Topic 5 - Energy Changes Flashcards
What can changes in heat be determined and measured by?
The changes in heat content can be determined and measured with a thermometer.
What is the law of conservation of energy? (3)
- The overall amount of energy does not change as energy is conserved in reactions.
- This means that it cannot be created or destroyed but it can be transferred.
- So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred.
What is an exothermic reaction?
An exothermic reaction releases heat energy. This means that the temperature increases.
What is an exothermic reaction in summary?
In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls.
What are 2 examples of exothermic reaction?
- Neutralisation
- Combustion
What is an endothermic reaction?
An endothermic reaction takes heat energy in. This means that the temperature decreases.
What is an endothermic reaction in summary?
In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases.
What are some typical endothermic reactions?
- Electrolysis
- Thermal decomposition reactions
- The first stages of photosynthesis
- The reaction between citric acid and sodium hydrogencarbonate.
How do sports injury treatments use endothermic reactions?
Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling.
What happens to energy in displacement reactions?
These can either take energy in or give it out.
What materials are needed for Required practical 4: Investigating temperature changes? (5)
- Dilute hydrochloric acid
- Dilute sodium hydroxide solution
- Styrofoam (polystyrene) calorimeter & lid
- 25 cm3 measuring cylinder
- Thermometer & stirrer
What is the 6 step method for Required practical 4: Investigating temperature changes?
- Using a measuring cylinder, place 25 cm3 of the NaOH solution into the calorimeter.
- Measure and record the temperature of the solution.
- Add 5 cm3 of the dilute HCl and stir.
- Measure and record the highest temperature reached by the mixture.
- Repeat steps 1 – 4 increasing the amount of acid added by 5 cm3 each time.
- Record results in a suitable table => one column for volume of acid, one for temperature.
What conclusion can be made from Required practical 4: Investigating temperature changes?
The larger the difference in the temperature the more energy is absorbed or released.
What must happen for atoms/ particles to react?
For atoms or particles to react with each other in a chemical system they must first come into contact with each other in a collision.
What 3 factors come into play when analysing collisions?
- Energy
- Orientation
- Number of collisions per second.
What is activation energy?
- In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful (for the particles to react together).
- This minimum amount of energy is called the activation energy.
- It is given the symbol E(a) => the a is in subscript.
What does the activation energy tell us (high or low)?
Reactions which have higher activation energies require more energy to start than those with lower activation energies.
Why do all reactions have an activation energy?
All reactions have an activation energy as the chemical bonds in the reactant molecules have to be broken first.
What is an energy profile?
Energy profiles are graphical representations of the relative energies of the reactants and products in chemical reactions.
How are energy profiles set out?
The energy of the reactants and products are displayed on the y-axis and the progress of the reaction is shown on the x-axis.
What does the difference in height between the energy of reactants and products represent?
The difference in height between the energy of reactants and products represents the overall energy change of a reaction.
What does an energy profile look like for exothermic reactions? (2)
- The energy of the products will be lower than the energy of the reactants, so the change in energy is negative.
- This is represented on the energy profile with a downwards arrow as the energy of the products is lower than the reactants.
What does an energy profile look like for endothermic reactions? (2)
- The energy of the products will be higher than the energy of the reactants, so the change in energy is positive.
- This is represented on the energy profile with an upwards arrow as the energy of the products is higher than the reactants.
What is a reaction being endothermic vs exothermic dependent on?
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed.
What kind of reaction is bond breaking? Why?
Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds.
What kind of reaction is bond making? Why?
Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed.
What kind of change in energy does an endothermic reaction have? Why? (4)
- If more energy is absorbed than is released, this reaction is endothermic.
- More energy is required to break the bonds than is released from making the new bonds.
- The change in energy is positive since the products have more energy than the reactants.
- Therefore an endothermic reaction has a positive change in energy.
What kind of change in energy does an exothermic reaction have? Why? (4)
- 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 change in energy.
What is a way to remember bond breaking vs bond making and the different types of reactions?
Remember bond breaking is ENDothermic and results in the END of the bond.
What does each chemical bond have?
Each chemical bond has a specific bond energy associated with it.
What is bond energy?
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.
What can bond energy be used to calculate? What must you know in order to do this?
This energy can be used to calculate how much heat would be released or absorbed in a reaction. To do this it is necessary to know the bonds present in both the reactants and products.
What must you know to calculate the total change in energy for a reaction?
We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved.
How do you calculate the total change in energy for a reaction? (3)
- Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.
- Add together the bond energies for all the bonds in the products – this is the ‘energy out’.
- Calculate the energy change using the equation:
Energy change = Energy taken in - Energy given out
What are some key things to remember when doing bond energy questions? (2)
- It is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.
- Don’t forget to take into account the balancing numbers when working out how many of each type of bond is being broken/ formed.
What are chemical cells?
Chemical cells or simple cells are a source of electrical energy.
What do simple chemical cells look like?
The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit.
What is a common example of metals used in simple chemical cells?
A common example is zinc and copper.
How does a chemical cell work using the example of zinc and copper? (6)
- Zinc is the more reactive metal and forms ions more easily, readily releasing electrons.
- The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes.
- The electrons then flow around the circuit to the copper electrode which is now the more positive electrode.
- The difference in the ability of the electrodes to release electrons causes a voltage to be produced.
- The greater the difference in the metals reactivity then the greater the voltage produced.
- The electrolyte used also affects the voltage as different ions react with the electrodes in different ways.
What are batteries?
Batteries consist of two or more cells connected together in series to provide a greater voltage.
What happens in non-rechargeable cells and batteries?
In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up.
What is an example of a non-rechargeable cell/ battery?
Alkaline batteries.
Why can rechargeable cells and batteries be recharged?
Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
What is a fuel cell?
A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode.
Why/where are fuel cells popular?
These cells are becoming more common in the automotive industry to replace petrol or diesel engines.
How does the fuel cell work? (2)
- As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell.
- Different electrolytes and fuels can be used to set up different types of fuel cells.
What is an important fuel cell?
An important cell is the hydrogen-oxygen fuel cell which involves the oxidation of hydrogen to produce water.
What is important to remember about fuel cells?
In fuel cells the polarity on the electrodes are the other way around: the anode is the negative electrode and the cathode is the positive electrode.
What do hydrogen fuel cells offer a potential alternative to?
Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.
What are the advantages of hydrogen fuel cells? (5)
- They do not produce any pollution.
- They produce more energy per kilogram than either petrol or diesel.
- No power is lost in transmission as there are no moving parts, unlike an internal combustion engine.
- No batteries to dispose of which is better for the environment.
- Continuous process and will keep producing energy as long as fuel is supplied.
What are the disadvantages of hydrogen fuel cells? (4)
- Materials used in producing fuel cells are expensive.
- High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle.
- Fuel cells are affected by low temperatures, becoming less efficient.
- Hydrogen is expensive to produce and store.
Where do hydrogen and oxygen enter the fuel cell?
Hydrogen enters at the anode where it is oxidised and oxygen enters at the cathode where it is reduced.
What reaction occurs at the anode? (half equation)
2H(2) → 4H+ + 4e–
What reaction occurs at the cathode? (half equation)
4H+ + O(2) + 4e– → 2H(2)O
What is the overall, full ionic reaction/ equation for the fuel cell?
2H(2) + O(2)→ 2H(2)O
How do electrons move within the fuel cell? How is this used?
- The electrons move around the external circuit from the anode to the cathode.
- This movement of electrons is used to drive an electric motor.
