Chem 101 Exam 4
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If the heat of melting of a sample is 135 J/g, what would the heat of freezing for the sample be?
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-135 J/g
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What is an endothermic reaction?
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all of the above
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What is a reaction rate?
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It is the speed at which reactants are consumed or product is formed.
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What is the gas found within a bubble of boiling water?
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water vapor
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Why does a glowing splint of wood burn only slowly in air, but rapidly in a burst of flames when placed in pure oxygen?
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There is a greater number of collisions between the wood and oxygen molecules.
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According to the following balanced chemical equation, if you want to generate two moles of H2O, how many moles of O2 do you need? 2 H2 + O2 → 2 H2O
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1
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Small samples of oxygen gas needed in the laboratory can be generated by any number of simple chemical reactions, such as 2 KClO3 (s) → 2 KCl (s) + 3 O2 (g) What mass of oxygen (in grams) is produced when 122.6 g of KClO3 (formula mass = 122.6 amu) takes part in this reaction?
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48.00 grams
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Why does evaporation cool a liquid?
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The departing gas molecule takes some kinetic energy with it as it leaves the surface.
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An adhesive force is best described as
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attraction between two different molecules
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Pictures for question . . For the above energy profiles, which reaction has the highest activation energy?
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D
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Water coming out of volcanic sea vents at the bottom of the ocean can reach temperatures in excess of 300°C without boiling. How is this possible?
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The extreme pressure exerted by the miles of ocean water above prevent the formation of any bubbles of water vapor
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How many oxygen molecules are needed to make 10 carbon dioxide molecules according to the following balanced chemical equation? 2 CO + O2 → 2 CO2
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5
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Unlike fresh water, ocean water contracts as it is cooled all the way down to its freezing point, which is about -18°C. Why?
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The presence of a solute disrupts the rate of ice microcyrstal formation.
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What is the formula mass of sulfur dioxide, SO2?
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about 64 amu
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Why does water expand when it goes from a liquid to a solid?
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The water molecules become aligned in a way that creates extra volume.
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Given the following specific heats and that each sample had the same mass, which would require the greatest amount of heat energy to increase its temperature 50°C?
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ammonia, 4.70 J/(g°C)
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What is the mass of one mole of H2?
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2 g
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What is an exothermic reaction?
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none of the above
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A cohesive force is best described as the
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attraction between two liquid molecules
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Why does evaporation of a liquid also cool the gas above it?
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The gas molecules need to transfer their kinetic energy to the new slower gas molecule
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Does soap increase or decrease water's surface tension?
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decreases
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Heat is absorbed when a substance
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evaporates
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If the specific heat of aluminum is 0.90 J/(g°C), how much heat is needed to raise the temperature of a 10.0-gram piece by 20.0°C?
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180 J
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Picture for question . . .Given the following energy profiles, which of the following reactions is endothermic? R= reactants P= products
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D
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How does formula mass differ from atomic mass?
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The formula mass of a substance is the sum of the atomic masses of the elements is its chemical formula. The atomic mass is the mass of a single atom.
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Why does sweat cool you off?
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The water molecules leaving the skin as vapor absorb the heat from your skin.
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What is a chemical reaction?
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when one or more new compounds are formed by rearranging atoms
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If it takes energy to break bonds and you gain energy in the formation of bonds, how can some reactions be exothermic while others are endothermic?
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It is the total amount of energy that matters. Sometimes some bonds are stronger than others and so you gain or lose energy when you form them.
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Why does a paper clip float when placed on water?
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The molecules of water hold together and keep it from sinking.
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Which statement best describes what is happening at the surface of liquid water?
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all of the above
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For the following balanced equation, which has the highest coefficient? 4 H2 + 2 C → 2 CH4
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H2
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At the molecular level, when a piece of ice is slowly melting, water molecules are being
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both A and B
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Given the following generic chemical reaction, which is the product? X → Y
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Y is the product.
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If you keep a mixture of water and ice at 0°C what will happen?
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Nothing, the ice and water will stay the same
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What is a chemical equation?
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It is a shorthand notation for illustrating a chemical reaction.
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Balance the following equation. ____ NO → ____ N2O + ____ NO2
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3, 1, 1
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Why does a lake freeze from the top down?
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As cold water cools towards freezing it gets less dense.
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Which of the following contains the most heat energy?
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1 gram of steam at 100°C
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How does a catalyst increase the rate of a reaction?
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It lowers the activation energy.
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What is the activation energy?
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the minimum amount of energy to break the bonds in reactants
