Physical Change - alters the form or appearance of a substance but does not change it into a new, different substance

Transcription

1 Chemical Reactions

2 Physical Change - alters the form or appearance of a substance but does not change it into a new, different substance

3 Chemical Change - forms one or more new substances with properties different from those of the original substances also called a chemical reaction

4 Evidence for Chemical Reactions color change odor

5 Evidence for Chemical Reactions formation of a precipitate, a solid that forms when two liquids are mixed

6 Evidence for Chemical Reactions production of a gas (seen as bubbles in liquid)

7 Evidence for Chemical Reactions energy changes energy is absorbed in an endothermic reaction energy is released in an exothermic reaction

8 Chemical formula - tells you how many atoms of each element are in a compound example: CO 2 (carbon dioxide) has one carbon atom and two oxygen atoms

9 Chemical Equation - a short, easy way to show a chemical reaction, using symbols The substances you begin with are called the reactants. The new substances formed from the reaction are called the products. = yields

10 The subscript (small number to the right of the element s symbol) tells you how many atoms of that element are in the molecule.

11 The coefficient (number to the left of the element s symbol) tells you how many molecules of that compound or element are present.

12 Count the Atoms C6H12O6 = C atoms = H atoms = O atoms = total atoms

13 Count the Atoms 3H2O = H atoms = O atoms = total atoms

14 Count the Atoms 5H2SO4 = H atoms = S atoms = O atoms = total atoms

15 Count the Atoms (NH 4 ) 2 SO 4 = N atoms = H atoms = S atoms = O atoms = total atoms

16 Conservation of Mass matter is not created or destroyed during a chemical reaction

17 Balancing Chemical Equations a chemical equation must show the same number of each type of atom on both sides of the equation

18 When balancing a chemical equation, you can change the coefficient, but you can t change the subscript as that would form a new substance. H 2 O vs. H 2 O 2 Two scientists walk into a bar...

19 Steps to balancing a chemical equation: 1. Write the equation. H 2 + O 2 H 2 O

20 Steps to balancing a chemical equation: 1. Write the equation. 2. Count the atoms of each type of element on both the reactant and product sides of the equation. H 2 + O 2 H 2 O H = H = O = O =

21 Steps to balancing a chemical equation: 1. Write the equation. 2. Count the atoms of each type of element on both the reactant and product sides of the equation. 3. Use coefficients to balance the number of atoms on both sides of the reaction. H 2 + O 2 H 2 O H = H = O = O =

22 Steps to balancing a chemical equation: 1. Write the equation. 2. Count the atoms of each type of element on both the reactant and product sides of the equation. 3. Use coefficients to balance the number of atoms on both sides of the reaction. 4. Look back and check that the equation is balanced. 2H 2 + O 2 2H 2 O

23 Let s try another one: CH 4 + O 2 CO 2 + H 2 O Count the atoms: C = C = H = H = O = O =

24 Change the coefficients to balance the hydrogen atoms: Now, change the coefficients to balance the oxygen atoms: CH 4 + O 2 CO 2 + H 2 O C = 1 C = 1 H = 4 H = 2 O = 2 O = 3

25 Check to make sure the equation is balanced: CH 4 + 2O 2 CO 2 + 2H 2 O C = C = H = H = O = O =

26 Kahn Academy Balancing Chemical Equations M

27 Balance the Equations Mg + O 2 MgO FeS + HCl FeCl 2 + H 2 S

28 Five types of chemical reactions Synthesis - two or more simple substances combine to form a more complex substance A + B AB (two or more reactants; one product) ex: formation of water

29 Decomposition - a compound breaks down into simpler substances AB A + B (one reactant; two or more products) ex: hydrogen peroxide decomposes to form water and oxygen 2H2O2 2H2O + O2

30 Single Replacement - one element replaces another in a compound A + BC B + AC C + 2Cu2O 4Cu + CO2

31 Double Replacement - two elements in different compounds trade places AB + CD AD + CB FeS + 2HCl FeCl 2 + H 2 S (iron sulfide + hydrochloric acid iron chloride + hydrogen sulfide)

32 Combustion - oxygen combines with a carbon compound to form water and carbon dioxide C # H # + O 2 CO 2 + H 2 O exothermic

33 Activation Energy - the minimum amount of energy needed to get a chemical reaction started

34 Activation Energy - the minimum amount of energy needed to get a chemical reaction started

35 Rates of Chemical Reactions - chemists can speed up or slow down chemical reactions by changing one or more of the following factors: Surface Area - increasing the surface area where a reaction takes place will increase the rate of the reaction Chewing food provides increased surface area; faster digestion

36 brain is folded to increase surface area; more chemical reactions can take place simultaneously

37 grinding a reactant into a powder creates more surface area; increase the rate of a reaction

38 Temperature - increasing the temperature will increase the rate of the reaction; decreasing the temperature will decrease the rate of the reaction

39 Concentration - the amount of a substance in a given volume (its strength)

40 increasing the concentration of the reactants increases the rate of the reaction

41 Catalyst - a material that increases the rate of a chemical reaction by lowering the activation energy Elephant Toothpaste

42 an enzyme is a biological catalyst

43 Inhibitor - decreases the rate of a reaction by preventing reactants from coming together