Energy Changes Flashcards
True or false, energy is stored in chemical bonds
True
Different chemical bonds hold different amounts of …
Energy
What type of reaction releases thermal energy to the surroundings
Exothermic
In an exothermic reaction does the temperature of surroundings increase
Yes because (thermal) energy is released
What is an endothermic reaction
A reaction that takes in thermal energy from its surroundings
What happens to the temperature of the surroundings in an endothermic reaction and why
Decreases
Thermal energy is taken in
In an endothermic reaction do the reactants or products have more energy
Product as energy is taken in from surroundings
In an exothermic reaction do the reactants or products have more energy
Reactants as energy is released to the surroundings
What is meant by activation energy
Minimum amount of energy the reactant particles require in order to collide with each other and react
On reactant profile diagrams what’s on the x axis
Progress of reaction
In reaction profile diagrams what’s in the y axis
Energy
In a reaction profile diagram where is the activation energy
From the reactants to the peak of the curve
What represents the energy change in a reaction profile diagram
Line between reactants and products
Does breaking bonds release or require energy
Require
Is breaking bonds exothermic or endothermic and why
Endothermic because it requires energy (energy is taken in)
What is bond energy
The amount of energy required to break one mole of a particular covalent bond
What can be defined as the amount of energy required to break one mole of a particular covalent bond
Bond energy
If the total bond energy of the reaction is negative is it exothermic or endothermic and why
Exothermic-
Energy is being lost hence the negative
Is forming bonds endothermic or exothermic and why
Exothermic as it releases energy
Equation used to calculate overall energy change of reaction
Energy of reactants- energy of products
(Energy of bonds broken- energy of bonds formed)
What type of reaction produces a negative energy change
Exothermic
During a chemical reaction must the bonds of the products be broken or formed
Formed
Exothermic reactions transfer energy to their surroundings in the form of…
Heat
Examples of exothermic reactions
Combustion (burning)- where fuels are burnt by reacting with oxygen and release energy
Neutralisation reactions between acids and bases
Oxidation reactions
What do (electrochemical) cells use to produce electricity
Chemical reactions
3 factors that affect voltage of a cell
Metals used as electrode (greater difference in reactivity between metals means higher voltage)
Type and concentration of electrolyte
Conditions e.g temperature
If there is a higher difference in reactivity between the 2 metals used in the electrodes does this increase or decrease voltage of the cell
Increase
Out of rechargeable and non-rechargeable cells for which one can the chemical reaction be reversed
Rechargeable
Why can rechargeable cells and batteries be recharged
The chemical reaction is reversible when an external electrical current is supplied
2 examples of devices that contain rechargeable batteries
Laptop/ mobile phone
In non-rechargeable cells and batteries when does the chemical reaction stop
When one of the reactants has been used up
2 examples of what non-rechargeable batteries are used for
Smoke alarms
Tv remotes
True or false, alkaline batteries are rechargeable
False
Name for liquid that conducts electricity in the cell
Electrolyte
How do chemical cells transfer energy to their surroundings
Electricity
True or false, a battery has a higher voltage than a single cell
True
Word used to describe lots of cells connected together and that increases voltage
Battery
Electrochemical cells are called ‘electrochemical’ because they convert energy between what
Chemical and electrical forms
Fuel cells convert the energy of what into electrical energy
Fuel and oxygen
Most common type of fuel cell
Hydrogen - oxygen
What is a common electrolyte used in hydrogen - oxygen fuel cells
Potassium hydroxide (KOH)
Why are the anode and cathode in fuel cells different to in electrolysis
The positive and negative is the other way around
Cathode is positive
Anode is negative
In fuel cells which out of the anode and cathode is positive
Cathode
In fuel cells which out of the anode and cathode is negative
Anode
In fuel cells what are both electrodes made from
(Porous) Carbon
There’s lots of tiny holes + a catalyst to speed up the reaction
What is outside of the electrodes in fuel cells
A cathode compartment for the cathode
An anode compartment for the anode
Where does hydrogen enter the fuel cell
Anode compartment
Where does the oxygen enter the fuel cell
Cathode compartment
In the fuel cell which side is the anode and which side is the cathode
Positive cathode is right
Negative anode is left
Which compartment of the fuel cell does hydrogen enter
Negative anode on left
Which compartment of the fuel cell does oxygen enter
Positive cathode on right
Where do the heat and water leave once the reaction has taken place in the fuel cell
Outlet of cathode compartment
In a fuel cell what do the hydrogen atoms do once they’ve entered the negative anode compartment on the left
Lose an electron to become a positive ion
(is oxidised as oxidation is loss of electrons)
What happens in the fuel cell after the hydrogen atoms have each lost an electron at the positive anode on the left and turned into a positive ion (been oxidised)
Electrons and hydrogen ions move to cathode
The electron each hydrogen atom has lost passes through the wire to the positive cathode on the right
At the same time the hydrogen ions move through the electrolyte to the positive cathode on the right
