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| Question | Answer |
|---|---|
| How does the conservation of energy apply to a chemical reaction? | the total energy before and after the reaction must be the same as energy cannot be created or destroyed in the reaction |
| Why is energy transferred in a chemical reaction? | because chemicl bonds are made which releases energy and chemical bonds are broken which requires energy |
| What is an exothermic reaction? | energy is transferred from the reacting chemicals to the surroundings |
| What is an endothermic reaction? | energy is transferred from the surroundings to the reacting chemicals |
| How would a test tube containing an exothermic reaction feel in your hand? | the test tube would feel warm, sometimes even hot |
| How would a test tube containing an endothermic reaction feel in your hand? | the test tube would feel cold, sometimes very cold |
| What does the term combustion mean? | burning in oxygen gas |
| Is the combustion of fuels exo- or endothermic? | when fuels are burned in oxygen, they give off energy so the reaction is exothermic |
| Name two reactions that are exothermic. |
• the combustion of fuels • neutralisation reactions |
| What happens when methane gas burns? | it is oxidised and gives off energy |
| Write the equation for the combustion of methane? | CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) |
| Compare the energy content of the products and reactants of an exothermic reaction. | the products have a lower energy content than the reactants |
| Why does an exothermic reaction give off energy? | the products have a lower energy content than the reactants with the difference being given off as energy |
| The reaction of citric acid and sodium hydrogencarbonate is endothermic. What will happen to the temperature of this reaction as it progresses? | it will fall |
| Name a group of reacations that is endothermic. | thermal decomposition reactions |
| Write the equation for the thermal decomposition of calcium carbonate. | CaCO3(s) → CaO(s) + CO2(g) |
| Write the equation for the combustion of methane? | CH4(g) + 2 O2(g) → 2 H2O(l) + CO2(g) |
| Compare the energy content of the products and reactants of an endothermic reaction. | the products have a higher energy content than the reactants |
| Why does an endothermic reaction get cold? | the products have a higherer energy content than the reactants so must take energy from the surroundings making them cold |
| What is a reaction called that transfers energy to the surroundings? | exothermic |
| The temperature of a reaction changes from 27 °C to 19 °C as it progresses. What type of reaction is it? | the temperature is falling so endothermic |
| Question | Answer |
|---|---|
| What type of reaction is used in chemical hand and body warmers? | exothermic |
| When are hand and body warmers used? |
• for the relief of aches and pains • to keep warm at outdoor events on cold days |
| How often can hand warmers be used? | some can be used once, and some can be used again and again |
| Describe the workings of a hand warmer that can only be used once. |
• one type uses the oxidation of iron • iron turns into hydrated iron (III) oxide • sodium is used as a catalyst • the exothermic reaction lasts for many hours • the hand warmer is disposable as it can only be used once |
| What reactions are reusable hand warmers built on? | they are based on the formation of crystals from solutions of a salt |
| Describe the operation of a reusable hand warmer. |
• CH3COO-Na+ sodium ethanoate is often the salt used • a supersaturated solution of the salt is made by disolving the salt in hot water • the solution is then allowed to cool • the pack contains a small metal disc which must be pressed to start the warmer • when the metal disc is pressed, small amounts of metal are scraped off • the small pieces of metal start the crystalisation which is an exothermic reaction • the reaction lasts for about 30 minutes • placing the warmer in hot water will disolve the crystals and make it ready for re-use once cool |
| Name another use of exothermic reactions other than hand and body warmers. | self-heating cans to make drinks like hot coffe without an external heating device like a kettle |
| State the reaction usually used in self-heating food and drink packs | calcium oxide + water → calcium hydroxide |
| How is the heating activated in a self-heating can? |
• the water and calcium hydroxide are stored in separate compartments • a button on the bottom of the can breaks the seal between the two • the water and calcium hydroxide mix • the exothermic reaction happens transferring heat to the drink |
| How are endothermic reactions useful? | they can be used to cool things down |
| What chemicals do chemical cold packs usually contain? | ammonium nitrate and water |
| What happens when ammonium nitrate dissolves in water? | it absorbs energy from the surroundings making the surroundings colder |
| Where are cold packs useful? | they are useful as emergency treatments for sports injuries so often found in first aid kits for venues of sporting events |
| How do cold packs help with sports injuries? | the decrease in temperature reduces swelling and numbs pain |
| How does an instant cold pack work? |
• the ammonium nitrate and water are kept separate in the pack • squeezing the bag inside the water breaks releasing the ammonium nitrate • the instant cold pack works for around 20 minutes • instant cold packs can only be used once |
| Where else can the endothermic reaction of nitrate and water be used? | in drinks to cool them |
| Question | Answer |
|---|---|
| What is a reaction profile? | a diagram that shows the relative amounts of energy contained in the reatants and the products measured in kilojoules per mole (kJ/mol) |
What does the line on a reaction profile show?
