chemsitry - energetics and electrolysis Flashcards
what is the symbol for the energy change that accompanies a reaction?
delta(change in) H
in exothermic reactions:
energy is given ? or ? when products form and the ? of the surroundings gets hotter/ increases
out, released, temperature
in exothermic reactions:
energy is taken ? when products form and the ? of the surroundings gets colder/ ?
in, temperature, decreases
Why are delta H values for Exothermic reaction negative?
As energy is released to the surroundings, so the products have less/lower energy than the reactants
Why are delta H values for endothermic reactions positive?
As energy is taken in from the surroundings, the products have more/higher energy than the reactants
what is the activation energy?
the minimum amount of energy that must be supplied in order to break the bonds in the reactants particles to initiate the reaction
old bonds between atoms in reactants are ?
broken
new bonds between atoms in products are ?
formed/made
what does bond breaking require and what type of process is this?
energy, so it is an endothermic reaction
what does bond breaking release and what type of process is this?
energy, it’s an exothermic reaction
Overall whether a reaction is endothermic or ? depends on the difference of ? needed to break old ? and the ? released in forming/? new bonds
exothermic, energy, bonds, energy, making
Exothermic reactions - when more ? is released in the formation of NEW bonds than is needed to ? old bonds. so delta H values for Exothermic ? are ?
energy, break, reactions, negative
Endothermic reactions - when less ? is released in the formation of NEW bonds than is needed to ? old bonds. so delta H values for Endothermic ? are ?
energy, break, reactions, positive
Exothermic reactions - greater than equation
energy released when new bonds are formed > energy required when old bonds are broken
Endothermic reactions greater than equation
Energy required to break the old bonds / when old bonds are broken> energy released when new bonds are formed
Energy profiles : what does an exothermic profile look like?
- have two axis - only y axis is labelled with energy
- start with a horizontal line at the top with the reactants
- draw a lower horizontal line at the bottom for the products
then draw a looping arrow up from the reactants down to the products -the peak of the arrow to the reactants should be the activation energy - the distance between reactants and products is delta H
energy profiles - what does an endothermic one look like?
- have two axis - only y axis is labelled with energy
- start with a horizontal line at the bottom with the reactants
- draw a higher horizontal line at the bottom for the products
then draw a looping arrow up from the reactants up to the products -the peak of the arrow to the reactants should be the activation energy - the distance between reactants and products is delta H
equation for delta H
sum of bonds broken - sum of bonds formed
4 steps to bond energy calculations
1 - write out symbol ? using displaced formulae showing all ?
2. work out energy needed to ? bonds
3. work out energy ? when bonds are ?
4. work out energy ? - delta ? using :
either :
• energy ? to break bonds - energy released when bonds are ?
or
• ? of bonds broken - sum of bonds ?
equation, bonds
break
released, formed
change, H
needed/required, made
sum, made
what is the alternative method for bond energy calculations?
- highlight the bonds that change between the reactants and the products
- only calculate the energy needed to break these bonds/ the energy released when new bonds are made
- then use the delta H equation to work out whether the equation is exothermic or endothermic (by the negative or positive delta H)!!!
what is the half equation for hydroxide being oxidised
4(OH-) –> 2(H20) +2(02) + 4e-
solid ionic ? do not conduct ? as there are no free ? to flow through the structure and all the charged ? are locked into a lattice ? in the solid ; the ions are not ? to move
compounds, electricity, electrons, particles/ions, structure, free
why is the cathode negative and therefore which ions go to it (and what is formed at it)?
because it has an excess of electrons, the cations go to it and so the metal element is formed at the cathode