Changes Click the SciLinks button for links on chemical changes. - Observing Chemical Change

Chemical Reactions - Describing Chemical Reactions Book L Ch 2.2 pgs(56-63)- Describing Chemical Reactions Cellular phone messages make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

Chemical Reactions - Describing Chemical Reactions Book L Ch 2.2 pgs(56-63)- What Are Chemical Equations? Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction.

Chemical Reactions - Describing Chemical Reactions Book L Ch 2.2 pgs(56-63)- What Are Chemical Equations? The formula of a compound identifies the elements in the compound and the ratios in which their atoms are present. For example: CO 2 Must contain: 1 molecule of carbon (C) 2 molecules of oxygen (O)

Chemical Reactions Book L Ch 2.2 pgs(56-63)- What Are Chemical Equations? Chemical equation - tells you the substances you start with in a reaction and the substances you get at the end. Reactants- refers to the substances you have at the beginning of a reaction Products- refers to the new substances created when the reaction is complete Reactant + Reactant  Product + Product Read the arrow as “yields”

Chemical Reactions Book L Ch 2.2 pgs(56-63)- What Are Chemical Equations? The formulas for the reactants are usually written on the left followed by an arrow. The formulas for the products are written on the right. Plus signs are used to separate two or more products and reactants. Reactant + Reactant  Product + Product C 6 H 12 O 6 + O 2  CO 2 + H 2 O + ATP (Energy) Chemical Formula for: Cellular Respiration

Chemical Reactions - Describing Chemical Reactions Book L Ch 2.2 pgs(56-63)- Conservation of Mass Conservation of mass - states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. Iron and Sulfur yields Iron Sulfide

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Conservation of Mass During a chemical reaction, matter is not created or destroyed. (all the atoms present at the start of the reaction are present at the end) Sometimes difficult to measure ail the matter involved in a reaction. Ex: When burning a match, oxygen comes from the surrounding air and products escape into the air.

Chemical Reactions Open system matter can enter from or escape to the surroundings Ex: A burning match Closed system matter is not allowed to enter or leave Ex: Chemical reaction inside a sealed plastic bag Book L Ch 2.2 pgs(56-63)- Conservation of Mass To measure all the matter before and after a reaction, you have to be able to contain it.

Chemical Reactions - Describing Chemical Reactions Book L Ch 2.2 pgs(56-63)- Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Balancing Chemical Equations How to Write a Balanced Chemical Equation: 1.Write the Equation Place the reactants on the left side of the arrow separated by a plus sign, then write the product on the right side of the arrow. H 2 + O 2  H 2 O

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Balancing Chemical Equations 2. Count the atoms Count the number of atoms of each element on each side of the equation. You find two atoms of oxygen in the reactants, but only one atom of oxygen in the products. H 2 + O 2  H 2 O 2 Hydrogen 2 Oxygen 2 Hydrogen 1 Oxygen

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Balancing Chemical Equations 3. Use Coefficients to Balance Atoms Coefficient- a number placed in front of a chemical formula in an equation Tells you how many atoms or molecules of a reactant or a product take part in the reaction. If the coefficient is 1, you do not need to write it. H 2 + O 2  2 H 2 O

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Balancing Chemical Equations H 2 + O 2  2 H 2 O There are now 2 hydrogen atoms in the reactants and four in the product. How can you balance the hydrogen? Double the number of hydrogens in the reactants by writing the coefficient 2 for hydrogen. 2 H 2 + O 2  2 H 2 O 2 Hydrogen 2 Oxygen 4 Hydrogen 2 Oxygen 4 Hydrogen Balanced!

Chemical Reactions 2 H 2 + O 2  2 H 2 O 4. Look Back and Check The equation is balanced: Two molecules of hydrogen react with one molecule of oxygen to yield two molecules of water. Book L Ch 2.1 pgs(56-63)- Balancing Chemical Equations

Chemical Reactions Balancing Equations Activity Click the Active Art button to open a browser window and access Active Art about balancing equations. - Describing Chemical Reactions

Chemical Reactions Balancing Chemical Equations Magnesium metal (Mg) reacts with oxygen gas (O 2 ), forming magnesium oxide (MgO). To write a balanced equation for this reaction, first write the equation using the formulas of the reactants and products, then count the number of atoms of each element. - Describing Chemical Reactions

Chemical Reactions Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O 2 ), forming sodium oxide (Na 2 O). - Describing Chemical Reactions

Chemical Reactions Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl 2 ), forming tin chloride (SnCl 2 ). - Describing Chemical Reactions

Chemical Reactions - Describing Chemical Reactions Book L Ch 2.1 pgs(56-63)- Classifying Chemical Reactions Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.

Chemical Reactions Synthesize - means to put things together (to create) In chemistry, when two or more elements or compounds combine to make a more complex substance, the process is called synthesis. The reaction of hydrogen and oxygen to make water is a synthesis reaction. Book L Ch 2.2 pgs(56-63)- Classifying Chemical Reactions

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Classifying Chemical Reactions Decomposition - breaking down of compounds into simpler products. Ex: Hydrogen peroxide (H 2 O 2 ) will decompose, or break down, into water and oxygen gas over a long period of time. 2 H 2 O 2  2 H 2 O + O 2

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Classifying Chemical Reactions Replacement – is when one element replaces another in a compound, Also when two elements in different compounds trade places Copper metal can be obtained by heating copper oxide with carbon. Carbon takes the place of copper. 2 Cu 2 O + C  4 Cu + CO 2

Chemical Reactions Book L Ch 2.2 pgs(56-63)- Classifying Chemical Reactions Single replacement - one element replaces another element in the compound Ex: 2 Cu 2 O + C  4 Cu + CO 2 Double replacement - elements in one compound appear to “trade places” with elements in another compound. Ex: FeS + 2 HCl  FeCl 2 + H 2 S