0620 · 5.1Exothermic and endothermic reactions

Exothermic and endothermic transfers

0:00A reaction hand warmer warms your fingers, while a reaction cold pack cools an injury. Both involve chemical changes, but thermal energy moves in opposite directions. The important difference is where that energy goes. By the end of this video, you will connect temperature changes, reaction pathway diagrams and chemical bonds into one explanation. We need three starting ideas about chemical reactions. Reactants are the starting substances, and products are the substances formed.

0:26Atoms are rearranged rather than created or destroyed. In reactions involving molecules, this rearrangement includes breaking existing bonds and forming new ones, and both processes involve energy transfers. To describe those transfers clearly, separate the reacting substances from their surroundings. The surroundings include anything that can exchange thermal energy with them. That might be the water in a reaction mixture, the container or the nearby air. A thermometer measures the temperature change of the surroundings, giving us evidence about the direction of energy transfer.

0:58An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings. Burning a fuel is a familiar example. Energy moves out from the reacting substances and warms nearby objects. The temperature rise is therefore a consequence of the transfer, rather than the definition on its own. Energy is not created when an exothermic reaction happens. Instead, the reacting substances finish with less chemical energy than they had initially.

1:24The difference is transferred to the surroundings as thermal energy. This explains how a reaction can warm a container even though its products have less energy than its reactants. An endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings. The reacting substances gain energy, while nearby material loses it. This is the principle behind a reaction cold pack. It does not send out coldness; it takes in thermal energy from the material around it.

1:52The word surroundings matters because it identifies whose temperature changes. An exothermic reaction makes the surroundings warmer, while an endothermic reaction makes them cooler. In a laboratory mixture, the liquid around the reacting particles can provide this evidence. Always connect the observation to a direction of thermal energy transfer, rather than simply calling a reaction hot or cold.

Reading reaction pathway diagrams

2:15A reaction pathway diagram shows the energy of the reacting substances as the reaction progresses. Energy belongs on the vertical axis, and progress of reaction goes along the horizontal axis. Read the reactants on the left and the products on the right. This is not a temperature graph, and its horizontal axis does not represent time. For an exothermic reaction, the products are lower than the reactants. That lower position means the products have less energy.

2:41The difference has been transferred to the surroundings. Although the curve first rises to a peak, its overall change is downwards because we compare the starting and finishing levels. For an endothermic reaction, the products are higher than the reactants. Energy has been taken in from the surroundings, leaving the products at a higher level. Again, compare the two ends rather than the peak. Both types of reaction can have a curve that rises before falling, so that shape alone does not identify the energy transfer.

3:09This next part is for the Extended paper. The transfer of thermal energy during a reaction is called the enthalpy change of the reaction. Its symbol is spoken as delta H. At this level, it tells us the overall energy change between reactants and products, including whether thermal energy enters or leaves the reacting substances. Think of delta H as the energy of the products minus the energy of the reactants.

3:32For an exothermic reaction, subtracting the higher starting value from the lower finishing value gives a negative result. An endothermic reaction gives a positive result because its products are higher. The sign describes the change for the reacting substances, not for the surroundings. Enthalpy changes are commonly given in kilojoules per mole. The amount relates to the reaction as written in its balanced equation. A negative value indicates thermal energy transferred out, and a positive value indicates thermal energy taken in.

4:01The size tells us how much energy is transferred for that stated amount of reaction. The rise at the start of a pathway diagram represents an energy barrier. Particles need enough energy during a collision for a reaction to occur. Activation energy is the minimum energy that colliding particles must have to react. A collision with insufficient energy cannot overcome that barrier, even if the products would end at a lower level.

4:24On the diagram, activation energy is the vertical distance from the reactant level to the peak. It is not measured upwards from the bottom of the page. Delta H connects the reactant and product levels instead. These arrows describe different changes, so they must not both point towards the peak. A fuel can release thermal energy overall but still need ignition to start reacting. The initial spark or flame helps particles overcome the activation energy barrier.

4:51Once the reaction starts, its energy release can help further particles react. Needing an initial energy input therefore does not make the overall reaction endothermic. Imagine walking over a hill to reach a valley lower than your starting position. You must first climb, even though your final position is lower. That separates the initial barrier from the overall change. The analogy helps us read the graph, but the curve represents chemical energy rather than the physical movement of particles over a hill.

Calculating values from pathways

5:17Suppose a question provides relative energy levels for a reaction. The reactants are at one hundred kilojoules per mole, the products at forty, and the peak at one hundred and eighty. First place the two end levels, then draw a curve passing through the peak. The products are lower, so the reaction is exothermic. For activation energy, subtract one hundred from one hundred and eighty, giving eighty kilojoules per mole.

5:40For delta H, subtract one hundred from forty, giving negative sixty kilojoules per mole. The upward arrow spans eighty, while the downward one covers only sixty. Their different lengths show that the initial barrier and overall energy transfer are not the same quantity. When drawing a pathway diagram, label both axes before adding the energy levels. Put reactants on the left and products on the right, with a peak above both.

6:05Draw the activation energy arrow from the reactant level to that peak. Finally, connect the starting and finishing levels with the enthalpy change arrow, directed from reactants towards products.

Bond energies and enthalpy change

6:15Chemical bonds explain why different reactions have different overall energy changes. A bond holds atoms together through attraction. Separating those atoms requires energy to overcome that attraction. Bond breaking is therefore an endothermic process, regardless of whether the complete reaction ultimately warms or cools the surroundings. Bond making is an exothermic process because energy is released when a bond forms. The bonded arrangement has less energy than the separated atoms.

