Exothermic and Endothermic Reactions Flashcards
If a reaction transfers energy to the
surroundings, what must be the case
If a reaction transfers energy to the
surroundings the product molecules must have less energy than the reactants, by the amount transferred.
define exothermic reaction
An exothermic reaction is a reaction that transfers energy from the reacting molecules to the surroundings so the temperature of the surroundings increases
describe and explain what happens/the change in the surroundings when an exothermic reaction occurs
the temperature of the surroundings increases- in an exothermic reaciton because these reactions release/transfer energy to the surroundings
define endothermic reactions
Endothermic reactions are reactions that take in energy from the surroundings so the temperature of the surroundings decreases
describe and explain what happens/the change in the surroundings when an endothermic reaction occurs
the temperature of the surroundings decreases in an endothermic reaction because these reactions take in energy from the surroundings
Exothermic reactions include:
Combustion (burning)
Many oxidation reactions
Neutralisation reactions
Examples of endothermic reactions
thermal decompositions and the
reaction of citric acid and sodium hydrogencarbonate
everyday uses of exothermic reactions
Self-heating cans e.g. containing food or drink
hand warmers
every day uses of endothermic reactions
Some sports
injury packs are based on endothermic reactions.
draw the energy profile diagram for an exothermic reaction
https://bam.files.bbci.co.uk/bam/live/content/zpnwpbk/medium
y axis - energy
x - axis - course of reaction
Describe and explain the shape for an energy profile diagram for exothermic reaction
The products have less energy than the reactants..
This is because energy has been transferred from the reaction (reacting particles) to the surroundings
What does the difference between the energy of the reactants and the energy of the products tell us in an exothermic reaction
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
Describe and explain the shape for an energy profile diagram for endothermic reaction
The products have more energy than the reactions
This is because energy has been taken in from the suroundings
What does the difference between the reactants and the products tell you - in an energy profile diagram for an endothermic reaction
The difference in energy between the reactants, and the products tells us the amount of energy that has been taken in by the reaction
draw the energy profile diagram for an endothermic reaction
https://bam.files.bbci.co.uk/bam/live/content/zgkfk2p/medium
y axis - energy
x -axis - course of rection
What does the peak show in the energy profile diagrams for both endothermic and exothermic reactions
This is called the activation energy
what is activation energy
the minimum amount of energy that particles must have in order to react
Show the activation energy on the energy profile diagram
activation energy is from the reactants to the peak of the curve
Explain why exothermic energy changes are shown as negative
Exothermic energy changes are shown as negative, because energy has been transferred to from the chemicals (to the surroundings)
Explain why endothermic energy changes are shown as negative
Endothermic energy changes are shown as positive because energy is gained from the surroundings
Describe what process is bond formation and breaking bond
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)
Every chemical bond has an energy value
What does this show
Every chemical bond has an energy value
This tells us the energy required to break that bond
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
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
Temp. changes practical
Describe how to investigate the variables that affect temperature changes in chemical reactions
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
Results from the Describe how to investigate the variables that affect temperature changes in chemical reactions experiment
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
Explain the reason for using a polystyrene cup with a lid
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
