C7 Flashcards
1st law of thermodynamics
In a chemical reaction, energy can’t be created or destroyed as the total energy before a reaction(of the reactants) is the same as the total energy after a reaction(of the products).
Exothermic reactions
Transfer energy from the reacting chemicals to their surroundings, causing a rise in temperature in the surroundings
e.g.
Neutralisation
Respiration
Displacement reactions
Oxidation
Endothermic reactions
Transfer energy from the surroundings to the reacting chemicals,causing a fall in temperature in the surroundings
e.g.
Thermal Decomposition - e.g. heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide
Electrolysis
Experiment to investigate temperature changes
1) This method works for neutralisation reactions or reactions between metals and acids, or carbonates and acids.
2) Place the reactants/reagents in separate 250cm3 beakers(e.g. 25cm3 of 0.25 mol/dm3 of hydrochloric acid and sodium hydroxide).
3)Place the beakers in a water bath set to one temperature(e.g. 25C) until both the reactants/reagents are at the same temperature.
4) Add the reactants to a polysterene cul with a lid, which reduces energy lost by evaporation and heat loss by convection currents.The polysterene cup can be put in a beaker of cotton wool to give more insulation.
5) Take the temperature of the mixture every 30 seconds(use a stopwatch), record the highest temperature.
6)Repeat the experiment by changing different variables to see if they affect the amount of energy transferred e.g. mass/concentration of reactants(e.g, now 1 mol/dm3 of hydrochloric acid)
What happens in a self-heating can?
1)You press a button in the base of the can.
2)This breaks a seal, letting the water and calcium oxide mix.
3)The exothermic reaction then occurs.
How do chemical cold packs/sports injury packs work?
1)Ammonium nitrate dissolves, absorbing energy from the surroundings, making them cooler.
2)The ammonium nitrate and water are kept separate in the pack.
3)When squeezed, the bag inside the water pack breaks, releasing ammonium nitrate.
4)Cold packs are emergency treatments for sports injuries. This chemical reaction means the pack becomes instantly cooler without having to put it in the freezer
Reaction profile
Diagrams with the relative difference in the energy in the reactants and products(kJ/mol) in a reaction and how the energy changes over the course of the reaction. A curved line shows the course of the reaction and the difference in energy between the reactants and products and the peak of the curve shows the energy input required for the reaction. The initial rise in energy represents the activation energy.
The difference in height between the energy levels of products and reactions shows the overall energy change in the reaction(the energy given out) per mole.
Exothermic reaction(reaction profile)
The products are at a lower energy level than the reactants.
Endothermic reaction(reaction profile)
The products are at a higher energy level than the reactants.
Activation energy
Minimum energy needed to start a reaction before colliding particles of reactants have sufficient energy to cause a reaction
What happens if the particles of reactants collide with less energy than the activation energy?
They just bounce off each other
What process is breaking bonds?
An endothermic process - energy’s supplied to break existing bonds
What process is making bonds?
An exothermic process - energy’s released when new bonds are formed
Bond energy
Energy needed to break the specific chemical bonds between two atoms
Uses of exothermic reactions
Hand warmers - use the exothermic oxidation of iron in air(with a salt solution catalyst) to release energy
Self-heating cans of hot chocolate and coffee rely on exothermic reactions between chemicals in their bases.
