Energy Change During Chemical Reactions

TLW identify the sources of energy changes in chemical reactions Energy Change During Chemical Reactions Endothermic and Exothermic Changes

When we finish you should be able to • Define exergonic reaction • Define exothermic reaction • Define endergonic reaction • Define endothermic reaction • Know what is a catalyst • Know what is a inhibitor

Exothermic & Endothermic Exothermic & Endothermic

Exothermic reactions Exothermic reactions • An exothermic reaction is one which releases heat energy to the surroundings • The temperature of the surroundings increases

Examples include: Burning reactions including the combustion of fuels. Detonation of explosives. Reaction of acids with metals. Magnesium reacting with acid Thermit reaction Exothermic Reactions Exothermic reactions increase in temperature.

Activity Say whether these processes are exothermic. Charcoal burning A candle burning. A kettle boiling Ice melting A firework exploding yes yes no no yes You have to put heat in for boiling and melting. You get heat out from all the other processes

Magnesium + Hydrochloric acid 45o C 25o C magnesium Hydrochloric acid Exothermic Reactions Heat energy given out Gets hot

Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. Exothermic Reactions 45o C 25o C Chemical energy becomes heat energy. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings. It follows that reaction products have less chemical energy than the reactants had to start with.

Reactants have more chemical energy. reactants Some of this is lost as heat which spreads out into the room. Energy / kJ) Products now have less chemical energy than reactants. products Progress of reaction (time) Energy Level Diagram for an Exothermic Reaction

H is how much energy is given out reactants H is negative because the products have less energy than the reactants. H=negative Energy / kJ products Progress of reaction Energy Level Diagram for an Exothermic Reaction 2. Energy Level Diagram for an Exothermic Reaction

Energy / kJ) Progress of reaction reactants products Exothermic Reaction - Definition Exothermic reactions give out energy. There is a temperature rise and H is negative. His negative

Activity

Endothermic reactions Endothermic reactions • An endothermic reaction is on which takes in heat energy from the surroundings • The temperature of the surroundings decreases

Endothermic chemical reactions are relatively rare. A few reactions that give off gases are highly endothermic - get very cold. Dissolving salts in water is another process that is often endothermic. Endothermic Reactions Endothermic reactions cause a decrease in temperature.

Heat energy taken in as the mixture returns back to room temp. Ammonium nitrate Water Endothermic Reactions Endothermic reactions cause a decrease in temperature. Cools Starts 25°C Cools to 5°C Returns to 25°C

The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. Endothermic Reactions The reactants gain energy. 25o C 5o C 25o C This comes from the substances used in the reaction and the reaction gets cold. Eventually heat is absorbed from the surroundings and the mixture returns to room temperature. Overall the chemicals have gained energy.

This is how much energy is taken in products This is positive because the products have more energy than the reactants. H=+ Energy / kJ) reactants Progress of reaction Energy Level Diagram for an Endothermic Process

Energy / kJ Progress of reaction products reactants Endothermic Reaction Definition Endothermic reactions take in energy. There is a temperature drop and H is positive. H=+

Are these endothermic or exothermic? A red glow spread throughout the mixture and the temperature rose. The mixture bubbled vigorously but the temperature dropped 150C. Hydrazine and hydrogen peroxide react so explosively and powerfully that they are used to power rockets into space. The decaying grass in the compost maker was considerably above the outside temperature. Activity exo endo exo exo

Endothermic and exothermic reactions Step 1: Energy must be SUPPLIED to break bonds: Step 2: Energy is RELEASED when new bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC

Energy level diagrams Activation energy Energy given out by reaction Using a catalyst might lower the activation energy Energy level Reaction progress

Exothermic vs endothermic: EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration) ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis)

Burning Methane CH4 + 2O2 2H2O + CO2 To burn methane you have to break all of these bonds: And then you have to make these ones:

Burning Methane CH4 + 2O2 2H2O + CO2 Methane Oxygen Water Carbon dioxide

Bond energies C-H = 435 Kj O=O = 497 Kj H-O = 464 Kj C=O = 803 Kj Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol Total energy change = 2734-3462 = -728 KJ/mol

Drawing this on an energy diagram: 2734 Kj 3462 Kj -728 Kj More energy is given out (3462) than is given in (2734) – the reaction is EXOTHERMIC. The total (“nett”) energy change is –728 Kj. An endothermic reaction would have a positive energy change.

Bond energy values C-H = 435 KJ/mol O-H = 464 KJ/mol O=O = 497 KJ/mol C=O = 803 KJ/mol C-O = 360 KJ/mol C-C = 346 KJ/mol

Hydrogen peroxide decomposes as shown: Calculate energy for bond breaking. Calculate the energy from bond making What is the value of H for the reaction shown Activity H H O O O O H H O O H H H H O O

Bond breaking. (endothermic) 4(O-H) + 2(O-O) =1856+292 = +2148kJ Bond forming: (exothermic) 4(O-H) + 1(O=O) =1856+498 = -2354kJ H = +2148 – 2354 = -206kJ (Exothermic) Answer H H O O O O H H O O H H H H O O

Copy the summary using the words from the box to fill in the gaps: Activity • Exothermic reactions are _____. • Reactions that get cold are called _____. • Bond forming is an _______ process. • Endothermic reactions have a _____ H. • In exothermic reactions the chemicals ___ chemical energy. common endothermic exothermic positive lose continued

Copy the summary using the words from the box to fill in the gaps: Activity • The energy needed to start off a reaction is called the ______ energy • In endothermic reactions bond breaking requires ___ energy than is produced by bond forming. • Bond breaking is an _________ process. activation more endothermic

Which of the following is an endothermic process? Burning wood Reacting an acid and alkali Reacting magnesium with acid Dissolving ammonium nitrate in water

Which of the following is true for an exothermic process? Energy / kJ Progress of reaction products reactants • The reactants lose energy to the surroundings • The reaction gets cold • The energy diagram will be as shown • The reaction will have a H that is positive (+).

Which of the following is true when chemical bonds are broken? • The process is exothermic • Energy is given out • A physical change is occurring • The reaction will have a H that is positive(+).

Which of the following is true for an exothermic reaction? • Bond breaking involves a bigger energy change than bond making • Bond making involves a bigger energy change than bond breaking • Bond making involves the same energy change as bond breaking

Which of the following is the activation energy: X Y Z

Energy Change During Chemical Reactions