Chapter 7: Chemical Reactions
the states used in chemical equations include the following...
(g) gas (l) liquid (s) solid (aq) aqueous **a substance is dissolved in a water; when a substance dissolves in water, it is called a water solution
a chemical reaction may result in the absorption or emission of heat
--> a chemical cold pack becomes cold when the plastic barrier separating two substances is broken and the reaction absorbs heat from the surrounding area --> in a car engine, hydrocarbons from gasoline combine with oxygen from the air and react to form carbon dioxide and water producing heat which expands the gases in the car's cylinder, accelerating it forward
a chemical reaction may result in the emission of light: example
--> a natural gas flame produces heat and light
a chemical reaction may result in the formation of a solid in a previously clear (unclouded) solution: example
--> aka precipitation reaction; laundry detergents contain carbonate ions that react with magnesium and calcium to form solid magnesium carbonate and solid calcium carbonate which settle at the bottom of the laundry mixture removing the ions
a chemical reaction may result in a color change: example
--> the color of a shirt fades (changes) when repeatedly exposed to sunlight because the color-causing molecules decompose --> the molecules embedded in a child's temperature-sensitive spoon change the color of the spoon upon warming
a chemical reaction may result in the formation of a gas when a substance is added to a solution: example
--> when you drive a car, hydrocarbons such as octane (in gasoline) react with oxygen from the air to form carbon dioxide gas and water --> in the classic grade school volcano, baking soda (which is sodium bicarbonate) reacts with acetic acid in vinegar to form carbon dioxide gas, water, and sodium acetate; the newly formed carbon dioxide bubbles out of the mixture, causing the eruption
what are the 5 pieces of evidence that shows a chemical reaction has taken place?
1. a color change 2. the formation of a solid in a previous clear (unclouded) solution 3. the formation of a gas when we add a substance to a solution 4. the emission of light 5. the emission or absorption of heat
3 characteristics of a redox reaction... meet any 1, and its a redox reaction.
1. a substance reacts w/ elemental oxygen 2. a metal reacts w/ a nonmetal 3. a substance transfers electrons to another substance
evidence at the molecular/atomic level proving a chemical reaction has taken place
1. atoms combine with other atoms to form compounds 2. new molecules form 3. original molecules decompose 4. atoms in one molecule change places with atoms in another molecule
writing the equation and predicting whether a precipitate forms for a precipitation reaction
1. write the formulas of the two compounds being mixed as reactants in a chemical equation. 2. below the equation, write the formulas of the potentially insoluble products that could form from the reactants; obtain these by combining the cation from one reactant with the anion from the other; make sure to write correct (charge-neutral) formulas for these ionic compounds. 3. use the solubility rules to determine whether any of the potential new products are insoluble. 4. if all of the potentially insoluble products are soluble, there will be no precipitate; write NO REACTION next to the arrow. 5. if one or both of the potentially insoluble products are insoluble, write their formula(s) as the product(s) of the reaction, using (s) to indicate solid. Follow any soluble products with (aq) to indicate aqueous. 6. balance the equation. Remember to adjust only coefficients, not subscripts.
steps to write a balanced chemical equation
1. write the unbalanced equation by writing chemical formulas for each of the reactants and products. 2. if an element occurs in only one compound on both sides of the equation, balance it first; if there is more than one such element, balance metals before nonmetals. 3. if an element occurs as a free element (not as part of a compound) on either side of the chemical equation, balance it last; always balance free elements by adjusting the coefficient on the free element. 4. if the balanced equation contains coefficient fractions, change these into whole numbers by multiplying the entire equation by the appropriate factor. 5. check to make certain the equation is balanced by summing the total number of each type of atom on both sides of the equation.
In photosynthesis, plants make the sugar glucose, C6H12O6, from carbon dioxide and water. The equation for the reaction is: 6 CO_2 + 6 H_2O → C_6H_12O_6 + x O2 In order for this equation to be balanced, the coefficient x must be 9 3 6 12
6 **there are 18 oxygen atoms on the left side of the equation, so the same number is needed on the right: 6 + 6(2) = 18.
chemical reactions can also be classified by what atoms do during the reaction. these include the following... synthesis or combination: decomposition: single displacement: double displacement:
A + B --> AB AB --> A + B A + BC --> AC + B AB + CD --> AD + BC
Which chemical equation is a net ionic equation? 2 K+(aq) + SO42−(aq) + Ba2+(aq) + 2 Cl−(aq) → BaSO4(s) + 2 K+(aq) + 2 Cl−(aq) Ba2+(aq) + SO42−(aq) → BaSO4(s) K2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2 KCl(aq)
Ba2+(aq) + SO42−(aq) → BaSO4(s)
Which reaction results in the formation of a precipitate? NaNO_3(aq) + CaS(aq) MgSO_4(aq) + CaS(aq) NaNO_3(aq) + MgSO_4(aq)
MgSO4(aq) + CaS(aq) **both of the possible products, MgS and CaSO4, are insoluble; the possible products of the other reactions—Na2S, Ca(NO3)2, Na2SO4, and Mg(NO3)2—are all soluble
balanced equation
a chemical equation in which the numbers of each type of atom on both sides of the equation are equal
molecular equation
a chemical equation showing the complete neutral formulas for every compound in a reaction AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
complete ionic equation
a chemical equation showing the reactants and products as they are actually present in the solution; when writing these, separate only the aqueous ionic compounds into their constituent elements, not gases, liquids, or solids Ag+ (aq) + NO3− (aq) + Na+ (aq) + Cl− (aq) → AgCl (s) + Na+ (aq) + NO3− (aq)
aqueous solution
a homogeneous mixture of a substance with water
precipitation reaction
a reaction in which a solid or precipitate forms when two aqueous solutions are mixed
acid-base reaction
a reaction that forms water & typically salt upon mixing of an acid & base
strong electrolyte solution
a solution containing a solute that dissociates into ions; therefore, a solution that conducts electricity well
Which observation is consistent with a chemical reaction occurring? Why? a. solid copper deposits on a piece of aluminum foil when the foil is placed in a blue copper nitrate solution. The blue color of the solution fades. b. liquid ethyl alcohol turns into a solid when placed in a low-temperature freezer. c. a white precipitate forms when solutions of barium nitrate and sodium sulfate are mixed. d. a mixture of sugar and water bubbles when yeasts are added. After several days, the sugar is gone and ethyl alcohol is found in the water.
