C7 - Energy Changes Flashcards
What is activation energy?
Activation energy is the minimum energy needed for a reaction to take place
What is bond energy?
bond energy is the energy required to break a specific chemical bond
What is an endothermic reaction?
An endothermic reaction is a reaction that takin in energy from the surroundings
What is an exothermic reaction?
An exothermic reaction is a reaction that transfers energy to the surroundings
What are fuel cells?
sources of electricity that are supplied by an external source of fuel
What is the first law of thermodynamics?
Energy cannot be created or destroyed
What happens to the energy during a chemical reaction?
energy is conserved
What happens to the temperature of the surroundings during an exothermic reaction?
The temperature of the surroundings increases
What happens to the temperature of the surroundings during an endothermic reaction?
The temperature of the surroundings decreases
List 4 types of exothermic reaction:
- Neutralisation
- Oxidation, e.g. Respiration
- Displacement
- Combustion
Is Neutralisation exothermic or endothermic?
Exothermic
Is Oxidation exothermic or endothermic?
exothermic
Is respiration exothermic or endothermic?
exothermic
Is displacement exothermic or endothermic?
exothermic
Is combustion exothermic or endothermic?
exothermic
List 2 uses of exothermic reactions:
- Hand Warmers
* Gas Stove
List 3 types of endothermic reactions:
- Thermal Decomposition
- Electrical Decomposition
- Photosynthesis
What is 1 use of an endothermic reaction?
An instant cool pack
What is a reaction profile?
A diagram showing the change in chemical energy between the reactants and the product. (It includes labels for the activation energy and energy change)
What type of reaction does this reaction profile show?
Exothermic reaction
What type of reaction does this reaction profile show?
Endothermic
What happens to the energy of the reactants and products in an exothermic reaction?
Reactants have more chemical energy than the products
What happens to the energy of the reactants and products in an endothermic reaction?
The reactants have less chemical energy than the products
If a reaction starts at 21C and has a final temperature of 65C, what type of reaction is it?
Endothermic
If a reaction starts at 20C and has a final temperature of 14C, what type of reaction is it?
Exothermic
What is the 4 step process for drawing a reaction profile?
1) Label the axes (Energy + Progress of reaction)
2) Add the symbols for the reactants and products on the horizontal lines. Include state symbols. (i.e. NH₄Cl ,(aq) and NH₄(aq) + Cl (aq))
3) Label the overall energy change and include specific data (i.e. Overall energy change = +10KJ/Mol)
4) Add a curved line to represent the course of the reaction and label the activation energy
What is the unit for energy change?
J/Mol or KJ/Mol
Describe the method for investigating temperature changes during displacement:
1) Using a 50cm³ measuring cylinder, place 25cm³ of 1.0mol/dm³ Copper Sulphate into the polystyrene cup.
2) Stand the polystyrene cup inside the 250cm³ beaker
3) Replace the lid and insert the thermometer
4) Stir then record the initial temperature
5) Add 1.0g of Zinc into the polystyrene cup and immediately replace the lid
6) Stir using the thermometer
7) Keep stirring until the temperature reaches a maximum and begins to decrease
8) Repeat this experiment 2 more times and record data in appropriate table/graph
Describe the method for investigating temperature changes during neutralisation:
1) Pour 30cm³ of 2.0 mol/dm³ HCl into the polystyrene cup
2) Place the polystyrene cup in the 250cm³ beaker and replace lid
3) Stir using the thermometer and record the starting temperature
4) Add 5cm³ of NaOH into the polystyrene cup and instantly replace the lid
5) Stir the solution and keep stirring until the temperature reaches a maximum and begins to decrease (record maximum temperature)
6) Repeat steps 4-5 until 40-50cm³ of NaOH has been added
7) Repeat this experiment 2 more times and record data in appropriate table/graph
Does breaking bonds require or release energy?
Breaking bonds requires energy
Does making bonds require or release energy?
Making bonds releases energy
If a reaction has a total energy change of -482 KJ/mol, is it endothermic or exothermic?
Exothermic - more energy is released creating new bonds than is required to break reactant bonds
If a reaction has a total energy change of -1070 KJ/mol, is it endothermic or exothermic?
Exothermic - more energy is released creating new bonds than is required to break reactant bonds
If a reaction has a total energy change of +971 KJ/mol, is it endothermic or exothermic?
Endothermic - more energy is required to break reactant bonds than released making new bonds
How do you calculate the energy required to break reactant bonds?
Use the bond energy data to calculate the total bond energy of the reactants
How do you calculate the energy released making new bonds?
Use the bond energy data to calculate the total bond energy of the products
How do you calculate total energy change?
Energy required to break reactant bonds - Energy required to make new bonds = Total energy change
What is the energy change of a substance if it requires 3868 KJ/mol to break reactant bonds and releases 3992 KJ/mol making new bonds?
3868 - 3992 = -124KJ/mol
What is the energy change of a substance if it requires 3231KJ/mol to break reactant bonds and releases 2260 KJ/mol making new bonds?
3231 - 2260 = +971 KJ/mol
What is the energy change of a substance if it requires 2208 KJ/mol to break reactant bonds and releases 2796 KJ/mol making new bonds?
2208 - 2796 = -588 KJ/mol
If the total energy change is negative, what type of reaction is it?
