C6 (RG) - The Rate And Extent Of Chemical Change 📚

Question 1

Formula for mean rate of reaction (2)

Answer 1

• Quantity of reaction used divided by time taken
Or
• Quantity of product formed divided by time taken

Question 2

What is the quantity of reactant or product measured by (2)

Answer 2

• grams

* volume cm(3)

Question 3

What are the units for rate of reaction (2)

Answer 3

• g/s

* cm(3)/s

Question 4

Ways to interpret graphs showing the quantity of product formed against time (3)

Answer 4

• The steeper the line on the graph, the faster the rate of reaction
• Over time the line becomes less steep as the reactants are used up
• The quickest reactions have the steepest lines and become flat in the least time

Question 5

Factors which affect the rate of chemical reactions (4)

Answer 5

• Temperature
• The concentration of a solution or pressure of gas
• Surface area of the solids
• The presence of a catalyst

Question 6

How does increasing temperature increase the rate of chemical reactions (3)

Answer 6

• When temperature increases the particles move faster
• Because they move faster they collide more frequently
• The faster they move the more energy they have, so the more collisions will have enough energy to make the reaction happen

Question 7

How does increasing the concentration or gas increase the rate of reaction (3)

Answer 7

• The more concentrated a solution is the mire particles knocking about in the same volume of water
• Similarly, when the pressure of a gas has increased, it means that the same number of particles occupy a smaller space
• This makes collisions between the reactant particles more frequent

Question 8

How does increasing surface area increase the rate of reaction (2)

Answer 8

• If one reactant is a solid, breaking it up into smaller pieces will increase its surface area to volume ration
• This means that for the same volume of the solid, the particles around it will have mire area to work on - so there will be more collisions more frequently

Question 9

How does using a catalyst increase the rate (3)

Answer 9

• A catalyst is a substance which speeds up a reaction without being used up in the reaction itself - not part of the overall reaction itself
• They all work by decreasing the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy

Question 10

Required practical - investigates how changes in concentration affect the rates of reactions (method) (4)

Answer 10

1) Start by adding a set volume of dilute sodium thiosulfate to a conical flask
2) Place the flask on a piece of paper with a black cross drawn on it. Add some dilute HCl to the flask and start the stopwatch
3) Now watch the Black cross disappear through the cloudy sulfur and time how long it takes to go
4) This can be repeated using different concentrations

Question 11

Required practical - investigates how changes in concentration affect the rates of reactions (results) (2)

Answer 11

• These results show the effect of increasing the concentration of HCl on the rate of reaction
• The higher the concentration the quicker the reaction therefore less time it takes for the mark to disappear

Question 12

Collision theory (3)

Answer 12

The rate of chemical reaction depends on:
• The collision frequency of reacting particles. The more collisions there are the faster the reaction is
• The energy transferred during a collision. Particles have to collide with enough energy for the collision to be successful

Question 13

Reaction profile for a catalysed reaction

Answer 13

Draw and check on Chegg flashcards

Question 14

Equation which shows a reversible reaction

Answer 14

A + B <=> C + D

Check photos

Question 15

Example of a reversible reaction being changed by the conditions

Answer 15

Ammonium chloride  (heat) ammonia + hydrogen chloride  
(Check photos)

Question 16

Example of a reversible reaction being exothermic in one direction and endothermic in the other but the same amount of energy is transferred

Answer 16

Hydrated copper sulfate (blue) (endothermic) Anhydrous copper sulfate (white) + water
(Check photos)

Question 17

When does an equilibrium occur in a reversible reaction

Answer 17

• When the forward and reverse reactions occur at exactly the same rate

Question 18

Conditions which the position of equilibrium depends on (3)

Answer 18

• Temperature
• Pressure
• Concentration

Question 19

Le Chatelier’s Principle

Answer 19

Is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

Question 20

What is the effect of changes in temperature on systems at equilibrium (2)

Answer 20

• If you decrease the temperature, the equilibrium will move in the exothermic reaction to produce more heat - meaning more products for the exothermic reaction and fewer products for the endothermic reaction
• If you raise the temperature, the equilibrium will move in the endothermic direction to try and decrease it - meaning more products for the endothermic reaction and fewer products for the exothermic reaction

Question 21

What is the effect of changes in pressure on systems at equilibrium (2)

Answer 21

• If you increase the pressure, the equilibrium tries to reduce it - moves in the direction where there are fewer molecules of gas
• If you decrease the pressure, the equilibrium tries to increase it - moves in the direction where there are more molecules of gas

Question 22

What is the effect of changes in concentration on systems at equilibrium (4)

Answer 22

• If you change the concentration of either the reactants or the products, the system will no longer be at an equilibrium
• System responds by bringing itself back to the equilibrium
• if you increase the concentration of the reactants the system tries to decrease it by making more products
• If you decrease the concentration of products the system tries to increase it again by reducing the amount of reactants