C7. Energy Changes Flashcards
What is an exothermic reaction?
A reaction in which energy is transferred from the reacting substances to their surroundings
What is an endothermic reaction?
A reaction in which energy transferred to the reacting substances from their surroundings
What are some exothermic reactions?
Combustion, many oxidation reactions and neutralisation
What are some endothermic reactions?
Thermal decomposition, the reaction of citric acid and sodium hydrogen carbonate
What are some everyday uses of exothermic reactioms
Self heating cans and hand warmers
What are some everyday uses of endothermic reactions
Instant cold packs
What are some endothermic reactions?
Thermal decomposition & the reaction of citric acid and sodium hydrogen carbonate
Why do chemical reactions need activation energy to start them?
Reactant molecules must bump into each other, so they must be moving and movement requires energy
What is activation energy?
The minimum energy needed for a reaction to take place
What is the reaction profile for an exothermic reaction?
The products are at a lower energy level than the reactions which means that when the reactants form the products, energy is transferred to the surroundings
What happens to the surroundings in an exothermic reaction?
The surroundings get hotter and hotter and the temperature rises because the difference in energy between the products and the reactants is transferred to the surroumdimgs
What happens to the surroundings in an endothermic reaction?
The surroundings get colder because energy is transferred from the surroundings to the reaction mixture
What is the reaction profile for an endothermic reaction?
The products are at a higher energy level than the reactants. As the reactants react to form products the energy is transferred from the surroundings to the reaction mixture
What is the energy change in terms of balance between bond making and breaking?
Breaking bonds is an endothermic process, energy is taken in from the surroundings
Making bonds is an exothermic process, energy is transferred to the surroundings
How does the balance between the energy needed to break bonds and the energy released when new bonds are made decide whether the reaction is exothermic?
The energy released when new bonds are formed (as the products are made) is more than the energy needed to break the bonds in the reactants. These reactions transfer energy to the surroundings
How does the balance between the energy needed to break bonds and the energy released when new bonds are made decide whether the reaction is endothermic?
The energy needed to break the bonds in the reactants is more than the energy released when new bonds are formed in the products. These reactions transfer energy to the reacting chemicals
How do you calculate the energy change?
- Work out how much energy change is needed to break the chemical bonds in the reactants
- Work out how much energy is released when new bonds are formed in the products.
What is the general relationship between the difference in reactivity and the voltage?
The greater the difference in reactivity between the two metals used, the higher the voltage produced
When happens when two metals are dipped in a solution and joined by a wire?
The more reactive metal will donate electrons to the less reactive metal. This forms a simple electrical cell.
What happens to hydrogen in the cathode a hydrogen fuel cell
Hydrogen gas is supplied as a fuel to the cathode. It diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit
What is the half equation for hydrogen gas in an hydrogen fuel cell?
2H₂ + 4OH- ➡ 4H₂O + 4e-
What happens to oxygen in the anode of a hydrogen fuel cell?
Oxygen gas is supplied to the anode. It diffuses through the graphite and reacts to form hydroxide ions accepting electrons from the external circuit
What is the half equation for oxygen in a hydrogen fuel cell?
O₂ + 2H₂O+ 4e- ➡ 4OH-
What is oxidised and what is reduced in a hydrogen fuel cell?
Hydrogen is oxidised
What is the equation for the oxidation of hydrogen?
2H₂ + O₂ ➡ 2H₂O
What are the advantages of hydrogen fuel cells?
Do not need to be electrically charged
No pollutants are produced
Can be a range of sizes for different uses
What are the disadvantages of hydrogen fuel cells?
Hydrogen is highly flammable
Hydrogen is sometimes produced for the cell by non-renewable sources
Hydrogen is difficult to store
