C7 Energy Changes
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what is an exothermic reaction?
a reaction that transfers energy from the reacting molecules to the surroundings this means the temperature of the surroundings increases
what are some examples of exothermic reactions?
combustion certain oxidation reactions neutralisation
what is an energy profile diagram?
a way to represent the energy changes that take place in reactions
where do you find exothermic reactions?
hand warmers self heating cans
what is the key fact for an energy profile diagram of an exothermic reaction?
the products have less energy than the reactants the difference in energy between the products and the reactions is the amount of energy transferred to the surroundings
what is an endothermic reaction?
a reaction that takes energy from the surroundings this means the temperature of the surroundings decreases
what is an example of an endothermic reaction?
thermal decomposition
what is the key fact about the energy profile diagram for an endothermic reaction?
the products have more energy than the reactants as it has taken in energy from the surroundings
what is the peak on an energy profile diagram? (for bot exothermic and endothermic reactions)
the activation energy
what is the activation energy?
the minimum amount of energy the particles must have in order to react
how do you represent the activation energy on an energy profile diagram?
it is the energy from the reactants to the peak of the curve
are an exothermic reaction’s energy changes positive or negative?
negative - energy has been transferred from the chemicals
are an endothermic reaction’s energy changed positive of negative?
positive - energy has been gained
what type of reaction is breaking a bond?
endothermic - it requires energy
what type of reaction is making a bond?
exothermic - it releases energy
what is the energy value of a chemical bond?
the energy required to break the bond
what is required to calculate the energy change of a reaction?
the structures of the molecules involved the energy value for each bond
how do you calculate the energy change for a reaction?
1) put the energy value for each bond under the structures of the molecules on the left 2) if the bonds are being broken, put a + in front. then add them together 3) put the energy value for the bond on the right 4) if the bonds are being made, put a - in front 5) put the right value under the left value and carry out the sum
how do you investigate the temperature change in the neutralisation reaction of hydrochloric acid and sodium hydroxide?
add increasing volumes of sodium hydroxide solution to hydrochloric acid and measure the maximum temperature reached
what are the variables of the investigation of the neutralisation reaction of hydrochloric acid and sodium hydroxide solution?
independent - volume of sodium hydroxide solution added dependent - maximum temperature reached control - volume of HCl, concentrations of HCl and sodium hydroxide solution
what is the trend for the neutralisation reaction of HCl and sodium hydroxide?
as the volume of sodium hydroxide increases, the maximum temperature reached increases this is because as more particles of sodium hydroxide are added, they react with the HCl at a certain point, the maximum temperature begins to decrease. this is because there is not enough HCl for the amount of sodium hydroxide being added so some is left unreacted. the maximum temperature has reached a maximum
why is a polystyrene cup and plastic lid used in the investigation of the neutralisation of HCl and sodium hydroxide?
the experiment is measuring temperature so the amount of heat loss needs to be minimised. polystyrene is a good insulator and prevents heat lost from the sides and bottom the lid reduces heat lost from the air
what happens if two different metals are put into an electrolyte?
they can produce electricity
what are two key facts about cells?
they can only produce electricity for a certain period of time. the reaction will eventually stop when the chemicals run out cells only produce electricity if metals with different reactivities are used