Topic 6-The Rate and Extent of Chemical Changes

Question 1

How can rate of reaction be calculated?

Answer 1

Rate of reaction=amount of reactant used/time

Rate of reaction=amount of product formed/time

Question 2

What are the factors that affect the rates of chemical reactions?

Answer 2

Concentration
Pressure
Surface area
Temperature
Catalysts

Question 3

What is the collision theory?

Answer 3

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy

Question 4

What are catalysts?

Answer 4

They are substances that speed up chemical reactions without being changed or used up during the reaction

Question 5

What do catalysts do?

Answer 5

They decrease the activation energy which increases the proportion of particles with energy to react

Catalysts provide a different pathway with a lower activation energy for a chemical reaction

Question 6

What are three ways of measuring rate of reaction?

Answer 6

• Loss in mass of reactants
• Volume of gas produced
• Time for a solution to become opaque

Question 7

Describe and explain the effect of increasing temperature on the rate of reaction

Answer 7

As the temperature increases, kinetic energy of the particles increases (more energetic collisions)

If they’re moving faster, they will collide more frequently

They are not directly proportional to each other

Question 8

Describe and explain the effect of increasing concentration on the rate of reaction

Answer 8

If the concentration increases, there will also be more reactants meaning more frequent collisions meaning a faster reaction

Question 9

Describe and explain the effect of increasing pressure of a gas on the rate of reaction

Answer 9

If you increase the pressure of reacting gases, it increases the number of gas molecules in the same volume and so it increases the frequency of collisions and therefore increases the rate of reaction

(Volume and pressure are inversely proportional to each other-increasing the volume slows down the reaction)

Question 10

Describe and explain the effect of increasing surface area

Answer 10

If solid reactants are in smaller pieces, they have a greater surface area.

Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction

Question 11

When is dynamic equilibrium reached?

Answer 11

In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant

Question 12

Describe Le Chatelier’s Principle

Answer 12

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.

Question 13

What happens to the equilibrium when you change the concentration?

Answer 13

Increase-shifts to the opposite side

Decrease-shifts to the same side

Question 14

What happens to the equilibrium when you change the pressure?

Answer 14

Increase-shifts to side with less moles

Decrease-shifts to side with more moles

Question 15

What happens to the equilibrium when you change the temperature?

Answer 15

Increase-Shifts in endothermic direction

Decrease- Shifts in exothermic direction

Question 16

Why does adding a catalyst not affect the equilibrium?

Answer 16

It just speeds up both forward and backward reactions equally. Equilibrium is achieved faster

Question 17

What are the various units for the rate of reaction?

Answer 17

Can include g/s or cm^3 or mol/s

Generally, mass/time, volume/time, moles/time

Question 18

Describe the method needed for the disappearing cross reaction

Answer 18

Use a measuring cylinder to put 10cm^3 of sodium thiosulfate solution into a conical flask

Place the conical flask onto a printed black cross

Next add 10^3 of hydrochloric acid into the conical flask

Swirl the solution and start a stopwatch

Look down the through the top of the flask-after a certain time, the solution will turn cloudy

Stop the clock when we can no longer see the cross

Carry out the experiment again using lower concentrations of sodium thiosulfate solution

Question 19

What should you do after you conduct the disappearing cross reaction?

Answer 19

Repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution

Do not include any anomalous results when calculating a mean

Question 20

What does it mean when a measurement is reproducible?

Answer 20

A measurement is reproducible if it can be repeated by another person or using a different technique or equipment and still get the same result

Question 21

What is the equation for the disappearing cross reaction?

Answer 21

Sodium thiosulfate+hydrochloric acid—>sulphur (solid)

Question 22

What does the sulphur do during the disappearing cross reaction?

Answer 22

The sulphur makes the solution go cloudy.

Scientists call this cloudiness turbidity

Question 23

What is a hypothesis?

Answer 23

A proposal that could explain a fact or an observation. In science, a hypothesis must be testable

Question 24

What is the problem when it comes to the disappearing cross method and how is it responded to?

Answer 24

The problem with the disappearing cross experiment is that different people have different eyesights

That means that some people can see the cross for longer than others so they may not get the same results

However because all the students use the same size printed cross, this problem may not be too great

Question 25

Describe the ‘measuring volume of gas’ method

Answer 25

Use a measuring cylinder to place 50cm^3 of hydrochloric acid into a conical flask

Attach the conical flask to a bung and delivery tube

Now place an upturned measuring cylinder also filled with water over the delivery tube

Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch. The reaction produced hydrogen gas which is trapped in the measuring cylinder

Every 10 seconds, measure the volume of hydrogen gas in the measuring cylinder. Continue until no more hydrogen is given off

Repeat the experiment using different concs of hydrochloric acid

Question 26

What is the equation for the ‘measuring volume of gas’ method?

Answer 26

Magnesium+hydrochloric acid—>magnesium chloride + hydrogen

Question 27

What is an enzyme?

Answer 27

An enzyme is a molecule that acts as a catalyst in a biological system

Question 28

What is a reversible reaction?

Answer 28

A reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants