Physical Science - Chapter 7
Balance the following equation: K + Br2 -> KBr
2K +Br2 -> 2KBr
Write a balanced chemical equation for the formation of magnesium oxide, MgO, from magnesium and oxygen.
2Mg + O2 -> 2MgO
Ammonia, NH3, can be made by reacting nitrogen with hydrogen. N2 + 3H2 ->2NH3 How many moles of NH3 can be made if 7.5 moles of H2 react with enough N2?
7.5 mol H2 X 2 mol NH3/3 mol H2 = 5.0 mol NH3
How does a catalyst make a reaction go faster?
A catalyst makes a reaction go faster by lowering the energy barrier required to break the bonds of the reacting particles.
Why does a chemical equation need to be balanced?
A chemical equation must be balanced in order to demonstrate that mass is conserved.
Explain why, If you want to store uncooked hamburger meat for a month, you put it in a freezer rather than a refrigerator.
A freezer is colder than a refrigerator, so the reactions responsible for spoiling the meat will take place more slowly in the freezer.
What is a mole ratio?
A mole ratio is a conversion factor that relates two substances that take part in a chemical reaction. A mole ratio can be determined from a balanced chemical equation.
What happens when a physical change does not go to completion? What happens when a reaction does not go to completion?
A physical equilibrium is reached between the forward and reverse changes. A chemical equilibrium is reached between the forward and reverse reactions.
Explain why methane does not react with oxygen at room temperature.
At room temperature, the collisions between methane molecules and oxygen molecules are not energetic enough to cause a reaction.
Why do chemists use the mole as a counting unit?
Because chemical reactions often involve large numbers of small particles, the mole (6.02X10^23 particles) is a practical counting unit for scientists to use.
Propane, C3H8, is frequently used in camping stoves. When propane undergoes combustion, what are the products formed?
Carbon dioxide and water
What happens to chemical bonds as a chemical reaction occurs?
Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.
How do chemical reactions involve energy?
During chemical reactions, energy is either released or absorbed.
Methane reacts with oxygen in the following combustion reaction. CH4 + 2O2 -> CO2 + 2H2O What bonds are broken when one molecule of methane reacts with two molecules of oxygen?
Four C-H single bonds and 2 O=O double bonds are broken when one molecule of methane burns.
What does a reaction rate tell you?
How fast a chemical reaction is going
Explain the difference between a single-replacement reaction and a double replacement reaction.
In a single-replacement reaction, an element replaces another element in one compound. In a double-replacement reaction, two elements replace each other in two different compounds.
What information do you need to predict the mass of a reactant or product in a chemical reaction?
In order to calculate the mass of a reactant or product in a chemical reaction, you need the molar mass of the reactant, the molar mass of the product, and a balanced chemical equation (which tells you the mole ratio between the reactant and product).
The synthesis of water is described by the equation of 2H2 + O2 -> 2H2O. How is the decomposition of water related to this reaction? Explain, using a chemical equation.
In the reaction 2H2 + O2 -> 2H2O, water is formed from its elements. During the decomposition of water, water is broken down into its elements, according to the equation 2H2O -> 2H2 + O2.
Suppose the following reaction is allowed to come to equilibrium. 2NO2(g) <=> N2O4(g) How will increasing the pressure on this system affect the amount of N2O4 formed?
Increasing the pressure of the system will cause the equilibrium to shift to the right, so as to decrease the total number of gas molecules. As a result, the amount of N2O4 in the system will increase.
What is the product of synthesis reaction between magnesium and iodine? Explain your answer
MgI2. In ionic compounds, magnesium has a charge of 2+ and iodine has a charge of 1-
What mass of NH3 can be made from 35 g of N2? N2 + 3H2 -> 2NH3
Molar mass of N2 is 28 g. 35 g N2 X 1 mol N2/28 g N2 = 1.25 mol N2 Mole ratio of NH3:N2 is 2:1 1.25 mol N2 X 2 mol NH3/1 mol N2 = 2.50 mol NH3 Molar mass of NH3 is 17.0 g. 2.50 mol NH3 X 17.0 g NH3/1 mol NH3 = 42.5 g NH3
The reaction between magnesium and hydrochloric acid produces hydrogen. If you increase the concentration of HCl, the reaction takes place faster. Could HCl be considered a catalyst for this reaction? Explain you answer.
No, HCl cannot be considered a catalyst for the reaction because it is a reactant (and is used up during the reaction). A catalyst is neither a reactant nor a product.
Explain why reactions take place faster at higher temperatures.
Reacting particles move faster on average at higher temperatures. As a result, they collide with each other more often and with greater energy, and the reaction rate increases.
What are five general types of reactions?
Synthesis reaction, decomposition reaction, single-replacement reaction, double-replacement reaction, and combustion reaction.
What five factors affect reaction rates?
Temperature, concentration, surface area, stirring, and catalysts
When you add baking soda to vinegar, the mixture fizzes as carbon dioxide gas is produced. Suppose you added water to the vinegar before you mixed it with the baking soda. What do you think would happen to the rate of carbon dioxide production?
The CO2 production would occur at a slower rate because you have diluted the vinegar. Decreasing the concentration of a reactant decreases the reaction rate.
Is the combustion of propane endothermic or exothermic?
The combustion of propane is exothermic because it releases energy.
How did the discovery of subatomic particles affect the classification of reactions?
The discovery of subatomic particles allowed scientists to classify certain reactions as transfers of electrons between atoms.
What is the law of conservation of mass?
The law of conservation of mass states that mass is neither created nor destroyed.
What does the double-ended arrow indicate in the following chemical equation? CaCo3 <=> Ca²⁺ + CO3²⁻
The reaction is reversible, meaning that both the forward and reverse reactions are taking place at the same time.
For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete? Explain. Mg + O2 -> 2MgO Hf + H2O <=> H3O⁺ + F⁻
The second reaction. The double-ended arrow indicates a reaction that reaches equilibrium. This means that both reactants and products are present when the reaction appears to be complete.
Once a chemical reaction has reached equilibrium, how does the system respond to change?
The system shifts in the direction that relieves the change.
When propane reacts with oxygen, does the surrounding area become warmer or cooler?
Warmer. An exothermic reaction releases heat to the surrounding area.
The following equation describes how sodium and chlorine react to produce sodium chloride. 2Na + Cl2 -> 2NaCl Is the equation balanced? Explain your answer
Yes, there are two sodium atoms and two chlorine atoms on each side of the equation.
Is the reaction represented by the following equation a redox reaction? Explain your answer 2Hg + O2 -> 2HgO
Yes, this is a redox reaction because electron transfer takes place. The charge of mercury charges from 0 to 2+ (oxidation). The charge of oxygen changes from 0 2- (reduction).
Platinum is a catalyst for the decomposition of hydrogen peroxide into water and oxygen. Pt 2H2O2 -> 2H2O + O2 What would you expect to see if platinum were added to hydrogen peroxide solution?
You would see oxygen bubbles because platinum catalyzes the reaction.
Identify these reactions as synthesis, decomposition, single replacement, double replacement, or combustion. a. Pb(NO3)2 + 2HCl -> PbCl2 + 2HNO3 b. 2C2H6 + 7O2 -> 4CO2 +6H2O c. Ca + 2HCl -> CaCl2 + H2 d. 2SO2 + O2 -> 2SO3
a. Double replacement b. Combustion c. Single replacement d. Synthesis (and combustion)
