topic 5 - energy changes

Question 1

what is an exothermic reaction

Answer 1

when a reaction transfers energy to the surroundings
it gives out heat
so surrounding temperatures get hotter

Question 2

what is an endothermic reaction

Answer 2

when a reaction takes in heat energy from surroundings
it takes in heat
so surrounding temperatures get colder

Question 3

what happens to bonds during chemical reactions

Answer 3

bonds between atoms are broken (by putting energy in) and new bonds between atoms are formed (they release energy)

Question 4

what does making new bonds do

Answer 4

it gives out energy - exothermic process

Question 5

what does breaking bonds do

Answer 5

energy needs to be put in to break bonds so it is an endothermic reaction

Question 6

how would you set up an energy level diagram with an exothermic reaction

Answer 6

in an exothermic reaction, there are less products than reactants as energy has been given off
–> this means the line you draw for the products would be lower than the one for reactants

• draw a graph, energy on the y-axis and progress of reaction on the x-axis
• draw a line for reactants and write the reactant side of the equation underneath that
• draw a line lower down to show a lesser energy level for the products and write the products side of the equation underneath that
• draw an upwards curved line from the reactants line to the products line
• the distance between the reactants line and the products line is the energy change, draw a downwards arrow to the products line and write the energy change (Δ𝐻) - negative in exo
• the distance from the reactants line and the curve of the curved line is the activation energy (Ea), draw an upwards arrow towards the curve and label

Question 7

how would you set up an energy level diagram with an endothermic reaction

Answer 7

in an endothermic reaction, there are more products than reactants as energy has been taken in from surroundings
–> this means the line you draw for the products would be higher than the one for reactants

• draw a graph, energy on the y-axis and progress of reaction on the x-axis
• draw a line for reactants and write the reactant side of the equation underneath that
• draw a line higher up to show a higher energy level for the products and write the products side of the equation underneath that
• draw an upwards curved line from the reactants line to the products line
• the distance between the reactants line and the products line is the energy change, draw a upwards arrow to the products line and write the energy change (Δ𝐻) - positive in endo
• the distance from the reactants line and the curve of the curved line is the activation energy (Ea), draw an upwards arrow towards the curve and label

Question 8

what is a cell

Answer 8

a system that connects chemicals which react and produce electricity

Question 9

what does a cell contain

Answer 9

• two different electrodes (must be able to conduct electricity - usually metal)
  the electrodes have to have different reactivity to get a reaction and be different metals so we don’t get a voltage of 0
• electrodes need to be in contact with an electrolyte a liquid that contains electricity and contains ions that react with electrodes

Question 10

what happens if you connect cells in series

Answer 10

if you connect several cells in series, the total voltage will be the sum of the individual cell voltages

Question 11

how do you increase cell voltage

Answer 11

• use metals which have a bigger reactivity difference
• increase concentration of electrolyte
• increase temperature

Question 12

what is a fuel cell

Answer 12

-an electrical cell
- supplied with a fuel and oxygen
- uses energy from the reaction between them to produce electrical energy efficiency

Question 13

hydrogen-oxygen fuel cell
explained with formulas

Answer 13

hydrogen enters the cell at the negative electrode where it is OXIDISED

H₂ —> 2H+ + 2e-

oxygen gains electrons and reacts with H+ ions - REDUCTION

O₂ + 4H+ + 4e- —> 2H₂O
O₂ + 2H₂ —> 2H₂O