Unit 5 - exothermic and endothermic reactions Flashcards
(11 cards)
What happens with energy during chemical reactions?
Energy is given out or taken in, usually in the form of heat energy.
Exothermic vs endothermic reactions definitons
Exothermic - a reaction that releases energy into the surroundings resulting in a temperature increase. Chemical energy (in substances) -> thermal energy (in surroundings). The products have less energy than the reactants.
Endothermic - a reaction that absorbs energy from the surroundings resulting in a temperature decrease. Thermal energy (in surroundings) -> chemical energy (in the substances). Products have more energy than the reactants.
What are reaction pathway diagrams?
Graph which shows the changes in energy levels during a chemical reactions (as a reaction progresses to change reactants into products)
E.g. in an exothermic reaction, energy of reactants is higher than energy of products, so there’s straight line high up at first then a steep drop with an arrow pointing downwards to show the energy change and a straight line for the energy of the products. For an endothermic, it’s the other way round
What is the activation energy?
The minimum amount of energy needed for bonds in the reactants to break.
Particles need to have some energy when they collide to react (for a chemical reaction to take place). This means that in a reaction pathway diagram (for an exothermic reaction) first there is a straight line showing the energy of the reactants, then a slope up for the activation energy, and then a slope down until it becomes a straight line to show the energy of the products.
For an endothermic reaction, the energy levels would start low, then rise until the peak (showing the activation energy) and then drop slightly, so the energy of the products is below the activation energy but above the reactants.
Abbreviation: Ea
What does it determine when the activation energy is low or high?
Low - fast reaction because lots of particles will collide with sufficient energy for it to occur
High - slow reaction because fewer particles will collide with sufficient energy or a reaction to occur
State if bond making and bond breaking are exothermic or endothermic
Bond breaking - endothermic
Bond making - exothermic
What is enthlapy change + formula?
The overall energy change of a reaction; the difference in energy between reactants & products. Unit is kJ/mol with symbol triangle H.
In exothermic reactions, the enthlapy change is negative because the products have less energy than the reactants.
In endothermic reactions, the energy of the products is higher than the reactants so the enthlapy change is positive.
Formula: total energy of bonds broken - total energy of bonds made
Give examples of exothermic and endothermic reactions
Exothermic : freezing of water, combustion, neutralization reactions, metal displacement reactions, respiration, etc.
Endothermic: electrolysis, melting of ice, thermal decomposition, photosynthesis
How does the energy taken to break/make bonds affect whether a reaction is exothermic or endothermic?
If the total amount of energy absorbed to break bonds is greater than the total energy released during bond making, the reaction is endothermic. Vice versa exothermic.
Describe an experiment for investigating temperature changes during a reaction between magnesium and hydrochloric acid
DV: the change in temperature of the reaction mixture
IV: the mass of magnesium added to the acid
CV: type of metal, type of acid, concentration of acid, volume of acid, surface area of metal pieces
1. Measure out and pour 50ml of HCl solution into a beaker using a measuring cylinder
2. Measure out 0.2g of magnesium using a weighing boat on the balance
3. Record initial temperature of HCl
4. Add the magnesium to the acid and gently stir it
5. Record the highest temperature reached on the thermometer
Repeat steps 1-5 with 0.3,0.4,..0.7g of magnesium
The reaction between magnesium and HCl is exothermic, which means that the products have less energy than the reactants. Some of that excess energy is released as heat. The more magnesium is added, the bigger the temperature change but not infinitely as the hydrogen atoms (the bonds between H-H atoms is actually what releases the energy) will eventually run out as there is a fixed amount of HCl.
What was the mixture stirred after magnesium was added?
To spread heat evenly throughout the whole mixture which allows us to get more accurate results.
