Energetics
temperature goes up EXOTHERMIC
temperature goes down ENDOTHERMIC
INTRODUCTION
When chemicals react together, the reactions cause energy changes. This is obvious when a fuel is burnt and heat energy is released into the surroundings. Heat changes in other reactions may be less dramatic but they still take place. A knowledge of chemical bonding can really help to understand how these energy changes occur.
Knowledge check shows the ideas you should have already encountered in previous work before starting the topic.
magnesium ribbon
Energy changes in reactions like these can be measured using a polystyrene cup as a calorimeter. If a lid is added to the cup, very little energy is transferred to the air and quite accurate results can be obtained. A simple equation is used to calculate the energy change. Energy = mass of the × specific heat × change in transferred to solution capacity of temperature the solution water kilojoules, kg (or g) 4.2k J/kg/°C °C kJ (or J) (or 4.2 J/g/°C)
KNOWLEDGE CHECK ✓ Know that atoms in molecules are held together by covalent bonds. ✓ Know that many common fuels are organic compounds called alkanes. ✓ Be able to write and interpret balanced chemical equations.
In short, heat change = m × SHC × ∆T Note: In this calculation it is assumed that all liquids or solutions have the same specific heat capacity as water and the same density (so 1000 cm3 has a mass of 1 kg, 1 cm3 has a mass of 1 g).
LEARNING OBJECTIVES ✓ Know the difference between exothermic and endothermic reactions and how they are represented on simple energy level diagrams. ✓ Be able to describe simple calorimetric experiments to measure heat or enthalpy changes. ✓ Understand the use of ∆H to represent a molar enthalpy change. ✓ Understand how to calculate ∆H from calorimetric results and bond energies.
WORKED EXAMPLE 1 g of magnesium ribbon was added to 200 g of 1M hydrochloric acid in a polystyrene beaker. When the magnesium had completed reacted and none was left, the temperature of the acid had risen by 30 °C. Calculate the energy change for the reaction.
ENERGY CHANGES IN CHEMICAL REACTIONS In most reactions, energy is transferred to the surroundings and the temperature goes up. These reactions are exothermic. In a minority of cases, energy is absorbed from the surroundings as a reaction takes place and the temperature goes down. These reactions are endothermic. For example, when magnesium ribbon is added to dilute hydrochloric acid, the temperature of the acid increases – the reaction is exothermic. In contrast, when sodium hydrogen carbonate is added to hydrochloric acid, the temperature of the acid decreases – the reaction is endothermic.
Equation:
energy change = mass of hydrochloric acid × 4.2 × temperature change
Substitute values: energy change = 200 × 4.2 × 30 energy change = 25 200 J or 25.2 kJ for 1 g of magnesium
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ENERGETICS
Calculate:
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PHYSICAL CHEMISTRY
hydrochloric acid
∆ Fig. 4.13 Measuring energy changes in a reaction.
∆ Fig. 4.12 Fireworks, a carefully controlled chemical reaction.
Learning objectives cover what you need to learn in this topic.
sodium hydrogencarbonate
hydrochloric acid
Worked examples take you through how to apply formulae that you need to know how to use.
reactants
products
Examples of investigations are included with questions matched to the investigative skills you will need to learn.
Developing investigative skills Two students wanted to calculate the molar enthalpy change for the neutralisation reaction between sodium hydroxide and hydrochloric acid. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
energy
energy
They decided to use a simple calorimetric method. ∆H = – (loss of energy)
products
∆H = + (energy put in)
reactants
course of reaction
course of reaction
∆ Fig. 4.15 Energy level diagrams for exothermic and endothermic reactions.
An exothermic reaction. Energy is being lost to the surroundings. ∆H is negative.
An endothermic reaction. Energy is being absorbed from the surroundings. ∆H is positive.
All ∆H values should have a + or – sign in front of them to show if they are exothermic or endothermic. Activation energy is the minimum amount of energy required for a reaction to occur. This diagram shows the activation energy of a reaction. The energy profile can now be completed as shown. The reaction for this profile is exothermic, with ∆H negative.
They put 50 cm3 of 1M sodium hydroxide (NaOH) solution in a large polystyrene cup. They took the temperature of the solution. They then measured the temperature of 50 cm3 of 1M hydrochloric acid (HCl) in a conical flask. Carefully but quickly they added the hydrochloric acid solution and stirred the resulting solution with the thermometer. They recorded the highest temperature reached. The results are shown in Table 4.5. Volume of 1M sodium hydroxide solution Volume of 1M hydrochloric acid solution Initial temperature of sodium hydroxide Initial temperature of hydrochloric acid Final temperature of the mixture
50 cm3 50 cm3 18 °C 18 °C 25 °C
∆ Table 4.5 Results of experiment.
Devise and plan ➊ What apparatus do you think was used to measure out the volumes of sodium hydroxide and hydrochloric acid?
Demonstrate and describe techniques ➋ 1M sodium hydroxide solution is highly corrosive. What safety precautions should have been used when carrying out the experiment?
activation energy
➌ How could the loss of heat energy from the polystyrene cup have been Analyse and interpret ➍ Calculate the enthalpy change that occurred in the 100 cm3 of the mixture. ➎ Use your answer in 4 to calculate the molar enthalpy change (ΔH) for the
∆H = – (energy lost)
reaction. (Do not forget to give your answer a positive or negative sign.)
products
◁ Fig. 4.16 An energy profile for an exothermic reaction.
QUESTIONS 1. The molar enthalpy change for a reaction is positive. Is the reaction endothermic or exothermic?
Evaluate ➏ How would you have changed the calculation if the temperatures of the sodium hydroxide and hydrochloric acid before mixing were different?
➐ List some possible sources of error in the experiment. Which do you think would be the greatest?
2. On an energy profile, what is the name given to the minimum
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amount of energy required for a reaction to occur?
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PHYSICAL CHEMISTRY
course of reaction
ENERGETICS
energy
reduced during the experiment?
reactants
5
Questions to check understanding.
GETTING THE BEST FROM THE BOOK
ENERGY PROFILES AND ΔH Energy level diagrams show the enthalpy difference between the reactants and products.
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