|
it shows the course of the reaction
|
| What does the difference in energy between the reactants and the peak of the curve indicate? | the energy required to make the reaction happen |
| What does the difference in energy between the reactants and the products of the curve indicate? | the energy released to or taken in from the surroundings |
| What is the difference in energy between the reactants and the peak of the curve called? | the activation energy |
| What are the units for the difference in energy between the reactants and the products on the curve? | kJ/mol |
| What is a reaction called where the reactants are at a higher energy than the products? | an exothermic reaction |
| What is a reaction called where the reactants are at a lower energy than the products? | an endothermic reaction |
| What is an exothermic reaction? | a reaction that releases energy to the surroundings |
| What is an endothermic reaction? | a reaction that takes energy from the surroundings |
| What would you feel if you were holding a beaker with an exothermic reaction taking place in it? | your hand would get hotter |
| What would you feel if you were holding a beaker with an endothermic reaction taking place in it? | your hand would get colder |
| What is the effect of an exothermic reaction on the surroundings? | the surroundings get hotter and its temperature rises |
| Sketch the reaction profile for an exothermic reaction. |
|
| Sketch the reaction profile for an endothermic reaction. |
|
| What is the energy required to start a reaction called? | the activation energy |
| What is the activation energy? | the minimum amount of energy before colliding particles have sufficient energy to react |
| What will happen to colliding particles if they have less than activation energy? | they simply bounce off each other |
| What happens to the bonds when a reaction takes place? |
• first the chemical bonds between the atoms or ions in the reactants are broken • then new chemical bonds are formed to make the products |
| What is supplied to break bonds? | energy has to be supplied |
| What type of process is breaking bonds? | an endothermic reaction |
| What happens when bonds are made? | energy is transferred to the surroundings |
| What type of process is making bonds? | an exothermic reaction |
| How can bond breaking and making to determine the type of reaction? | by comparing the energy required to break and make the bonds in a reaction, it is possible to determine whether a reaction is exothermic and endothermic |
| Question | Answer |
|---|---|
| What happens to energy when new bonds are formed? | energy is released |
| What happens to energy when existing bonds are broken? | energy is absorbed |
| What determines whether a reaction is exothermic or endothermic? | the difference in energy between making and breaking bonds |
| Describe bond energy changes in an exothermic reaction. | the energy needed to break the bonds in the reactants is less than the energy released when forming new bonds in the products |
| Describe bond energy changes in an endothermic reaction. | the energy needed to break the bonds in the reactants is greater than the energy released when forming new bonds in the products |
| What is bond energy? | the energy needed to break the bond betweeen two atoms is called the bond energy |
| What are the units for bond energies? | kJ/mol |
| How do you calculate the energy change for a chemical reaction using bond energies? |
•calculate the energy needed to break the bonds in reactants •calculate the energy released to form the bonds in products •take the difference |
| Is the energy for breaking a specific bond different to forming the same bond? | no, the energy for breaking and forming a bond is the same |
| The bond energy to break a C-C bond is 347 kJ/mol. What is the energy released when forming a C-C bond? | 347 kJ/mol |
| Question | Answer |
|---|---|
| What chemical concept drives electrical cells and batteries? | the differing reactivity of metals |
| What determines displacement in a chemical reaction? | the reactivity of the metals involved |
| What ions do metals form? | they tend to lose electrons and form positive ions |
| Complete the following reaction: Zn(s) + CuSO4(aq) → |
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) |
| What type of reaction is Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) |
a displacement reaction |
| What is a spectator ion? | the ions that do not change in a displacement reaction |
| What are the spectator ion(s) in the reaction Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) |
SO42- |
| What must be done to write a displacement reaction as an ionic equation? | leave out the spectator ions |
| Write the equation as an ionic equation Zn(s) + CuSO4(aq) → ZnO4(aq) + Cu(s) |
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) |
| Write the half equations for Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) |
Cu2+(aq) + 2e- →Cu(s) Zn(s) → Zn2+(aq) + 2e- |
| What is happening to Cu is the following equation Cu2+(aq) + 2e- →Cu(s) |
the reduction of copper i.e. copper ions are gaining electrons and becoming copper atoms |
| What is happening to Zn is the following equation Zn(s) → Zn2+(aq) + 2e- |
the oxidation of zinc i.e. zinc atoms are losing electrons and becoming zinc ions |
| What is a battery? | two or more cells |
| Why are cells combined to form a battery? | to increase the voltage |
| Where does the voltage in a battery come from? | the difference in reactivity between the two metals |
| What are the basic components of a cell? | two metals connected by a wire and dipped in an electrolyte |
Explain what is happening in the Daniel Cell below.