What happens once the positive hydrogen ions and electrons that they have lost reach the positive cathode on the right of the fuel cell
They react with the oxygen there to produce water
What part of the reaction in the fuel cell is what generates the electricity
The electrons lost by the hydrogen atoms passing through the wire from the negative anode on left to positive cathode on right
In fuel cells what is reduced and what is oxidised out of oxygen and hydrogen
Oxygen is reduced (gains electrons when it reacts to form water at the positive cathode on the right)
Hydrogen is oxidised (loses electrons at the negative anode on the left before it moves through the electrolyte to react with the oxygen in the cathode)
In a hydrogen - oxygen fuel cell what is the fuel
Hydrogen
One key use of hydrogen - fuel cells
Vehicles like cars
(Replaces fossil fuel engines)
He is it good that hydrogen - fuel cells only require hydrogen and oxygen as the reactants
They are abundant
(There’s lots of hydrogen and oxygen)
They don’t produce any carbon dioxide/ other pollutants as waste
True or false, fuel cells are relatively simple devices
True- they last longer than batteries and are less polluting to dispose of
This is an advantage of them
Advantages of hydrogen - fuel cells
Only require hydrogen and oxygen which are in abundance
Less polluting than batteries to dispose of and last longer
Hydrogen and oxygen don’t produce any pollutants like CO2/ any waste
Disadvantages of hydrogen - oxygen fuel cells
Because hydrogen is a gas it takes up much more space to store than fossil fuels/ batteries
Hydrogen is explosive when mixed with air so can be dangerous to store
The energy required to make hydrogen fuel usually comes from burning fossil fuels
Disadvantages of hydrogen - oxygen fuel cells
Because hydrogen is a gas it takes up much more space to store than fossil fuels/ batteries
Hydrogen is explosive when mixed with air so can be dangerous to store
The energy required to make hydrogen fuel usually comes from burning fossil fuels
What is the overall reaction for a hydrogen - oxygen fuel cell (word equation)
Hydrogen + oxygen -> water
True or false, hydrogen and oxygen are both renewable
True
True or false, hydrogen is dangerous to store
True
(Disadvantage of hydrogen - oxygen fuel cells)
Describe how to calculate temperature change in an exothermic reaction (RPA) (in this case the reaction is the neutralisation of hydrochloric acid and sodium hydroxide)
Summary: Add increasing volumes of sodium hydroxide solution to the hydrochloric acid and for each different experiment (different volume of hydroxide) measure maximum temperature reached
IV- volume of sodium hydroxide
DV- maximum temperature reached
CV- volume of hydrochloric acid, concentration of acid and hydroxide solution
1) measure 30cm cubed of dilute hydrochloric acid in measuring cylinder
2) Transfer acid to cup inside beaker (stops cup from falling over)
3) use thermometer to measure temperature of acid
4) Use measuring cylinder to measure 5cm cubed or sodium hydroxide solution and add to the acid in the cup
5) place plastic lid over cup and place thermometer through hole in lid making sure it is in the solution
6) Gently stir solution with thermometer
7) look carefully at the temperature rise on the thermometer and record temperature when it stops rising (highest temperature reached)
8) Keep repeating the experiment, each time adding 5 more cm cubed of the hydroxide solution, stirring and then recording highest temperature reached
9) Stop once a maximum of 40cm cubed of hydroxide solution has been added to the acid
10) Repeat the whole experiment one more time to obtain 2 full sets of results and calculate mean value of maximum temperature reached for each volume of sodium hydroxide added
11) plot results on graph
Explain the graph produced plotting results from seeing how volume of sodium hydroxide added to hydrochloric acid affects temperature change
As volume increases so does temperature change (because when more particles of sodium hydroxide are added they react with the acid and as it’s an exothermic reaction so thermal energy (heat) is given off (more reaction means more heat given off so maximum temperature increases)
However at certain volume of hydroxide, maximum temperature starts to decrease (there is so much hydroxide compared to acid that some of the hydroxide is unable to react and the maximum temperature reached by the reaction has already reached its maximum) Also as volume increases the temperature becomes more spread out
Why should you use a lid when investigating temperature change (RPA)
To reduce heat loss through air which could make the maximum temperature reached appear lower than it should be
For cup use insulating material like polystyrene to prevent heat loss for sides and bottom
Fuel cell half equation at negative electrode
H2 goes to 2H+ + 2e-
Fuel cell half equation at positive electrode
O2 + 4H+ + 4e- goes to 2H2O
Overall fuel cell equation
2H2 + O2 goes to 2H2O
Describe overall process of fuel cell including half equations and overall equation
1) hydrogen enters inlet and is oxidised at negative electrode to form hydrogen ions
H2 goes to 2H+ + 2e-
2) lost electrons travel through wire to positive anode (generates electricity)
3) hydrogen ions travel through electrolyte to positive electrode and oxygen enters through inlet
4) hydrogen ions, electrons that have travelled round wire and oxygen all react to produce water
4H+ + 4e- + O2 goes to 2H2O
2 Advantages of fuel cells over rechargeable cells and batteries (e.g for powering cars)
Rechargeable batteries eventually run out after a certain number of charges so need replacing (but fuel cells last for as long as they are supplied with fuel for
Rechargeable batteries have a smaller capacity so must be charged more frequently than fuel cells
