Physical or Chemical Change?

Transcription

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2 Chemical Formulas and Equations Physical or Chemical Change? Matter can change physically or chemically; a process that produces a chemical change is a chemical reaction.

3 Chemical Formulas and Equations Chemical Equations A chemical equation is a shorthand form of what reactants are used and what products are formed in a chemical reaction.

4 Chemical Formulas and Equations Chemical Equations Reactants are the substances that exist before the reaction begins. The substances that form as a result of the reaction are called the products.

5 Chemical Formulas and Equations Using Words Some equations use words or chemical names to identify reactants and products.

6 Chemical Formulas and Equations Using Words The reactants are listed on the left side of an arrow, separated from each other by plus signs.

7 Chemical Formulas and Equations Using Words The products are placed on the right side of the arrow, also separated by plus signs.

8 Chemical Formulas and Equations Using Words The arrow between the reactants and products represent the changes that occur during the chemical reaction.

9 Chemical Formulas and Equations Using Chemical Names Chemical formulas represent chemical names of substances in a chemical equations (Ex: H 2 O instead of water)

10 Chemical Formulas and Equations Using Formulas The chemical equation for the reaction between baking soda and vinegar can be written as follows.

11 Chemical Formulas and Equations Subscripts When you look at chemical formulas, notice the small numbers written to the right of the atoms. These numbers called subscripts, tell you the number of atoms of each element in that compound. If an atom has no subscript, it means that only one atom of that element is in the compound.

12 Chemical Formulas and Equations Conservation of Mass The mass of reactants and mass of products in a chemical reaction is always the same due to the law of conservation of mass.

13 Chemical Formulas and Equations Balancing Chemical Equations Chemical equations are balanced when the number of atoms is the same on each side of the equation.

14 Chemical Formulas and Equations Balancing Chemical Equations

15 Chemical Formulas and Equations Count the Atoms

16 Chemical Formulas and Equations Energy in Chemical Reactions Energy is released or absorbed during a chemical reaction. For example, energy of a welding torch is released when hydrogen and oxygen combine to form water.

17 Chemical Formulas and Equations Energy Released When reactions release energy (exothermic reactions), the products have bonds with less energy than those of the reactants.

18 Chemical Formulas and Equations Energy Absorbed When reactions absorb energy (endothermic reactions), the reactants are more stable and their bonds have less energy than those of the products.

19 Chemical Formulas and Equations Heat Absorbed An example of an endothermic physical process that absorbs heat energy is the cold pack shown. The heavy plastic cold pack holds ammonium nitrate and water. The two substances are separated by a plastic divider. When you squeeze the bag, you break the divider so that the ammonium nitrate dissolves in the water. The dissolving process absorbs heat energy.

20 Chemical Formulas and Equations Rapid Release Heat energy may be absorbed or released; the rate of heat release can be rapid or slow.

21 Chemical Formulas and Equations Slow Release Other materials also combine with oxygen but release heat so slowly that you cannot see or feel it happen. This is the case when iron combines with oxygen in the air to form rust.

22 Chemical Formulas and Equations Energy in the Equation The word energy can be written in a chemical equation as a reactant or product.

23 Question 1 Section Check In a chemical reaction, the substances that react are called the. Answer The substances that react are called reactants. The substances that form as a result of the reaction are known as the products.

24 Question 2 Section Check What kind of expression is Acetic acid + Sodium hydrogen carbonate Sodium acetate + Water + Carbon dioxide? Answer This is a word equation. It spells out the actual words that are used to describe the reactants rather than writing them as chemical formulas.

25 Question 3 Section Check What kind of expression is CH 3 COOH + NaHCO 3 CH 3 COONa + H 2 O + CO 2? Answer This is an example of a chemical equation. It uses chemical formulas instead of word names.

26 2 How Fast? Different chemical reactions take different amounts of time Fireworks explode in rapid succession on a summer night. Old copper pennies darken slowly while they lie forgotten in a drawer. Rates of Chemical Reactions

27 2 Rates of Chemical Reactions Activation Energy Starting a Reaction Activation energy the amount of energy needed to start a chemical reaction

28 2 Rates of Chemical Reactions Activation Energy Starting a Reaction Activation energy the amount of energy needed to start a chemical reaction Most fuels need energy to ignite. The Olympic Torch provided the activation energy required to light the fuel in the cauldron.

29 2 Rates of Chemical Reactions Reaction Rate The rate of reaction measures how quickly a reactant is disappearing or how quickly a product is appearing. Tells how quickly the amount of a substances changes per unit of time Importance in industry: the faster a product can be made, the lower its cost. Temperature can change the rate of reaction.

30 2 Rates of Chemical Reactions Temperature Changes Rate You can keep the food you buy at the store from spoiling so quickly by putting it in refrigerator or freezer. Food spoiling is a chemical reaction. Lowering the temperature of the food slows the rate of this reaction.

31 2 Rates of Chemical Reactions Temperature Affects Rate Most chemical reactions speed up when temperature increases, because the molecule move faster. Lowering temperature usually slows down reactions.

32 2 Rates of Chemical Reactions Concentration Affects Rate Concentration, the amount of a substance present in a specific volume, affects the rate of reaction. Higher concentration = increase rate of reaction

33 2 Rates of Chemical Reactions Surface Area Affects Rate Particle size affects the speed of a reaction.

34 2 Rates of Chemical Reactions Slowing Down Reaction Inhibitor a substance that slows down the rate of a chemical reaction.

35 2 Rates of Chemical Reactions Speeding Up Reactions Catalyst a substance that speeds up a chemical reaction; enzymes are special protein molecules that act as catalysts in the body.

36 2 Rates of Chemical Reactions Catalytic Converters Catalysts speed the reactions that change incompletely burned substances that are harmful, such as carbon monoxide, into less harmful substances, such as carbon dioxide.

37 2 Question 1 Section Check The minimum amount of energy required to start a reaction is called the. Answer Activation energy is the minimum amount of energy required to start a reaction.

38 2 Question 2 Section Check Most chemical reactions when temperature increases. A. abate B. remain the same C. slow down D. speed up

39 2 Answer Section Check The answer is D. Temperature has a direct effect on the reaction rate.

40 2 Question 3 Section Check Most chemical reactions when concentration increases. A. remain the same B. slow down C. speed up D. stop

41 2 Answer Section Check The answer is C. Higher concentration means a greater likelihood of collisions between the atoms in the reactants.

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43 End of Chapter Summary File