Energy Changes Flashcards
What is an Exothermic reaction?
A reaction that transfers energy to the surroundings, usually by heating
- shown in a rise in temperature
What is are examples of an Exothermic reaction?
Combustion- burning fuels
- gives out lots of energy
Neutralisation reactions (acid + alkali)
Many oxidation reactions
What are everyday uses of Exothermic reactions?
- handwarmers
- self heating cans of hot chocolate or coffee
- freezing
What is an Endothermic reaction?
A reaction that takes in energy from the surroundings
- shown by fall in temperature
What is are examples of an Endothermic reaction?
- citric acid + sodium hydrogencarbonate
- thermal decomposition
What are everyday uses of Endothermic reactions?
- Sports injury packs
- melting
What is a Reaction Profile?
Diagrams that show relative energies of reactants and products in a reaction
- show how energy changes over the course of the reaction
What is Activation energy?
The minimum amount of energy the reactants need to collide with eachother and react
-The energy needed to start the reaction
Is bond breaking an endothermic or exothermic reaction?
Endothermic- energy must be supplied
Is bond formation an endothermic or exothermic reaction?
Exothermic- energy is released
What happens to bonds in a chemical reaction?
Old bonds are broken and new bonds are formed
What is the equation to calculate Overall Energy Change using known Bond energies?
Overall energy change= energy required to break bonds - energy released by forming bonds
What is an electrochemical cell?
A basic system made up of 2 different electrodes in contact with an electrolyte
- electrodes need to conduct electricity-> usually metal
-charge difference between electrodes, charge flows through wire, electricity is produced - connected voltmeter, measures voltage of the cell.
What factors does the voltage of a cell depend on?
- type of electrode (different metals react differently with the same electrolyte)
- difference of reactivity between electrodes (bigger the difference, bigger the cell voltage)
- electrolyte used (reacts differently with the electrodes)
- battery formed by connecting 2 or more cells in series (combined voltage)
How do non-rechargeable batteries work?
- reactions that happen at the electrodes are irreversible
- reacting particles get used up and turned into products over time (electrolyte ions + metal ions on electrode)
- once reactants are used up, reactions can’t happen -> no electricity is produced
- reactants can’t reverse back into products, cell cannot be recharged (alkaline batteries)
What is a fuel cell?
An electrical cell supplied with a fuel and oxygen, uses energy from the reaction to produce electrical energy
How does a fuel cell work?
- Fuel enters a cell, becomes oxidised and sets up PD within the cell
How does a Hydrogen-Oxygen fuel cell work?
Electrolyte- acid
Electrode- porous carbon with catalyst
- Hydrogen goes to the anode, Oxygen goes to the cathode
-Hydrogen looses electrons at anode (-ve), producing H+ ions (oxidation)
-H+ ions move to the cathode (+ve)
-Oxygen gains electrons from the cathode (+ve), reacts with H+ ions (from electrolyte) to make H2O. (reduction) - Electrons flow through external circuit (anode to cathode) (-ve to +ve), creating electrical current
- Overall reaction: hydrogen + oxygen-> water
What charge does an anode have in fuel cells?
Negative (opposite to electrolysis)
What charge does a cathode have in fuel cells?
Positive (opposite to electrolysis)
What are advantages of Hydrogen-Oxygen Fuel Cells?
- produce less pollutants, only water and heat
- Electric vehicle batteries are polluting to dispose of
- Rechargeable batteries
- Less expensive than Batteries to produce
- Batteries store less energy, charge more often
What are disadvantages of Hydrogen-Oxygen Fuel Cells?
-Hydrogen is a gas, takes more storage space than rechargeable battery
- Hydrogen is explosive (in air), hard to store safely
- Hydrogen fuel often made from hydrocarbons (fossil fuels) or electrolysis (fuelled by fossil fuels)