a. chemical reaction; evidenced by a color change b. not a chemical reaction; a state change is a physical change c. chemical reaction; evidenced by the formation of a solid in a previously clear solution d. chemical reaction; evidenced by the formation of a gas when yeast is added to the solution
Which of these are chemical reactions? a. copper turns green on exposure to air. b. when sodium bicarbonate is combined with hydrochloric acid, bubbling is observed. c. liquid water freezes to form solid ice. d. a pure copper penny forms bubbles of a dark brown gas when dropped into nitric acid. The nitric acid solution turns blue.
a. chemical reaction; evidenced by the color change b. chemical reaction; evidenced by the evolution of gas c. not a chemical reaction; solid ice is still H_2O d. chemical reaction; evidenced by the evolution of gas and by the color change
Which changes involve a chemical reaction? Explain your answers. a. butane burning in a butane lighter b. butane evaporating out of a butane lighter c. wood burning d. dry ice subliming
a. chemical reaction; heat and light are emitted b. not a chemical reaction; gaseous and liquid butane are both butane c. chemical reaction; heat and light are emitted d. not a chemical reaction; solid dry ice is mad eof carbon dioxide, which sublimes (evaporates) as carbon dioxide gas
These images portray molecular views of various substances before and after a change. Determine whether a chemical reaction has occurred in each case. (look at image in answer)
a. no chemical reaction took place; the molecules endured a state change, which is physical, from solid to gas (melting) (the molecules are the same before and after the change) b. a chemical reaction took place; the molecules in the first picture decomposed into different molecules (the molecules have changed) c. a chemical reaction took place; water molecules decomposed into their constituent elements hydrogen and oxygen (the molecules have changed)
Which changes involve a chemical reaction? Explain your answers. a. ice melting upon warming b. an electric current passing through water, resulting in the formation of hydrogen and oxygen gas that appears as bubbles rising in the water c. iron rusting d. bubbles forming when a soda can is opened
a. not a chemical reaction; the solid water molecules become liquid water molecules, but they are still H_2O molecules b. chemical reaction; water decomposes into hydrogen and oxygen, as evidenced by the bubbling c. chemical reaction; iron changes into iron oxide, changing color in the process d. not a chemical reaction; even though there is bubbling, it is just carbon dioxide coming out of the liquid
Which of these are redox reactions? a. 2 Mg(s)+O2(g)→2 MgO(s) b. 2 HBr(aq)+Ca(OH)2(aq)→2 H2O(l)+CaBr2(aq) c. Ca(s)+Cl2(g)→CaCl2(s) d. Zn(s)+Fe2+(aq)→Zn2+(aq)+Fe(s)
a. redox reaction; Mg reacts w/ elemental oxygen b. acid-base reaction c. redox reaction; metal reacts w/ nonmetal d. redox reaction; Zn transfers 2 electrons to Fe
Which quantity must always be the same on both sides of a balanced chemical equation? a. the number of each type of atom b. the number of each type of molecule c. the sum of all of the coefficients
a. the number of each type of atom
net ionic equation
an ionic equation that includes only the particles that participate in the reaction --> Ag+ (aq) + Cl- (aq) → AgCl (s)
Which image best depicts a mixture of BaCl_2 and water? (look at image in answer)
c. **since chlorides are usually soluble and Ba2+ is not an exception, BaCl2 is soluble and will dissolve in water; when it dissolves, it dissociates into its component ions
to balance a chemical equation, ____________ are added; the __________ of an atom are NEVER changed.
coefficients; subscripts
soluble: insoluble:
dissolves in a solution does not dissolve in water
Both precipitation reactions and acid-base reactions can also be classified as: single-displacement reactions double-displacement reactions synthesis reactions decomposition reactions
double-displacement reactions
spectator ions
ions that do not participate in a reaction
do all ionic compounds dissolve in water?
no
the substances on the left side of a chemical equation are called __________, and the stances on the right side of the chemical equation are called __________; the _______ of each reactant and product is usually specified in parentheses next to each compound/atom
reactants, products; state
chemical reaction
the transformation of one or more substances into different substances
do polyatomic ions dissociate when they dissolve in water, or do they remain intact?
they remain intact
when balancing an equation with a polyatomic ion, the polyatomic ions are balanced as a _____.
unit
When a commercial hair bleaching mixture is applied to brown hair, the hair turns blond. Has a chemical reaction occurred? Explain your answer.
yes; the color change of the hair evidences a chemical reaction
Vinegar forms bubbles when it is poured onto the calcium deposits on a faucet, and some of the calcium dissolves. Has a chemical reaction occurred? Explain your answer.
yes; the presence of bubbles is evidence for the formation of a gas
