C5 - Energy Changes Flashcards
Energy is ______ in chemical reactions. If a reaction transfers energy to the surroundings, the product molecules must have _____ energy than the reactants.
Conserved
Less
What is an exothermic reaction?
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases
What is an endothermic reaction?
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases
Give two every day uses of exothermic reactions
Self-heating cans
Hand warmers
Give two examples of endothermic reactions
Thermal decompositions
Reaction of citric acid and sodium hydrogencarbonate
Give an everyday use of endothermic reactions
Sport injury packs
Students should be able to evaluate uses and applications of exothermic and endothermic reactions given appropriate information
AT 5: Opportunity to measure temperature changes when substances react or dissolve in water
RP4: Design an investigation for the variables that affect temperature changes in reacting solutions such as:
- Acid + metals
- Acid + carbonates
- Neutralisations
- Displacement of metals
When can chemical reactions only occur?
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
What is the activation energy?
The minimum amount of energy that particles must have to react is called the activation energy
What three things can reaction profiles be used to show?
- Relative energies of reactants and products
- The activation energy
- Overall energy change of the reaction
Students need to explain that the activation energy is the energy needed for a reaction to occur.
This could be a ‘suggest why the reaction did not occur/start’ question
During a chemical reaction:
Energy must be supplied to ______ bonds in the reactants
Energy is released when bonds in the ____ are formed
Break
Products
How can the energy needed to break bonds and the energy released when bonds are formed be calculated?
By bond energies
In an exothermic reaction, the energy released from forming _______ is greater than the energy needed to break _______ bonds
New bonds
Break
In an endothermic reaction, the energy needed to break ______ bonds is greater than the energy released from forming new bonds
Existing
Cells contain ____ which react to produce _____
Chemicals
Electricity
What is the voltage produced by a cell dependant on?
Type of electrode
Type of electrolyte
How is a simple cell made?
A simple cell can be made by connecting two different different metals in contact with Electrolyte
How is a battery made?
Batteries consist of two or more cells connected together in series to provide a greater voltage
How can the voltage of a simple cell and battery be increased?
Increase the reactivity difference between the two metals
Breaking bonds in reactants is an ____ process
Endothermic process
Making bonds in products is an _____ process
Exothermic
When one of the reactants has been used up, the chemical reactions ___ in non-rechargeable ___ and ____
Stop
Cells
Batteries
Are alkaline batteries non-rechargeable or rechargeable?
Alkaline batteries are non-rechargeable
Rechargeable cells and batteries can be ______ because the chemical reactions are _____ when an external ______ ______ is supplied
Recharged
Reversed
Electrical current
Students should be able to interpret data for relative reactivity of different metals and evaluate the use of cells
How are fuel cells supplied?
Fuel cells are supplied by an external source of fuel such as hydrogen and oxygen or air
How is a potential difference produced in a fuel cell?
The fuel is oxidised electrochemically within the fuel cell to produce a potential difference
Advantages of using hydrogen fuel cells in comparison with rechargeable cells and batteries?
Hydrogen can be renewable
Produces a constant voltage
No toxic chemicals released after disposal
What do hydrogen fuel cells offer?
Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries
Suggest why using hydrogen as a fuel could help reduce the human impact on global warming
The reaction does not produce carbon dioxide is a greenhouse gas
Write two half equations that show what happens to the hydrogen and oxygen gases in a hydrogen fuel cell
2H2 (g) + 4OH- (aq) —> 4H20 (l) + 4e-
Hydrogen gas is supplied as a fuel to the negative electrode and it diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit
O2 (g) + 2H20 (l)+ 4e- —-> 4OH- (aq)
Oxygen gas is supplied to the positive electrode. It diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from the external circuit
What are three disadvantages of hydrogen fuel cells?
Hydrogen is highly flammable
Hydrogen is difficult to store
Hydrogen is sometimes produced for the cell by nonrenewable sources
Suggest why hydrogen fuel cells are useful
Offer a potential alternative to rechargeable cells and batteries
What is the overall reaction in a hydrogen fuel cell?
The oxidation of hydrogen to form water
Water is the only product, so this is useful! No separation! Better alternative as no toxic products!
