MU #1 Chemical and physical changes can be exothermic or endothermic.

Energy is absorbed (endothermic, ΔH+, see below table I) when bonds or intermolecular forces are broken and energy is released (exothermic, ΔH-, see below table I)) when bonds or intermolecular forces are formed.

MU #2 A physical change results in the rearrangement of existing particles in a substance. 

Example

Note: In endothermic reactions HEAT is on the reactants side and ΔH is positive, see below table I.

 Note: In exothermic reactions HEAT is on the products side and ΔH is negative, see below table I.

Problems

Find other examples

MU #3 A chemical change results in the formation of different substances with changed properties.

Example

Problems

Find all exothermic and endothermic reactions in table I.

TEST YOUR UNDERSTANDING

6/02

Base your answers on the following information below. A hot pack contains chemicals that can be activated to produce heat. A cold pack contains chemicals that feel cold when activated.

     a. Based on energy flow, state the type of chemical change that occurs in a hot pack.

     b. A cold pack is placed on an injured leg. Indicate the direction of the flow of energy between the leg and the cold pack.

     c. What is the Law of Conservation of Energy? Describe how the Law of Conservation of Energy applies to the chemical reaction that occurs in the hot pack.

8/02

In which equation does the term "heat" represent heat of fusion?

(1) NaCl(s) + heat --> NaCl(l)                                                     (3) H₂O(l) + heat -> H₂O(g)

(2) NaOH(aq) + HCl(aq) --> NaCl(aq) + H₂O(l) + heat          (4) H₂O(l) + HCl(g) --> H₃O⁺(aq) + Cl^-(aq) + heat

6/03

Which type of change must occur to form a compound? (1) chemical (2) physical  (3) nuclear (4) phase

8/04

14 Based on Reference Table I, which change occurs when pellets of solid NaOH are added to water and stirred?

                (1) The water temperature increases as chemical energy is converted to heat energy.

                (2) The water temperature increases as heat energy is stored as chemical energy.

                (3) The water temperature decreases as chemical energy is converted to heat energy.

                (4) The water temperature decreases as heat energy is stored as chemical energy.

1/05

10 Which statement describes a chemical property that can be used to distinguish between compound A and compound B?

                (1) A is a blue solid, and B is a white solid. (2) A has a high melting point, and B has a low melting point.

                (3) A dissolves in water, and B does not dissolve in water. (4) A does not burn in air, and B does burn in air.

6/05

6 An example of a physical property of an element is the element’s ability to 

                    (1) react with an acid   (2) react with oxygen  (3) form a compound with chlorine (4) form an aqueous solution 

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