Exothermic and Endothermic Reactions

Question 1

If a reaction transfers energy to the
surroundings, what must be the case

Answer 1

If a reaction transfers energy to the
surroundings the product molecules must have less energy than the reactants, by the amount transferred.

Question 2

define exothermic reaction

Answer 2

An exothermic reaction is a reaction that transfers energy from the reacting molecules to the surroundings so the temperature of the surroundings increases

Question 3

describe and explain what happens/the change in the surroundings when an exothermic reaction occurs

Answer 3

the temperature of the surroundings increases- in an exothermic reaciton because these reactions release/transfer energy to the surroundings

Question 4

define endothermic reactions

Answer 4

Endothermic reactions are reactions that take in energy from the surroundings so the temperature of the surroundings decreases

Question 5

describe and explain what happens/the change in the surroundings when an endothermic reaction occurs

Answer 5

the temperature of the surroundings decreases in an endothermic reaction because these reactions take in energy from the surroundings

Question 6

Exothermic reactions include:

Answer 6

Combustion (burning)
Many oxidation reactions
Neutralisation reactions

Question 7

Examples of endothermic reactions

Answer 7

thermal decompositions and the
reaction of citric acid and sodium hydrogencarbonate

Question 8

everyday uses of exothermic reactions

Answer 8

Self-heating cans e.g. containing food or drink
hand warmers

Question 9

every day uses of endothermic reactions

Answer 9

Some sports
injury packs are based on endothermic reactions.

Question 10

draw the energy profile diagram for an exothermic reaction

Answer 10

https://bam.files.bbci.co.uk/bam/live/content/zpnwpbk/medium

y axis - energy
x - axis - course of reaction

Question 11

Describe and explain the shape for an energy profile diagram for exothermic reaction

Answer 11

The products have less energy than the reactants..
This is because energy has been transferred from the reaction (reacting particles) to the surroundings

Question 12

What does the difference between the energy of the reactants and the energy of the products tell us in an exothermic reaction

Answer 12

the difference between the energy of the reactants and the energy of the products tells us the energy being that has been released to the surroundings

Question 13

Describe and explain the shape for an energy profile diagram for endothermic reaction

Answer 13

The products have more energy than the reactions
This is because energy has been taken in from the suroundings

Question 14

What does the difference between the reactants and the products tell you - in an energy profile diagram for an endothermic reaction

Answer 14

The difference in energy between the reactants, and the products tells us the amount of energy that has been taken in by the reaction

Question 15

draw the energy profile diagram for an endothermic reaction

Answer 15

https://bam.files.bbci.co.uk/bam/live/content/zgkfk2p/medium

y axis - energy
x -axis - course of rection

Question 16

What does the peak show in the energy profile diagrams for both endothermic and exothermic reactions

Answer 16

This is called the activation energy

Question 17

what is activation energy

Answer 17

the minimum amount of energy that particles must have in order to react

Question 18

Show the activation energy on the energy profile diagram

Answer 18

activation energy is from the reactants to the peak of the curve

Question 19

Explain why exothermic energy changes are shown as negative

Answer 19

Exothermic energy changes are shown as negative, because energy has been transferred to from the chemicals (to the surroundings)

Question 20

Explain why endothermic energy changes are shown as negative

Answer 20

Endothermic energy changes are shown as positive because energy is gained from the surroundings

Question 21

Describe what process is bond formation and breaking bond

Answer 21

When a chemical bond is broken, this requires energy (bond breaking is therefore an endothermic process)

Making bonds releases energy (bond formation is an exothermic process)

Question 22

Every chemical bond has an energy value
What does this show

Answer 22

Every chemical bond has an energy value
This tells us the energy required to break that bond

Question 23

Hydrogen + chlorine —> hydrogen chloride
H2 + Cl2 —> 2HCl
Calculate the energy required to break the bonds in the reactants
The energy released when the bonds in products are made
The energy change for the reaction
State whether the reaction is exothermic or endothermic
Explain why the reaction is exothermic or endothermic in terms of bond breaking and making

H-H = 436 kJ/mol
Cl - Cl 242 kJ/mol
H-Cl - 431 kJ/mol

Answer 23

Break
H-H = +436
Cl-Cl = +242
Total = 678

Make = H-Cl = 431
431 x 2 = -862

Energy change = break - make
= 678 - 732 = -184 kJ

Exothermic
More energy is released making bonds than the energy (absorbed) needed to break bonds

if it was endothermic
More energy is absorbed to break the bonds than released when new bonds are formed

if answer = 0
Neither exothermic nor endothermic
Same amount of energy released making bonds as is needed to break bonds

Question 24

Temp. changes practical
Describe how to investigate the variables that affect temperature changes in chemical reactions

Answer 24

In this reaction we are investigating the temperature change in an exothermic reaction

This is the neutralisation reaction between hydrochloric acid and the alkali sodium hydroxide

We are going to add increasing volumes of NaOH solution to the hydrochloric acid, and in each experiment we will measure the maximum temperature reached

Independent variable - volume of NaOH solution
Dependent variable - maximum temperature reached
Control variable - volume of HCl and the concentrations of both the hydrochloric acid and the sodium hydroxide solution

Use a measuring cylinder to measure 30cm3 of dilute hydrochloric acid

Transfer the acid into a polystyrene cup
Stand the polystyrene cup inside a beaker - this stops the cup from falling over

Use a thermometer to measure the temperature of the acid, record this in a table

Use a measuring cylinder to measure 5cm3 of NaOH solution and transfer this to the polystyrene cup

At this point we fit a plastic lid to the cup and place the thermometer through the hole in the lid
The bulb of thermometer must be in the solution

Use the thermometer to gently stir the solution

Because this reaction (a neutralisation reaction) is exothermic - it releases energy
This means that the temp. of the solution will increase

Look carefully at the temperature rise on the thermometer

When the reading on the thermometer stops changing we record the highest temperature reached

Rinse out and dry the polystyrene cup
At this point repeat the whole experiment using 10cm3 of sodium hydroxide solution

Carry out the experiment several more times
Each time increase the volume of NaOH solution by 5cm3 until we reach a maximum of 40cm3 of NaOH solution

At this point we repeat the whole experiment one more time so that we have two sets of results

We can use these results to calculate a mean value for the maximum temp. reached for each volume of NaOH solution

At the end we can plot a graph of our results

Question 25

Results from the Describe how to investigate the variables that affect temperature changes in chemical reactions experiment

Answer 25

As the volume of NaOH solution increases, the max. temp. reached increases

This is because when we add more particles of NaOH, they react with they HCl

This is an exo. reaction so more energy is released and the max. temp. reached increases

However at a certain volume of sodium hydroxide solution, the max. temp reached starts to decrease

At this point we are now adding so much sodium hydroxide that there is not enough hydrochloric acid

This means that some of the NaOH is unable to react
So because of this, the amount of energy released by the reaction has reached a maximum

The reason why the maximum temperature actually decreases:
we are adding a greater volume of solution in each experiment, so the energy released is now spread out into a greater volume
Because of this when we add large volumes of NaOH solution, the maximum temp. reached actually decreases

Question 26

Explain the reason for using a polystyrene cup with a lid

Answer 26

In this experiment we are measuring the temperature
This means that we want to reduce any heat losses

Polystyrene is a good thermal insulator so this reduces heat loss through the sides and the bottom

The lid reduces heat loss to the air