UNIT 6
What is Avogadro's number?
Avogadro's number, or the mole, is 6.02 x 1023.
Why can't we use subscripts to balance a chemical reaction?
Example: Why couldn't we change CO2 to CO?
Subscripts change the chemicals involved. Carbon dioxide (CO2) is exhaled from our lungs every time we take a breath, but carbon monoxide (CO) is deadly.
In a chemical reaction, what do we call the reactant that is entirely used up?
Example: 1 teaspoon of baking soda mixed into 3 GALLONS of vinegar. Baking soda is an example of this type of reactant.
This is the limiting reactant. It limits how much product can be made.
What is stored energy also called?
Stored energy, like the energy stored in chemical bonds, is also called potential energy.
Example: wood, dynamite, food
When you put honey in tea, what does the honey become? A solution, a solute, or a solvent?
When honey is added to tea it dissolves. The sugar molecules break apart and are dispersed into the water. This makes honey a solute.
P.S. Water is the solvent- it is the dispersion medium, or the thing that something else dissolves into.
How much do 0.5 moles of H2O weigh?
H - 1 g/mol
O - 16 g/mol
1 mole of H2O weighs 18 grams. 0.5 moles of H2O weighs 18 x 0.5 = 9 grams.
Balance this equation:
__ Na + __ O2 --> __ Na2O
There must be an equal number of each type of atom on either side; this is required by conservation of mass.
4 Na + O2 --> 2 Na2O
In a chemical reaction, what do we call the reactant that is leftover?
Example: 1 teaspoon of baking soda mixed into 3 GALLONS of vinegar. Vinegar is an example of this type of reactant.
Does ice experience an exothermic or endothermic change as it melts?
Heat is entering the ice to make it melt, so this is an endothermic change.
What is one property of an acid?
Acids are sour, corrosive, neutralize bases, act as electrolytes, increase H+ concentration in water, change the color of indicators, and have a pH less than 7.
How many atoms are in 3.7 moles of Unobtanium?
1 mole of anything is 6.02 x 1023.
3.7 moles = (6.02 x 1023) x 3.7 = 2.23 x 1024 atoms.
P.S. Unobtanium is a made-up element
What type of reaction is this?
2MgO --> 2Mg + O2
This is a decomposition reaction. Magnesium oxide decomposes by breaking down from one substance to multiple substances.
What is the percent yield of this reaction; it was possible to produce 14.7 grams of product, but the actual yield was only 9.9 grams.
We were only able to produce 67% of our theoretical maximum.
(9.9/14.8) x 100 = 67%
What causes gas pressure?
(explain by describing interactions involving atoms and molecules)
Gas pressure is caused by the collisions of billions of gas particles (atoms or molecules) with the walls of a container. The faster the collisions (temperature), the more collisions (moles), or the smaller the container (volume), the higher the pressure.
A solution of HF has a pH of 1.2. Is this an acid or a base?
This solution of HF is an acid due to its low pH. In water, HF donates a proton.
HF --> H+ + F-
How many atoms are in 24 grams of Carbon?
Hint: assume carbon weighs 12 g/mol
To find moles of Carbon, divide grams by molar mass. In this case, 24/12 = 2 means we have 2 moles.
1 moles = 6.02 x 1023
2 moles = (6.02 x 1023) x 2 = 12.04 x 1023 atoms
What do we use the symbol (g) to denote in a chemical equation.
Example: 2H2 (g) + O2 (g) --> H2O (l)
(g) denotes that a given chemical is in its gaseous state when reacting.
The other symbols are (s) for solid, (l) for liquid, and (aq) for aqueous or dissolved in water.
How many moles of H2 are required to produce 4.4 moles of NH3?
N2 + 3H2 --> 2NH3
According to the equation, for every 2 NH3 produced, 3 H2 are required. This means we must multiply by a mole ratio of 3/2.
4.4 x (3/2) = 6.6 moles H2
What is specific heat?
Specific heat is the energy required to increase the temperature of one gram of a substance by 1°C.
Sand and water have different specific heats. When heated by the sun, sand on the beach becomes much hotter than the water by the shore- water has a VERY high specific heat.
A solution of NaOH has a pH of 11.7. What type of solution is this?
Any pH above 7 is a base. This solution is very basic. In water, NaOH releases a hydroxide ion which can then accept a proton.
NaOH --> Na+ + OH-
OH- + H+ --> H2O
How many grams does 4.7 x 1024 molecules of H2O weigh?
We can only find the weights of moles of a substance from the periodic table, so we must convert to moles first.
(4.7 x 1024)/(6.02 x 1023) = 7.8 mol H2O
1 mole H2O weighs 18 grams, so 7.8 moles weighs 140.5 grams.
Predict the products of this double replacement reaction:
MgCl2 + 2HF --> ?
Mg: +2 H: +1
Cl: -1 F: -1
H isn't a metal, but it usually forms a positive ion, so it often acts like a metal.
The metals swap places and ionic charges cancel.
MgCl2 + 2HF --> MgF2 + 2HCl
How many grams of hydrogen are needed to react completely with 54 g of oxygen?
2H2 (g) + O2 (g) --> 2H2O (l)
6.8 grams of hydrogen are required. (values rounded)
54 g O2 ÷ 32 g/mol = 1.69 mol O2
1.69 mol O2 × (2/1) = 3.38 mol H2
3.38 g O2 × 2 g/mol = 6.8 mol O2
A 15.75-g piece of mystery metal absorbs 1086.75 joules of heat energy, and its temperature changes from 25°C to 175°C. (use q = m×c×𝚫T)
Is this material...
Iron: c = 0.46 J/g°C
Aluminum: c = 0.9 J/g°C
Coppper: c = 0.385 J/g°C
This material must be iron because it has a specific heat of 0.46 J/g°C
Which two substances are typically formed when an acid and a base react?
Hint: predict this double replacement
NaOH + HCl --> ?
Acids and bases react to form a salt and water.
NaOH + HCl --> NaCl + H2O