7.1 Describing Reactions
I Chemical Equations
Reactants Products
Example: Carbon + Oxygen Carbon Dioxide
A. Chemical equation- a representation of a chemical reaction in which the reactants and products are expressed as formulas.
C + O2 CO2
Arrow= ______
B. Law of Conservation of Mass- mass is neither created nor destroyed in a chemical reaction ______. The mass of the reactant must equal the mass of the ______
II. Balancing Chemical Equations- in order to show that mass is conserved during a reaction, a chemical equation must be balanced.
1. ______- the number that appears before the formula
2. Only coefficients can be changed when balancing a chemical equation. NEVER CHANGE THE SUBSCRIPT.
STEPS FOR BALANCING CHEMICAL EQUATIONS
1. Count the number of atoms of each element on each side of the equation.
2. Change one or more coefficients until the equation is balanced.
N2H4 + O2 N2 + H2O
Reactants Products
7.3 Notes Chemical energy and Thermal Energy (Heat)
Chemical Bond: ______holding ______together as a unit.
Chemical Energy: stored in chemical ______
Drawing:
Chemical reactions bonds are broken in the ______and new bonds are formed in the ______
Reactant Product
Law of conservation of energy: All energy must be ______.
Energy is not ______or ______just changed from one form to another.
Energy in the reactant must ______energy in the product.
Example:
Energy in a Chemical Reaction:
EXO –Thermic Reactions:Energy ______in Product
Energy is ______and reaction feels warm
Examples: Combustion
C3H8 + 5O2 3CO2 + 4H2O+ 2220kJ
· kJ (energy) is on the PRODUCTS side
This energy that is released as the ______is greater than the energy required to ______the bonds in the reactants.
Graphical representation: / ENDO- Thermic Reactions:
Energy ______in Reactant
Energy is ______and the reaction feels cold.
Examples:
C2 H6 + 137kJ C2 H4 + H2
· kJ (energy) in on the REACTANTS side
· More energy is ______to BREAK the bonds in the ______than is released when the products form.
Graphical Representation:
Chapter 7 Review
1. The substances that are present before a chemical reaction takes place are called
a. / reactants. / c. / coefficients.b. / products. / d. / elements.
2. Which of the following does NOT state what the arrow means in a chemical equation?
a. / forms / c. / conservesb. / produces / d. / yields
3. Which of the following is a balanced chemical equation for the synthesis of NaBr from Na and Br2?
a. / Na + Br2 ® NaBr / c. / Na + Br2 ® 2NaBrb. / 2Na + Br2 ® NaBr / d. / 2Na + Br2 ® 2NaBr
4 . In a compound, chemical energy is contained in the
a. / nuclei of the atoms. / c. / bonds.b. / unbonded electrons. / d. / movement of the electrons.
5. Which of the following statements is true about what happens during a chemical reaction?
a. / Bonds of the reactants are broken, and bonds of the products are formed.b. / Bonds of the reactants are formed, and bonds of the products are broken.
c. / The bonds of both the reactants and the products are broken.
d. / The bonds of both the reactants and the products are formed.
6. In terms of energy, how would you classify the following chemical reaction?
2Cu + O2 ® 2CuO + 315 kJ
a. / endothermic / c. / both endothermic and exothermicb. / exothermic / d. / neither endothermic nor exothermic
7. For the chemical reaction C2H6 + 137 kJ ® C2H4 + H2, the chemical energy of the
a. / reactant is greater than the chemical energy of the products.b. / products is greater than the chemical energy of the reactant.
c. / reactant and the chemical energy of the products are equal.
d. / reaction is conserved.
8. For the chemical reaction H2 + CO2 ® H2O + CO, the energy contained in the reactants is 352 kJ, and the energy contained in the products is 394 kJ. Which of the following statements is true?
a. / 42 kJ is released, and the reaction is exothermic.b. / 42 kJ is released, and the reaction is endothermic.
c. / 42 kJ is absorbed, and the reaction is exothermic.
d. / 42 kJ is absorbed, and the reaction is endothermic.
Figure 7-3
9. In an exothermic reaction as seen in Figure 7-3, do the reactants or the products contain more chemical energy?______
10. Cooking requires continuous addition of energy to the chemical reactions that are taking place. The chemical reactions involved in cooking can be described as ______.
11. Cooking involves an endothermic reaction. Which has more chemical energy in its chemical bonds—an uncooked egg white or a cooked egg white? Use Figure 7-3 to explain your answer.
12. If the bonds in the reactants of Figure 7-3 contained 432 kJ of chemical energy and the bonds in the products contained 478 kJ of chemical energy, what would be the amount of energy change during the reaction? Would this energy be absorbed or released?
13.A student balanced the chemical equation Mg + O2 ® MgO by writing Mg + O2 ® MgO2. Was the equation balanced correctly? Explain your answer
14 Using the law of conservation of mass explain the results that you found in the steel wool, balloon, flask system lab?
15. Why did you place the balloon over the flask in the conservation of mass lab?
16. What are the reactants and products in the following equation? C2H6 + 137 kJ ® C2H4 + H2
17. The statement that in chemical reactions, the total mass of the reactants equals the total mass of the products is the law of ______.
18. A(An) ______is the number that appears before a formula in a chemical equation.
Make sure you can balance chemical equations