Unit #5 - Chemical Reactions

How does a balanced chemical equation show that matter is never lost or gained in a chemical reaction?

Balanced chemical equations show that matter is never lost or gained in a chemical reaction because the number of atoms is the same for the reactants and the products. During a chemical reaction, atoms break apart and combine to form new substances (they aren't lost or gained).

Which class lab had an exothermic reaction? Explain how you know this occurred.

CaCl₂/H₂O lab had an exothermic reaction. An exothermic reaction is a reaction in which heat energy is released to its surroundings. We know that this happened because the substance got warmer. Heat energy was released in this experiment, so we felt the liquid warm up.

Why do chemical equations have to be balanced?

Chemical reactions must be balanced because of the law of conservation of mass. This law says that mass is not created or destroyed in regular chemical and physical changes. In chemical reactions, atoms are never lost or gained; instead, they break apart and rearrange themselves to form new substances. Therefore, all chemical equations must be balanced.

Coefficient

a number that is placed in front of a chemical symbol or formula; used to balance equations; must multiply subscript of each element by the coefficient; COEFFICIENTS CAN CHANGE

Subscript

a number that tells how many atoms of an element are in a molecule; always written below and to the right of the element's symbol; SUBSCRIPTS CANNOT BE CHANGED

Chemical reaction

a process in which one or more substances combine and change to form one or more new substances (chemical/physical properties of the new substances differ from those of the original substances)

Exothermic reaction

a reaction in which heat energy is released to its surroundings

Endothermic reaction

a reaction in which heat energy is taken in from its surroundings

Chemical bond

force of attraction that holds two atoms together

Covalent bonds

formed when atoms SHARE electrons to fill their outermost energy level

Chemical formula

shows which elements are found in a substance; shows how many atoms of each element are found in a molecule of the substance

Yield

the arrow symbol in a chemical equation; left side = reactants, right side = products; shows that reactants "yield" to become products

Chemical bonding

the joining of atoms to form new substances

Products

the new substance(s) that are formed or produced in a chemical reaction

Reactants

the original substance(s) that participate in or combine together in a chemical reaction

Write balanced equations for the following: -Al + CuSO₄ -> Al₂(SO₄) + Cu -Mg (OH)₂ + HCl -> MgCl₂ + H₂O

-2Al + 3CuSO₄ -> Al₂(SO₄)₃ + 3Cu -Mg(OH)₂ + 2HCl -> MgCl₂ + 2H₂O

Calculate the number of ATOMS of each element represented in each of the following formulas: -2Na₃PO₄ -4Al₂(SO₄)₃ -6PCl₅

-Na: 6 P: 2 O: 8 -Al: 8 S: 12 O: 48 -P: 6 Cl: 30

What happens to the atoms of the starting substances during a chemical reaction?

Atoms of the starting substances break apart and rearrange to form new substances.

Signs of chemical reactions

Gas formation - bubbles or fizzing, odor might change, inflation of bag, etc. Color change - new color than original substance(s) Energy change - change in heat, light, sound, etc. New formation - new solids are formed

Reactants and Products in the Alka Seltzer Lab

H₂O (water) + C₆H₈O₇ (citric acid) + NaHCO₃ (sodium hydrogen carbonate) -> CO₂ (carbon dioxide gas) + H₂O (water) + NaC₆H₅O₇ (sodium citrate) *the product is a weak base b/c the pH is 8 - often used to neutralize stomach acids

If the chemical properties of a substance have not changed, has a chemical reaction occurred?

If the chemical properties of a substance have not changed, then a chemical reaction hasn't occurred. In a chemical reaction, the new substances' chemical and physical properties are different than those of the original substances.

What were the signs that a chemical reaction occurred in the Alka Seltzer lab?

In the Alka Seltzer lab, we know that a chemical reaction occurred because there were gas formations (bubbles/fizzing), new formations ("crystals" formed on the inside of the test tube and there was white residue at the bottom of it), and a change in heat energy (the temperature dropped).

What were the signs that a chemical reaction occurred in the CaCl₂/H₂O lab?

In the CaCl₂/H₂O lab, we know that a chemical reaction occurred because there were gas formations (bubbles/fizzing and the bag inflated), a change in heat energy (the temperature rose), and a change in color (the substance turned white).

Reactants and Products in the CaCl₂/H₂O Lab

NaHCO₃ (sodium hydrogen carbonate) + CaCl₂ (calcium chloride) + H₂O (water) -> NaCl (sodium chloride - salt) + CaCO₃ (calcium carbonate) + CO₂ (carbon dioxide gas) + H₂O (water)

What was the purpose of the phenol red in the CaCl₂/H₂O lab?

Phenol red is an indicator, which means that it shows whether a substance is an acid or a base. When an acid is present, phenol red will change color and become yellow. In the lab, it showed us if an acid had formed. When the reactants combined to form CO₂ gas (the acid), there was a change in color. However, in the CaCl₂/phenol red experiment and the NaHCO₃/phenol red experiment, the color stayed pink. This happened because there was no acid present.

Which class lab had an endothermic reaction? Explain how you know this occurred.

The Alka Seltzer lab had an endothermic reaction. An endothermic reaction is a reaction in which heat energy is taken in from its surroundings. We know that this happened because the reaction took in heat energy from the water, glass (test tube), and our hands. The liquid felt colder to the touch because we lost heat energy and "gave" it to the reactants.