Energy changes

what is an exothermic reaction?

a reaction where heat energy is transferred to the surroundings therefore heating the surroundings

examples of exothermic reactions

combustion and respiration

what is an endothermic reaction?

a reaction where heat energy is taken in from the surroundings meaning the surroundings therefore get cold

endothermic reaction examples

instant icepacks and photosynthesis

What kind of enthalpy changes do exothermic reactions have>

negative enthalpy changes: - triangle H

What is the energy transfer in exothermic reactions?

Chemical energy turns into heat energy

What kind of enthalpy changes do endothermic reactions have?

positive enthalpy changes: + triangle H

What is the energy transfer in endothermic reactions?

Heat energy turns into chemical energy

What happens to bonds in an endothermic reaction?

They break

What happens to bonds in an exothermic reaction?

They are made

Calculation for total energy of reaction

breaking - making (endo-exo)

what is the unit for total energy of reaction

kJmol-1

temperature change during neutralisation practical steps

1. measure 30cm3 of dilute hydrochloric acid in measuring cylinder and add to polystyrene cup

2. Place cup in beaker for stability

3. measure initial temp and record in table

4. measure 5cm3 of sodium hydroxide solution

5. add sodium hydroxide to polysterene cup, fit lid and gently stir 

6. record max temp reached in table

7. repeat 1-4 to add 5cm3 of sodium hydroxide till 40cm3 added

8. repeat experiment and record in table

9. calculate mean temp for each of sodium hydroxide volumes

Why might there be anomalies from the experiment?

Heat may have been lost to surroundings in the time between the addition of the alkali and the lid being placed on

OR

incomplete stirring to evenly distribute heat

balanced chemical equation for reaction between sodium hydroxide and hydrochloric acid

NaOH + HCl → NaCl + H2O

Why might a slower group end up with worse results to a faster group?

More time for heat to be lost to surroundings → temp changes smaller

Why does starting the y axis from zero make the value of amount of alkali needed to achieve neutralisation less accurate

Larger scale means intersection reading is less accurate

How would using more dilute acid and alkali differ the results?

Larger volume of dilute acid/alkali required for neutralisation.

Smaller temperature changes -? same heat distributed in larger mass of solution

Why does temperature increase to begin with?

the reaction is exothermic and heat energy is being lost to the surroundings

Why does temperature decrease later in the reaction?

the neutralisation reaction is complete and the products are cooling down to the surrounding temp, not an endothermic reaction!!

Why does you need to draw two lines of best fit for this experiment?

First line demonstrates exothermic neutralisation reaction

Second line shows products cooling

Intersection gives idea of max temperature, minimising experimental errors

how would your results differ if a glass beaker was used instead of polystyrene?

Glass is a worse heat insulation so heat would be retained in beaker and the reaction mixture would not heat up as quickly, and max temp would be lower and not cool down as quickly