6:43You must put energy in to separate them, but forming the bond transfers energy out. This is why saying that breaking bonds releases energy reverses the actual process. During a reaction, breaking bonds takes in energy and making bonds releases it. The overall enthalpy change depends on which total is greater. If bond making releases more than bond breaking requires, the reaction is exothermic, and in the opposite case it is endothermic.

7:08Imagine an energy budget with a cost and a return. Breaking reactant bonds is the cost, while forming product bonds supplies the return. A larger return leaves energy available for transfer to the surroundings. This is only a way of organising the calculation; actual reactions do not necessarily break every bond before forming any new ones. A bond energy tells us the energy needed to break one mole of a particular type of bond.

7:32These values are usually supplied in kilojoules per mole. To calculate delta H, subtract the total energy of bonds made from the total energy of bonds broken. The subtraction accounts for energy taken in and energy released having opposite effects. Use the balanced equation to count how many bonds are involved. Calculate the breaking total from the reactants, and calculate the making total from the products. Multiply each bond energy by the number of corresponding bonds.

7:57The coefficient before a substance multiplies every bond in its molecule, so it must be included.

Worked bond energy calculation

8:03Our worked example is hydrogen plus chlorine gives hydrogen chloride. The balanced symbol equation is on the board. One hydrogen molecule reacts with one chlorine molecule to form two hydrogen chloride molecules. We therefore break one hydrogen to hydrogen bond and one chlorine to chlorine bond, then make two hydrogen to chlorine bonds. The hydrogen to hydrogen bond requires four hundred and thirty-six kilojoules per mole to break. Breaking the chlorine to chlorine bond requires two hundred and forty-two kilojoules per mole.

8:34Adding four hundred and thirty-six to two hundred and forty-two gives six hundred and seventy-eight kilojoules per mole. This is the total energy taken in for breaking the reactant bonds. Each hydrogen to chlorine bond releases four hundred and thirty-one kilojoules per mole when formed. Multiplying by two gives eight hundred and sixty-two kilojoules per mole released. Subtracting eight hundred and sixty-two from six hundred and seventy-eight gives negative one hundred and eighty-four kilojoules per mole.

8:59The negative sign identifies an exothermic reaction. The result also makes physical sense because forming the product bonds releases more energy than breaking the reactant bonds requires. The excess is transferred to the surroundings as thermal energy. Our answer refers to the balanced equation as written, which produces two moles of hydrogen chloride. It is not automatically the energy change for producing one mole of that substance. One common mistake is counting only one product bond because there is one type of product molecule.

9:29The coefficient tells us that two molecules are formed, so two bonds must be counted. Another mistake is reversing the subtraction and obtaining the wrong sign. Keep energy taken in first and energy released second to match the meaning of delta H. Do not confuse a strong bond with a large amount of energy released when it breaks. A stronger bond requires more energy to separate its atoms. Energy is released when bonds form instead.

9:54Likewise, an exothermic reaction still involves energy being taken in during bond breaking; its overall release is larger than that input.

Exam practice and recap

10:02An exam question might ask you to state the reaction type and explain your answer using this diagram. It could then ask you to calculate the enthalpy change and activation energy. The reactants are at forty kilojoules per mole, the products at one hundred, and the peak at one hundred and sixty. Read those three levels before deciding which differences to calculate. The reaction is endothermic. The products have a higher energy level than the reactants.

10:27Thermal energy is taken in from the surroundings. The temperature of the surroundings decreases. These sentences connect the diagram to the energy transfer and its observable effect. Subtracting forty from one hundred gives an enthalpy change of positive sixty kilojoules per mole. For activation energy, subtract forty from one hundred and sixty to obtain one hundred and twenty kilojoules per mole. If asked to define activation energy, use the minimum energy that colliding particles must have to react.

10:54Do not replace that definition with the overall energy transferred. To complete the drawing, add the two arrows to this endothermic diagram. The activation energy arrow again starts at the reactant level and points up to the peak. The enthalpy change arrow also starts at the reactant level, but it stops at the higher product level. Because the products are higher, this arrow points upwards, matching the positive value of delta H.

11:17A question may also ask you to explain an energy change in terms of bonds. For hydrogen and chlorine, a full answer has three creditworthy points. Energy is taken in to break the bonds in the reactants. More energy is released when the bonds in the products form. The overall change is therefore an energy release, so delta H is negative and the reaction is exothermic. In the practical paper, temperature readings can provide evidence about energy transfers.

11:43You may need to record observations and describe whether the surroundings become warmer or cooler. A rise supports an exothermic change, while a fall supports an endothermic change. Explain the observation through thermal energy transfer, rather than treating a thermometer reading as a direct measurement of activation energy. The whole topic fits together through one energy balance. Exothermic reactions transfer thermal energy to the surroundings, raise their temperature and finish with lower energy products.

12:11In contrast, an endothermic reaction takes in thermal energy, lowers the temperature of the surroundings and finishes with higher energy products. Their enthalpy changes are negative and positive respectively. Activation energy describes the initial barrier, not the difference between the starting and finishing levels. Breaking bonds takes in energy, while making bonds releases it. Subtract the total energy of bonds made from the total energy of bonds broken to calculate delta H.

12:36These relationships let you explain the temperature change, interpret the diagram and check whether your calculation makes sense.