Chem 101 Exam 4

question

If the heat of melting of a sample is 135 J/g, what would the heat of freezing for the sample be?
answer

-135 J/g
question

What is an endothermic reaction?
answer

all of the above
question

What is a reaction rate?
answer

It is the speed at which reactants are consumed or product is formed.
question

What is the gas found within a bubble of boiling water?
answer

water vapor
question

Why does a glowing splint of wood burn only slowly in air, but rapidly in a burst of flames when placed in pure oxygen?
answer

There is a greater number of collisions between the wood and oxygen molecules.
question

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
answer

1
question

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?
answer

48.00 grams
question

Why does evaporation cool a liquid?
answer

The departing gas molecule takes some kinetic energy with it as it leaves the surface.
question

An adhesive force is best described as
answer

attraction between two different molecules
question

Pictures for question . . For the above energy profiles, which reaction has the highest activation energy?
answer

D
question

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?
answer

The extreme pressure exerted by the miles of ocean water above prevent the formation of any bubbles of water vapor
question

How many oxygen molecules are needed to make 10 carbon dioxide molecules according to the following balanced chemical equation? 2 CO + O2 → 2 CO2
answer

5
question

Unlike fresh water, ocean water contracts as it is cooled all the way down to its freezing point, which is about -18°C. Why?
answer

The presence of a solute disrupts the rate of ice microcyrstal formation.
question

What is the formula mass of sulfur dioxide, SO2?
answer

about 64 amu
question

Why does water expand when it goes from a liquid to a solid?
answer

The water molecules become aligned in a way that creates extra volume.
question

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?
answer

ammonia, 4.70 J/(g°C)
question

What is the mass of one mole of H2?
answer

2 g
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What is an exothermic reaction?
answer

none of the above
question

A cohesive force is best described as the
answer

attraction between two liquid molecules
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Why does evaporation of a liquid also cool the gas above it?
answer

The gas molecules need to transfer their kinetic energy to the new slower gas molecule
question

Does soap increase or decrease water’s surface tension?
answer

decreases
question

Heat is absorbed when a substance
answer

evaporates
question

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?
answer

180 J
question

Picture for question . . .Given the following energy profiles, which of the following reactions is endothermic? R= reactants P= products
answer

D
question

How does formula mass differ from atomic mass?
answer

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.
question

Why does sweat cool you off?
answer

The water molecules leaving the skin as vapor absorb the heat from your skin.
question

What is a chemical reaction?
answer

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?
answer

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.
question

Why does a paper clip float when placed on water?
answer

The molecules of water hold together and keep it from sinking.
question

Which statement best describes what is happening at the surface of liquid water?
answer

all of the above
question

For the following balanced equation, which has the highest coefficient? 4 H2 + 2 C → 2 CH4
answer

H2
question

At the molecular level, when a piece of ice is slowly melting, water molecules are being
answer

both A and B
question

Given the following generic chemical reaction, which is the product? X → Y
answer

Y is the product.
question

If you keep a mixture of water and ice at 0°C what will happen?
answer

Nothing, the ice and water will stay the same
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What is a chemical equation?
answer

It is a shorthand notation for illustrating a chemical reaction.
question

Balance the following equation. ____ NO → ____ N2O + ____ NO2
answer

3, 1, 1
question

Why does a lake freeze from the top down?
answer

As cold water cools towards freezing it gets less dense.
question

Which of the following contains the most heat energy?
answer

1 gram of steam at 100°C
question

How does a catalyst increase the rate of a reaction?
answer

It lowers the activation energy.
question

What is the activation energy?
answer

the minimum amount of energy to break the bonds in reactants

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