Exothermic
If the total energy change is positive, what type of reaction is it?
Endothermic
What is the 4 step process to calculate energy change and whether a reaction is endo/exothermic?
1) Calculate energy required to break reactant bonds
2) Calculate energy released making new bonds
3) Calculate total energy change (break - make)
4) State whether it is endothermic or exothermic (endo is positive total energy change, exo is negative total energy change)
How do cells produce electricity?
They contain chemicals which react to produce electricity
What is the voltage produced by a cell dependent on?
The type of electrode and type of electrolyte
How can a simple cell be made?
by connecting 2 different metals in contact with and electrolyte
How do you increase the voltage produced by a cell?
Increase the difference in reactivity of the 2 metals
What is a battery?
2 or more cells connected together in series
How does a non-rechargeable battery work?
- Chemicals inside produce electricity
- Chemicals get used up and the battery goes flat (the voltage drops to 0) when the reactions inside stop because one of the reactants has been used up
How do rechargeable batteries work?
- Chemicals inside produce electricity
- Chemicals get used up but can be reproduced when recharged
- When recharged the chemical reactions can be reversed when connected to an external electric current
- The cell can be used over and over again
How do fuel cells work?
- They require a continuous supply of hydrogen and oxygen into the cell
- Inside chemical processes convert hydrogen and oxygen into water
- These have continuous fuel supply so do not go flat or need recharging
What are example uses of non-rechargeable cells?
*Alkaline cells (e.g. used in tv controls)
What are example uses of rechargeable cells?
*Lithium-ion cells (e.g. those used in mobile phones or laptops)
What are example uses of fuel cells?
*Hydrogen fuel cell (e.g. cars, spacecraft)
What are advantages of non-rechargeable cells?
*Cheap to produce
What are disadvantages of non-rechargeable cells?
- disposal of many cells (which contain toxic chemicals)
* Has a finite life
What are advantages of rechargeable cells?
- Can be used many times
- Cheaper than fuel cells
- No emissions
- Better for environment as they do not create as much waste
- No dangerous fuels are required
- Produce a greater voltage than hydrogen fuel cells
What are disadvantages of rechargeable cells?
- Takes time to recharge
- More expensive than non-rechargeable cells
- Has a finite life, they get less efficient and can store less energy the more times they are recharged and eventually have to be replaced
What are advantages of fuel cells?
- The only waste product is water (no pollutants)
- Continuous supply of electricity (as long as there is a continuous supply of fuel)
- Do not need to be recharged
- They do not get less efficient the longer they run
- They can be used a source of water (i.e. on a spacecraft)
What are disadvantages of fuel cells?
- Very expensive
- Often use hydrogen which is flammable
- Fossil fuels may be used to produce hydrogen
- Catalyst used in cell eventually poisoned
- Hydrogen can be difficult to store
- Produce a relatively low voltage so many are needed to be used together
What are more common, exothermic or endothermic reactions?
exothermic reactions
What type of process is breaking chemical bonds?
endothermic process
What type of process is making chemical bonds?
Exothermic
What are bond energies measured in?
KJ/mol
The greater the difference in reactivity between 2 metals used in a cell ……………………………….
The greater the voltage produced
What is voltage?
potential difference
What does the potential difference rely on?
the difference in reactivity between the 2 metals in a cell, and the electrolyte
What is an electrolyte?
An electrolyte is a solution that can conduct electricity
Why are batteries used over single cells?
They have more voltage than single cells
What happens if you use metals with the same reactivity in a cell?
No electricity will be produced
What is the equation for the reaction within a hydrogen fuel cell?
2H₂ + O₂ -> 2H₂O
What is the half equation at the positive electrode in a hydrogen fuel cell?
O2 + 2H₂O + 4e⁻ → 4OH⁻
What is the half equation at the negative electrode in a hydrogen fuel cell?
2H₂ + 4OH⁻ → 4H₂O + 4e⁻
Explain in detail the reaction that happens within a hydrogen fuel cell
Hydrogen gas is supplied as a fuel to the negatively charged electrode. It diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit. Oxygen gas is supplied to the positively charged electrode. It diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from the external circuit. If you add the 2 electrode reactions together, the electrons and the OH⁻ ions on either side of the equations cancel out. So you are left with the overall charge in the hydrogen fuel cell, that is the oxidation of hydrogen: 2H₂ + O₂ -> 2H₂O.
What happens to the temperature of the surroundings during an exothermic reaction?
Increase
What happens to the temperature of the reaction during an exothermic reaction?
Decrease
What happens to the temperature of the surroundings during an endothermic reaction?
decrease
What happens to the temperature of the reaction during an endothermic reaction?
Increase
Explain the shape of the graph in terms of the energy transfers taking place
As the citric acid is added to the solution, the temperature decreases. This is because an endothermic reaction occurs, taking in energy from the surroundings and decreasing the surrounding temperature. Once 1.5g of citric acid is added, the reaction is complete. Therefore, as excess citric acid is added, the temperature of the solution begins to increase as the citric acid has a higher temperature than the solution, causing energy to be transferred to the solution, increasing its temperature
Give 2 reasons why a burette is used to titre solutions:
- You can easily control the volume of acid added by using the tap on the burette
- You can add in small and precise increments
Calculate x
