C7 - Energy Changes Flashcards
What is an exothermic reaction? Example?
Energy transferred to surroundings
e.g combustion, oxidation reactions and neutralisation
What is an endothermic reaction? Examples?
Energy transferred from surroundings
e.g thermal decompositions, sports injury packs
What is the law of conservation of energy?
Energy cannot be created or destroyed
Describe how energy is transferred to or from the surroundings during a chemical reaction
- If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred
RP4 - Temperature changes - Investigate the variables that affect temperature change in chemical reactions
- Measure 25cm3 hydrochloric acid into polystyrene cup
- Place cup inside beaker to make it more stable
- Measure and record temperature of hydrochloric acid
- Measure 5cm3 of sodium hydroxide and add it to the polystyrene cup.
- Put lid on the cup and stir with thermometer through hole of lid
- When reading on thermometer stops changing, record temperature
- Repeat to add 5cm3 more until 40cm3 added
- Repeat to ensure reliability, calculate mean maximum temperature reached
What is the collision theory?
Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy
What is the activation energy?
- Minimum amount of energy that particles must have to react
What are reaction profiles used for?
To show
- relative energies of reactants and products
- activation energy
- overall energy change of a reaction
What would a reaction profile/energy level diagram of an exothermic reaction look like?
- Products lower energy than reactants
- Energy released when forming bonds greater than energy needed to break bonds
What would a reaction profile/energy level diagram of an endothermic reaction look like?
- Products higher energy than reactants
- Energy released when forming bonds is lower than energy needed to break bonds
What 2 things happen during a chemical reaction?
- Energy must be supplied to break bonds in reactants
- Energy is released when bonds in the products are formed
How do you calculate energy change when bonds are formed using bond energies?
Energy change = SUM of bonds BROKEN - SUM of bonds MADE
What is a cell?
- Contains chemicals which react to produce electricity
- voltage produced by it depends on factors like type of electrode and electrolyte
What is a simple cell?
- Made by connecting two different metals in contact with an electrolyte
- Bigger difference in reactivity = larger voltage
What is a battery?
- Consists of two or more cells connected together in series to provide a greater voltage
What are alkaline batteries called?
Non rechargeable - chemical reactions stop when one of the reactants has been used up
What are rechargeable cells and batteries?
Can be recharged because the chemical reactions are reversed when electrical current is supplied
What are fuel cells?
- Supplied by an external source of fuel and oxygen or air
- Fuel is oxidised electrochemically within fuel cell to produce potential difference
What does the overall reaction in a hydrogen cell involve and what is the symbol equation?
Oxidation of hydrogen to produce water
2H2 + O2 → 2H2O
Evaluate the use of hydrogen fuel cells compared to rechargeable cells and batteries.(ADVANTAGES)
Advantages of hydrogen fuel cells
- No pollutants produced
- No need to recharge - quick to refuel
- Travel further before refueling compared to rechargeable
- No toxic chemicals to dispose of at end of cell’s life
- Hydrogen can be renewable if made by electrolysis
Evaluate the use of hydrogen fuel cells compared to rechargeable cells and batteries.(DISADVANTAGES)
Disadvantages of hydrogen fuel cells
- hydrogen is highly flammable
- hydrogen is difficult to store
- hydrogen is not renewable if produced using fossil fuels
- not many hydrogen filling stations
Write half equations for the electrode reactions in the hydrogen fuel cell. (CATHODE)
Cathode (negative electrode)
- hydrogen gas reacts with hydroxide ions to produce water
- produces electrons
- 2H2 + 4OH- → 4e- + 4H2O
Write half equations for the electrode reactions in the hydrogen fuel cell. (ANODE)
Anode(positive electrode)
- oxygen gas reacts with water to produce hydroxide ions
- this uses up electrons
- 4e- + 2H2O + O2 → 4OH-