|
• zinc atoms from the zinc rod donate electrons to become Zn2+ ions • the zinc rod is known as the zinc electrode • the zinc rod is the negative terminal of the cell • the electrons travel through the wire to the copper rod • the travelling electrons are a current • Cu2+ ions in the salt solution accept the electrons to become copper atoms • the copper rod is known as the copper electrode • the copper rod is the positive terminal of the cell • the electrons will flow until the Zn atoms or Cu2+ ions are used up • the voltmeter will move to show a potential difference between the electrodes |
| What determines the reactivity of a metal? | metals have a tendency to give always electrons the greater the tendency to give away electrons, the greater the reactivity of the metal |
| What determines the voltage produced in a cell? | the difference in reactivity between the metals the greater the difference in reactivity between the metals, the higher the voltage produced |
| What is a reducing agent? | a substances that causes another substance to be reduced |
| Which metal is reduced in a Daniel Cell? | Cu2+ ions are reduced |
| Which metal is the reducing agent? | Zn is the reducing agent as it causes Cu2+ ions to be reduced |
| What is a primary cell? | a non-rechargeable battery |
| Are the dry cells made with zinc and carbon chargeable or non-rechargeable? | non-rechargeable |
| Are alkaline batteries chargeable or non-rechargeable? | non-rechargeable |
| How does a rechargeable battery work? | the reactions at each electrode is reversed when the battery is connected to a power supply |
| Question | Answer |
|---|---|
| Write the equation for the combustion of hydrogen. | H2 + O2 → H2O |
| What is the benefit of using hydrogen as a fuel? | it burns well and produces no pollutants so could help to reduce the human impact on global warming |
| How does hydrogen as a fuel assist with the problem of global warming? | it does not produce carbon dioxide |
| What are the issues with using hydrogen as a fuel? |
• there are problems with safety and storage of hydrogen for use in a car engine • producing hydrogen by electrolysis uses electricity produced using non-remewable fuels at a power station producing carbon dioxide |
| What is a more efficient use of the electricity from hydrogen to power vehicles than using hydrogen as a fuel? | oxidising hydrogen in a fuel cell |
| What happens in a hydrogen cell? | oxygen and hydrogen react to produce water |
| What makes a hydrogen cell more efficient than using hydrogen as a fuel? | most of the energy released in the reaction is converted to electrical energy |
| What is a use of a hydrogen cell? | hydrogen cells can be used to power a vehicle |
| What is the main disadvantage of a fuel cell? | it needs a constant supply of hydrogen |
| What is the challenge with vehicles powered by hydrogen as a fuel or that use a hydrogen fuel cell? | matching the performance, convenience and price of petrol and diesel vehicles |
| What is the negative terminal of a cell called? | the anode |
| What is the positive terminal of a cell called? | the cathode |
| Which direction do electrons flow? | the anode to the cathode i.e. from negative to positive |
| Which direction does conventional current flow? | the cathode to the anode i.e. from positive to negative |
Below is a simple hydrogen fuel cell. Describe how it works. 
|
• hydrogen gas is supplied to the negative electrode • hydrogen diffuses through the carbon electrode • hydrogen reacts with hydroxide ions to form water • the formation of water provides electrons to an external circuit • water diffuses through the electrode • oxygen gas is supplied to the positive electrode • oxygen diffuses through the carbon electrode • oxygen reacts with water to produce hydroxide ions • the formation of hydroxide ions accepts electrons from an external circuit |
| Write the half equation for the reaction at the anode of a hydrogen fuel cell. | 2H2(g) + 4OH-(aq) → 4H2O(l) + 4e- |
| Write the half equation for the reaction at the cathode of a hydrogen fuel cell. | O2(g) + 2H2O(l) + 4e- → 4OH-(aq) |
| Write the overall equation for the hydrogen fuel cell. | 2H2(g) + O2(g) → 2H2O(l) |
| State the advantages of hydrogen fuel cells. |
• do not need to be electrically recharged • no pollutants produced • can be a range of sizes for different uses |
| State the disadvantages of hydrogen fuel cells. |
• hydrogen is highly flammable • hydrogen is sometimes produced for the cell using non-renewable sources • hydrogen is difficult to store |
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