C6 + C7 - Energy changes and Chemical reactions

Question 1

Why and how do chemical reactions occur?

Answer 1

Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons
* done by chemical bonding, where old bonds are broken and new bond are formed
* it involvs the transfer of thermal energy into and out of reaction mixtures

Question 2

Exothermic reactions

Answer 2

Thermal energy is transferred to the surroundings so the temperature of the surroundings increases
* energy is transferred from the chemical energy store of the chemical system to the surroundings and so the energy of the system falls

Question 3

Examples of exothermic reactions (name 2)

Answer 3

• Combustion
• Oxidation
• Neutralisation

Question 4

Endothermic reactions

Answer 4

Thermal energy is taken in from the surroundings so the temperature of the surroundings decreases
* energy is transferred to the chemical energy store of the chemical system and so the energy of the system increases - this means the energy change is positive

Question 5

Examples of endothermic reactions

Answer 5

• Electrolysis
• Thermal decomposition
• First stages of photosynthesis

Question 6

Reaction pathway diagrams

Answer 6

Graphical representations of the relative energies of the reactants and products in chemical reactions

Question 7

Energy level diagram properties (on both exo. and endo. reactions)

Answer 7

• Y-axis: energy of the reactants and products
• X-axis: reaction pathways
• Overall energy change: the difference in height between the energy of reactants and products

Question 8

Exothermic reaction pathways diagram

Answer 8

• If more energy is released than is absorbed, then the reaction is exothermic
• More energy is released when new bonds are formed than energy required to break the bonds in the reactants
• The change in energy is negative since the products have less energy than the reactants
• Therefore an exothermic reaction has a negative ΔH value

Question 9

Endothermic reaction pathway diagrams

Answer 9

• If more energy is absorbed to break bonds than is released to form new bonds, this reaction is endothermic overall
• The change in energy is positive since the products have more energy than the reactants
• Therefore an endothermic reaction has a positive ΔH value, which is shown on the energy level diagrams and in calculations

Question 10

Activation enegry

Answer 10

The initial increase in energy represents the activation energy (Ea), which is the minimum energy that colliding particles must have in order to react

Question 11

Bond breaking

Answer 11

Endothermic process

Question 12

Bond forming

Answer 12

Exothermic process

Question 13

Enthalpy change formula

Answer 13

Enthalpy change (ΔH) = Energy taken in - Energy given out

Question 14

Physical change

Answer 14

Physical changes (such as melting or evaporating) do not produce any new chemical substances
* easy to reverse

Question 15

Chemical change

Answer 15

During chemical changes, new chemical substances are formed that have very different properties to the reactants
* usually difficult to reverse

Question 16

Rates of reaction factors (4)

Answer 16

• Concentration of the reactants in solution or the pressure of reacting gases
• Temperature at which the reaction is carried out
• Surface area of solid reactants
• The use of a catalyst

Question 17

How does an increased concentration or pressure affect reaction rate?

Answer 17

With increased concentration of a solution or increased pressure of a gas, the rate of reaction will increase

Question 18

How does surface area affect reaction rate?

Answer 18

With increased surface area of the solid, the rate of reaction will increase

Question 19

How does temperature affect reaction rate?

Answer 19

With increased temperature, the rate of reaction will increase

Question 20

How does a catalyst affect reaction rate?

Answer 20

With a catalyst, the rate of reaction will increase

Question 21

Collision theory

Answer 21

Collision theory states that in order for a reaction to occur:
* The particles must collide with each other
* The collision must have sufficient energy to cause a reaction i.e. enough energy to break bonds

Question 22

Successful collisions

Answer 22

Collisions which result in a reaction
* If they have sufficient energy (i.e. energy greater than the activation energy), they will react, and the collision will be successful

Question 23

Unsuccessful collisions

Answer 23

When the colliding species do not have enough energy to break the necessary bonds (i.e. they collide with energy less than the activation energy)

Question 24

Factors affecting number of successful collision (4)

Answer 24

• Number of particles per unit volume
• Frequency of collisions
• Kinetic energy of particles
• Activation energy - fewer collisions will have an energy that exceeds higher activation energy and fewer collisions will be successful

Question 25

Process of investigating the effect of surface area on the rate of reaction (5 steps)

Answer 25

1. Add dilute hydrochloric acid into a conical flask
2. Use a delivery tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement)
3. Add calcium carbonate chips into the conical flask and quickly put the bung back into the flask
4. Measure the volume of gas produced in a fixed time using the measuring cylinder
5. Repeat with different sizes of calcium carbonate chips (lumps, crushed and powdered)

Question 26

Results of investigating the effect of surface area on the rate of reaction

Answer 26

Smaller sizes of chips cause an increase in the surface area of the solid, so the rate of reaction will increase
* This is because more surface area of the particles will be exposed to the other reactant so there will be more frequent and successful collisions, increasing the rate of reaction

Question 27

Process of investigating the effect of concentration of a solution on the rate of reaction (6 steps)

Answer 27

1. Measure 50 cm3 of sodium thiosulfate solution into a flask
2. Measure 5 cm3 of dilute hydrochloric acid into a measuring cylinder
3. Draw a cross on a piece of paper and put it underneath the flask
4. Add the acid into the flask and immediately start the stopwatch
5. Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
6. Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)

Question 28

Results of investigating the effect of concentration of a solution on the rate of reaction

Answer 28

• With an increase in the concentration of a solution, the rate of reaction will increase
• This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction

Question 29

Process of investigating the effect of temperature on the rate of reaction (5 steps)

Answer 29

1. Dilute hydrochloric acid is heated to a set temperature using a water bath
2. Add the dilute hydrochloric acid into a conical flask
3. Add a strip of magnesium and start the stopwatch
4. Stop the time when the magnesium fully reacts and disappears
5. Repeat at different temperatures and compare results

Question 30

Results of investigating the effect of temperature on the rate of reaction

Answer 30

With an increase in the temperature, the rate of reaction will increase
* This is because the particles will have more kinetic energy than the required activation energy, therefore more frequent and successful collisions will occur, increasing the rate of reaction

Question 31

Process of investigating the effect of a catalyst on the rate of reaction (5 steps)

Answer 31

1. Add hydrogen peroxide into a conical flask
2. Use a delivery tube to connect this flask to a measuring cylinder upside down in a tub of water (downwards displacement)
3. Add the catalyst manganese(IV) oxide into the conical flask and quickly place the bung into the flask
4. Measure the volume of gas produced in a fixed time using the measuring cylinder
5. Repeat experiment without the catalyst of manganese(IV) oxide and compare results

Question 32

Results of investigating the effect of a catalyst on the rate of reaction (5 steps)

Answer 32

• Using a catalyst will increase the rate of reaction
• The catalyst will provide an alternative pathway requiring lower activation energy so more colliding particles will have the necessary activation energy to react
• This will allow more frequent and successful collisions, increasing the rate of reaction